Theory and Technique of Soaring

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PaTIsW


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THEORY AND TECHNIQUEOF SOARING


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THEORY ANDTECHNIQUEOF SOARING

BYJOHN KUKUSKI

LONDONSIR ISAAC PITMAN & SONS, LTD.


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First published 1952

SIR ISAAC PITMAN & SONS, LTD .PITMAN HOUSE, PARKER STREET, KINGSWAY, LONDON, W.C.2

THE PITMAN PRESS, BATHPITMAN HOUSE, LITTLE COLLINS STREET, MELBOURNE

27 BECKETTS BUILDINGS, PRESIDENT STREET, JOHANNESBURGASSOCIATED COMPANIES

PITMAN PUBLISHING CORPORATION2 WEST 45TH STREET, NEW YORK

SIR ISAAC PITMAN & SONS (CANADA), LTD.( INCORPORAT ING TH E C O M M E R C IA L TEXT B O O K COMPANY )PITMAN H O U S E , 381-383 CHURCH S T R E E T , TORONTO

MADE IN GREAT BRITAIN AT THE PITMAN PRESS, BATHE2 (A.i6i)


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PREFACET H IS book hasbeen written toprovidea guideand a quickre ferencefor soaring pilots who m ay feel in need of an explanation of th ephenom ena and of th eunfamiliarterms which th ey will meet duringtheir flying career. I have tr ied to keep this explanation as simpleas possible and have used m athematical formulae only where th eycould not beavoided.I should like to stress th e point that it is difficult to form ulateexact rules in gliding and soaring. The individual approach to flyingte chnique leaves an open field for research and investigation.The facts herese t down are based onmy own personal experiencegained in the past while gliding and soaring on the continent ofEurope and in Britain.I should like to acknowledge th e invaluable help which I havereceived from Mr. D . J. Farrar, M r. M. R . Chantrill, M r. R. F.Taylor, and M r. T. R. ^Y oung, all ofth e Bristol Gliding Club, andalso to express m y thanks to M r. J. F. Douglas of Sir Isaac Pitman& Sons, Ltd., for theworkhe has done in theimprovem ent of textand layout.


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CONTENTSCHAPTER *A OEPreface . . , . . . . . . . v

List ofSymbols.......xI. M E T E O R O L O G Y ........II. D I R E C T S U N T H E R M A L U P -C U R R E N T S . . . .45III. I N S T R U M E N T S ........6IV . M E C H A N IC S O F S O A R IN G F L I G H T . . . .71V. L O A D S A C T I N G O N S A I L P L A N E . . . . .90V I. L A U N C H IN G T E C H N IQ U E . . . . . .104V II. L A N D IN G T E C H N IQ U E . . . . . . .110VIII. C IR C L IN G T E C H N IQ U E . . . . .118IX . T E C H N IQ U E O F S O A R IN G . . . . .129X . S A I L P L A N E A E R O B A T IC S......4 9XI. A IR N A V IG A T IO N . . . . . . .167

X II. T H E P A R A C H U T E A N D I T S U S E . . . . .172X III. I N S T R U C T IN G . . . . . . . .178

Index.........8 1


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LIST OF SYMBOLSa alpha angle of attack, w ing chord to wind (L.E.-T .E . chord)a lso radial accelera tiona1 absolute angle of attackof aerofoil (from nolift line) =(a a^a0 no lif t angle of aerofoil, angle of a ttack of wingchordat no lift(usually ve)ay angle of attack of tail plane to local airstream

a.c . aerodynamiccentreA are a (ft2 )A aero dynam ic force/62 \A.R . aspect ratio I IA.S.I, indicate d airspeed in m.p.h. (diffe rs from E .A .S . by correctionfo rinstrumen t and posi tion errors )

b span ofw ing or ta il plane in feetfi beta sid e slip (angle of)B.Th.U . British Therm al U nitC chord of w ing or tail plane( S\-CD to ta l drag coefficientCDB para site drag coefficien t = ( \QX o/CD i induced dragcoeffi cient (of w in g) = (CDP + CD i )

profile drag coefficien t (o f w in g = ( ~ s = ( CDW ~CD i)D oCDP (parasi te + profile) drag coefficien t = ( ,1 =(C D CD i )

\Q X *3 /

CDW wing drag coefficie nt =Cv skin friction coefficie ntCL to ta l (w ing +tail) lift coefficient

CLT ta il plane liftcoefficien t = ( \ Q X O

CLW w in g lift coefficien t = ( i x


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LIST OF SYMBOLS

CM pitchingmoment co efficient = ( - -}M * & \q X S X clCA normalforce co efficient=(CL cos a+CD sin a)CP centreofpre ssure coefficient

' dis tance ofC.P . from aero foil L.E. \chord of aero foil /

CT tangential (chordwise) force co efficient=(CD cos a CLsina)C.G . ce ntre ofgravityC.P. ce ntre ofpre ssure

. specificheat atconstant pressurey gamma ratio of - specific heat at constant volume

als oangleof glided diameterd delta de flection or setting of co ntro l surfaceD totaldrag of sailplane =(D P + D^-D B+D 0 +A)D B body (plifs tail unit) parasite drag =(DP D 0 )

D F skin friction dragD i wing induced dragD 0 wing profile drag =(skin friction nduced drag)= (D F ~D )Dp (parasite plus wing pro file drag) = (DA)= (D B + D 0 )Dw wingdrag(+vebackwardsalongwind)=(D0 +A)e eps ilo n downw ash (+ve downw ards)E.A.S. equivalent airspeed=(A .S .I. +correction forinstrumentand positio n error)

ft footorfeetg accele ra tiondue to gravity =32-2 ft persec2h heightabovesea-leve l (f t)

hT total head of air stream =(p +q) Ib perft2H centrifugal forcei setting ofwingchord to fuselagedatum/ moment of inertiaI.A .S . indicate d airspeed (m .p .h.)6 theta angle of pitch (H-ve lo oking to starboard, nose rising)/ arm (e .g. C.P. of tail toC.G .)L lift(+ve up,normal to wind)L.E. leading edge


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LIST OF SYMBOLS X I,, . w eig ht in Ib m m ass (slugs) = 2=

also m etr e &m ax m axim um valuem in m inim um valueM pitching m om ent (+velo okingto starboard, noseris ing)ML ro lling m om entMT pitc hingm om entdue to tailplane =(M M N O TAII)M . A .C . m ean aerodynam icchord

fi m u coefficient offrictionv nu coefficient ofkinem atic viscosityN yaw in gm om ent (+ve lookin g dow n port w in g advancing) norm al accele ration fa cto rN .T .S. norm al top speed= 0-87 V m ax.< f > phi angle ofrollp staticpressu re of air (Ib perft 2 )P tail lo adq dynam ic pressu re of air stream =% p v 2 = \pQ v (Ib per ft 2 )r ra dius (ft)p rh o air m ass density (slu gs per ft 3 )s sem is pan of w in g= ( -als o secondS area of w in g (ft2 )

, . , / density at height \a sie m a air relativedensity = I -.: ^ f ,b J Vdensityatsea-level/t tim e (sec)also m axim um thickness of w ingT te m perature (F ) or (QT.A.S. true air speed (m .p .h.)T .E . trailingedgeu cir cum ference (ft )v velocity (ft/sec)

VD design div in g speed (m .p .h.) vv vertical velocity (ft/ sec)vw velocity of w indV velo city (m .p.h .)V landing speed (m .p.h.)

Vm m in im um speed (m .p.h .) m a x i m u m speed (m .p.h .)


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Xll LIST OF SYMBOLSvs sinking velo city (ft/sec)

VST stalling speed (m .p .h .)Vss sideslip speed (m .p.h.)VT te rminal velocity (m.p.h.)

VT8 towing velocity (m.p.h.)w wing loading (Ib /f t2 )W w eight o f sailplaneX axis (+veforwards)Y axis (+veto starboard at 90 to X axis)Z axis (+ ve downw ards at 90 to X and Y ax is)

axis ofreference fixed in C.G. of th esailplane


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CHAPTER IMETEOROLOGY

T H E source of the main energy available to th e soaring pilot is oftenan unknow n factor to him . The scarcity of really interesting andsuccessful flightsis largely due to lack of knowledge of this importantsubject. This part of th e book is not intended as a meteorologicaltreatise, however, and it is limited to the discussion of m atterswhic h may be useful to m any soaring enthusiasts during therm aland cross-country flights and nights in clouds.How often th e soaring pilot takes offwithou t having th e slightestidea ofthe condition of th e air or its behaviour; how often lack ofinformation about the weather is the main th ing that spoils a flightwhich m ight otherwise be perfect. It is of immense im portance fo rth e soaring pilot to know th e qualities of th e air when he makes aflight, and these can be explained quite easily in terms of th ermodynamics, a science in which the soaring pilot will find m uchfascinating m atter to be explored.

THE ATMOSPHEREThe atmosphere is a m ix ture of a number of gases and vapourswhich surroundth e earth and rotate withit. These gasesand vapoursdiffer radically from one another in every particular. The chiefindependent gases in the atmosphere at th e surface of th e earth arenitrogen (78 per cent), oxygen (21 per cent), argon, carbon dioxide,hydrogen, neon, helium and water vapour whic h is fo und in varyingquantities. For any given pressure, th e higher th e temperature th egreater the percentage of water vapour which may be present, butthis seldom amounts to m ore than 2 -5 per cent of t h f e total gasespresent. A t th e surface of the earth the average proportion of watervapour is 1 per cent although of course it is greate r over th e oceanthan over th e land. The percentages of all these gases are constantto a height of approximately 7 miles (except fo r wate r vapour),and this is th e average lim it of noticeable vertical movem ents ofth e air.As well asth e aboveconstituents th e air also containsth e followingim puritiesdust, soot and salts. Dust and soot are most frequentlyfound over industrial regio ns. The minute particles of salt in theair are formed by th e actionof th e win d in to ssing up spray from the

1


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2 THEORY AND TECHNIQUE OF SOARINGsea, and by evapora tion. These im purities are of great im portanceto the soaring pilot, as th ey change the visibility. If there are noimpurities in the air there w ill be no appreciable condensation ofwater va pour and visibility will be good.

PRESSUREThe a tm ospheric pressure is the pressure of a column of air actingon a unit area. The average pressure in th e British Isles is 14-7 lb/in. 2M ercury pla ced in a gla ss tube and turned upside-dow n in a vesselfull of mercury, w ill show that if th e atmospheric pre ssure is1 4-7 lb/in. 2 th e height of m ercury in th e glass tu be , above the levelof the mercury in th e vessel will be 30 in. If this apparatus is

-Vacuum

Pressure ofairAtmosphericPressure =30 n=1015m b

-Mercury

F IG . 1 . S I M P L E M E R C U R Y B A R O M E T E R S H O W I N G A G L A S S T U B E F I L L E DW IT H M E R C U R Y A N D I N S E R T E D I N A V E S S E Ltaken up in th e a ir th e he ight of th e mercury will decrease withaltitude because the smaller pressure of the air will be unable tosupport it.This set-up of glass tube and ve ssel fitted w ith a m illibar sca le isca lled a barometer. 1000 millibars equal 1 bar, which is equivalentto 29-53 in. In the British Isles th e average pressure a t sea-level is1013:2 m b. A s th e ch anges in the height of mercury w ith altitudeare directly proportionate , th e barometer may be used as an altitudein dica to r and is then ca lled an altimeter. A change of 1 m b isequivalent to 30 ft . A ltitude , how ever, as w ell as being a functionof pre ssure is als o a functio n of a ir density and te mperature , andnorm ally altimeters are calibrated to give readings based on astandardatmosphere .To simplify th e comparison of performances, the major countriesof th e w orld have adopted th e International Standa rd A tm osphere.


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METEOROLOGY J

STANDARD ATMOSPHEREThis is an arbitrarycondition oftem pera ture, pressu re and densitywith altitude whichis usedfor com paringperformances, as follows

Ground temperature T= 15C =59F Isotherm al tem peratureT= 55C = 67F Tem perature gradient a =0-0065 C/m=0-003566F/ftThe air a perfect, dry gas.

The anero id barometer can be substituted for th e mercury type,and as its name implies, this is a non-liquid instrum ent. The main

Clockwork A neroidFIG. 2 . B A R O G R A P H U S E D F O R A L T I T U D E R E C O R D I N G D U R I N G S O A R I N G FL IG H T

part of th is barometer is a disc-lik e vacuum cell, th e corrugatedflexible ends of which are held apart by a short, stiff spring. Iftheatmospheric pressure changes, th ere will be corresponding flexureofthe springand th e m otionof this spring will be indicated by meansof a poin te r or record ing pen. The movements of th e pointer orrecordin gpen are usually com pensated, by means of abim etal arm ,for temperature changes.The barograph is an automatically recording in strum ent whichprovides a continuous graphic record of the atm ospheric pressure.It is usuallyan aneroid which moves a pen instead ofa poin te r, andth e pen tra ces the pressure-record on a chart carried by a drumoperated by clock work . It can be seen that this in strum ent canin dicate a change of pressure due to height or due to weath er, andin the form er case it is called an altimete r and in th e la tte r abaro meter.


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4 THEORY AND TECHNIQUE OF SOARINGThe difference between an altimeter reading and th e standardatmosphere may be as much as 300 ft and for this reason a thermo

graph is required, as well as a barograph, for the accurate registration of altitude record flying.TEMPERATURE

The su n is the only source ofheat energy that is su pplie d to thesu rface of th e earth and to th e atmosphere. This is of th e greatestimportance to th e soaring pilot because th e heating of th e earth isresponsible for th e varying conditions of th e troposphere. Anychange in the intensity of th e radiation absorbed by th e earth

Clockwork inside Cylinder A mplifying LeversFIG . 3 . T H E R M O G R A P H T E M P E R A T U R E R E C O R D E R

during a day leads to about th e same change in th e average intensityof th e radiation lost by th e earth. Changes in th e temperature andtemperature-distribution of th e atmosphere provide the strength ofwinds and other weather elements. This subject will be discussed indetail in th e chapter on th ermo-currents. For the moment th ediscussion will be limited to temperature only.Normally, with increase of height the temperature of the atm osphere decreases, and any portion of the atmosphere transferredw ithout loss or gain of heat from one level to anoth er, has at everystage the same temperature and density as the surrounding air. Therate of change of air temperature per unit increase of height aboveth e earth's surface, is called the lapse rate. The low er regio n ofatmosphere, where the temperature fall averages 5-4F per 1000 ft iscalled the troposphere. The upper region, where th e temperatureremains constant with increase in height, is called th e stratosphere.


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METEOROLOGY

The temperatures of th e air are measured by means of a th ermograph. This is a self-recording th ermom eter w hich provides acontin uous graphic record of th e tempera ture. The type m ostcommonly used in meteorology is the bimetallic thermograph. TheFt

2OOO-I

53-6 F

FIG. 4. N EU T RA L C O NDITIO N SBubble 'of air mec hanically forced upw ards retains ch aracteristics of surroundingair.

Ft 53"F '{ *Sl 46'F (g?56 F

59 F

52-5 F

59 FFIG. 5 . S T A B L E C O N D I T I O N SThe atm ospheric lapse rate is 3F pe r1000 ft.The bubble of air has a tendency to fall to alower level.

FIG. 6 . D R Y A D I A B A T I C L A P S E R A T EC O N D I T I O N

The bubble ofair moves vertically, expandsand cools.

bim etallic strip, consisting of tw o curved th in sheets made frommetals of w idely different thermal expansions, is fixed to the frame.The rise in temperature changes th e curvature ofthis strip and itsfree end moves up or dow n.A decrease in temperature of th e atmosphere of 5-4F per 1000 fta(A.i6i)


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6 THEORY AND TECHNIQUE OF SOARINGincrease of height is known as th e dry adiabatic lapse ra te drybecause no cloud will develop in such a condition, and adiabaticbecause no heat is being transferred to or from it . If a bubble ofair has moved vertically with altitude (Fig. 4) it will expand and coolat th e dry adiabatic lapse rate. The surrounding air has a lapse rateof 5-4F per 100 0 ft. The result of equal temperature between th ebubble of air and the surrounding air wil l be that no change indensity, and no vertical motion, wil l be imposed on the bubble ofair. This condition is called neutral.Should th eatmospheric lapse rate be 3F per 1000 ft, i.e. less thanth e dry adiabatic lapse rate, th en the dry bubble of air which has

Ft2OOO 54F

Temperature Inversion ;

53 FFIG. 7. TEMPERATURE I N V E R S IO NWhen the temperature of air increaseswith height up to 1000ft and thendecreases, temperature inversion willbe formed at 1000 ft, and the bubbleof air moving upwardswill be stoppedat the inversionlevel.

5 10 15 FFIG. 8. E FF E C T O F C H A N G E IN D R YA DI A B A T I C LA PSE R A T E O N A T M O S PHERIC C O N DI T I O N S

It can be said that the dry adiabaticlapse rate drops steadily at the rate of5-4F per 1000 ft, which is shown as acontinuous line. All lines below thiswill indicate greater lapse rateunstablecondition.

been moved vertically (mechanically) as shown in Fig. 5 , will coolat th e dry adiabatic lapse rate which is 5-4F per 1000ft and wil lhave a tendency to fall to a lower level. This is because the bubblewill be colder and denser than the surroundin g air. This conditionof th e air is called stable. O n th e other hand, if the temperature ofth e surrounding air is lower than th e temperature of the air bubble,and th e lapse rate equal to 6-5 F, then it is natural that the bubbleof air wil l have a tendency to rise. It will cool at th e dry adiabaticlapse rate and, being warmer and lighter, will move upwards. Thiscondition ofthe air is called unstable. The greate r the temperatu redifference at the beginning of bubble-movement, the higher th ebubble will be able to travel before it cools to the same te m peratureas its surrounding air. Unstable conditions of air do not exist forvery long periods, because changes of weather restore the atmosphereto stable conditions.


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METEOROLOGY /

Som etim es the tem perature o f the air in cre ases with heig ht, afterwhic h it decre ases in the usual way . This condition is called te m pera turein versio n. Forinstance, thesurfaceof theearth , whichis am uch better rad iator than the atm osphere, very often cools to alo w er tem perature than the air 300ft or so above it, especial ly during clear nights. H ence, when the sky is clear and there is avery light w in d or none at all , the tem perature of the air near thegro und in cre ases w ith in cre ase in heig ht. E ven when th ere issufficient w in d to prevent this in versio n, the lo w er le ve l is sti llcolderthan itoth erw is e w ould be. T he lapserate o f ah - w henin versio n takes place is called the negativ e lapse rate .HUMIDITY

A ir whic h has m oved over the ocean contain s a quan tity of w ate r, and the am oun t ofw ater present in agiv en volum eof air is know nFtI5OOO

12 000

6O O O

3OOO

Lapse ratesmaller thandry adiabatic .S aturation odiabaticla pse rateC

Dry adicfbaticlapse rate^10 -22 -4 1 4 32 50FF IG . 9 . V A R I O U S C O N D I T I O N S O F T H E A T M O S P H E R E

as it s re la tive hum idity . This value is usually given as a percentage.The re lative hum idity over the Briti sh Isle s is appro xim ately 70per cent.W henever w ate ris evaporated in to theair it takes 318 B .Th.U .s to convert each 0 -035 oz of w ater in to vapour, and conversely it canbe said thatwhenever vapour is condensed in to w ater318 B .Th.U .sare rele ased from 0-035 oz of vapour. W hen a rising bubble ofairbecom es saturated due to ad ia baticcooling, the actualrate o fcoolingbecom es sm aller, as there is a re lease of heatdue to condensation .T he valu e of th is newrate is appro xim ately halfofthe dry adiabatic lapse ra te .


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8 THEORY AND TECHNIQUE OF SOARINGThe maximum amount of water-vapour which th e atmospherecan absorb depends entirely upon the te m perature, and the higherth e temperature of the air, the more water-vapour it can absorb.When th e air can contain no more water, it has a relative humidityequal to 100 per cent and is said to have reached saturation point.When th e temperature of saturated water-vapour decreases, th eresult will be th e condensation of th e water into small visible drops.If the pressure and tem perature are constant, then air which containssay 4 per cent water vapour is le ss dense than air containing only0-1 per cent.If the lapse rate is shown as AF (Fig. 9) and is smaller than the dry

adiabatic, and AS is the dry adiabatic (the lapse rate of th e upwardCompensatingComb

Clockwork AdiustinqScrew 3FIG. 1 0 . H Y G R O G R A P H H U M ID IT Y R E C O R D E R

moving unsaturate d bubble of air), then the saturation adiabaticlapse rate will be BCHDE. This means that th e rising air will firstcool at th e dry adiabatic lapse rate (along line AB) then from B to Cth e cooling rate will be th e saturated adiabatic. The air along ABCwill be colder than th e surrounding air and w ill be stable. AlongCHDEthe air will be warmer and unstable.The relative humidity is th e ratio of th e quantity of water-vapourpresent in th e air, to the saturation quan tity for a given temperature.The absolute humidity is th e ratio of th e volume of water-vapourpresent, to th e volume ofair. Dew po int is the tem perature at whichsaturation has been reached without change of pressure, and atwhich condensation of the water-vapour into liquid begins.The amount of moisture in th e air is measured by means of ahair hygrometer, or by a dry bulb and wet bulb thermom eter. Thehygrometer or hygrograph is a self-recording instrument and givesa continuous graphic record of th e relative humidity by means of


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METEOROLOGY 9

the m easurem entof the variation in th elength ofabundle of hum anhair. T he length of the hum an hair varies w ith the m oisture of th eair, increasing its length with an increase in rela tiv e humidity, andvic e versa. Each hair in th e bundle is fixed atth e ends,andthe centreis connected to a pen so that the connecting m echanism keeps th ehair in tension.The dry and wet th ermometer consists of tw o therm ometers.The bulbof one is keptwet by a strip of m uslinw hich is tied roundth e bulb w ith its ends im m ersed in a vessel ofw ater. The w et bulb

Wet Bu lbD ep re ssion

IFIG. 11 . W ET A N D D R Y B U L B H Y G R O M E T E R

is cooled by th e evaporation from it s surface, so that th e tw o therm om eters show different readings. The less the air is saturated w ithw ater-v apour, th e stronger is th e evaporation from the su rface ofth e w et bulb, and consequently the greater the difference betweenthe tw o th erm om ete r readings.

EVAPORATIONIf an upw ard m oving bubble of air is m oist but unsaturated it w il lcool, m ovingfrom A to B (F ig . 12), at the dry adiabatic lapse rate. AtB it w il l becom e saturated and form a cloud. Fro m B to C it w il lcoolat th e saturatedadiabaticrate andhold all th e w ater condensed.From C to D th e te m perature is constant and is equal to 32 F . Intravelling from CtoDthew ater in the airw il l freeze, andw hen thishappens th e tem perature w il l fall at the saturated adiabatic rateuntil all the vapour condenses at E .


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10 THEORY AND TECHNIQUE O F SOARINGSometimes when th e bubble reaches temperature B, dew point,it will continue to rise and cool at th e saturated adiabatic rate. Asits temperature is above freezing point, the water-vapour, which isin a condensed form, will be released as rain. When the curv eapproaches freezing poin t, however, there will not, as m ight beexpected, be hail, as there will not be enough water left to freezeand it m ay only change into snow and precipitation in th e form ofsnow. Hail can form only in thunderstorm conditions.The water vapour present in th e atmosphere comes from th e

Ftisooch

I2OOO-

6OOO

30OO

r ,; s Ic e an d water i? ' :" ' held in air

-22 -4 -14 32 50 68FFIG. 12 . M O I S T U N S A T U R A T E D B U B B L E O F A IR M O V IN G U P W A R D S

evaporation of water fro m oceans, lakes and th e active surfaces ofvegetation. When th e vapour pressure in th e air above th e watersurface is less than th e saturationvalue, water will evaporate fromth e surface and mix with th e air. The speed of evaporation dependson four fa ctors: (a) the difference between th e saturation vapourpressure at water level, and the actual vapour pressure in the air;(b) th e velocity of th e wind, which m eans that evaporationincreases with wind velocity; (c) th e area of th e surface; and (d] th etemperature of th e water, as an in crease in temperature acceleratesevaporation.

CONDENSATIONWhen the w ater-vapour in th e air is saturated, th e lo wering of th etemperature to below dewpointresults in condensation. When th esurface of th e earth is cooled during the night to a temperaturebelow dew point, there is a direct condensation of water on thesurface. Condensation in th e atmosphere is caused by th e cooling


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METEOROLOGY 11of the air m asses, either bya cold surfacebelo w it ,orby th e expansion of tn e air when it m oves up to regions of low er pre ssure. A bubble ofair m aytravelin ahorizonta l dire ctionover seas , lakesorbig rivers, when, natu rally , it will absorb m ore m oistu re. In thesecircum stances dew poin t appro aches th e te m perature of the air andm ay rem ain constan t, and condensatio n and pre cipita tion m ayoccur w ith out the bubble rising, especially if it m oves over cold gro undor m eets cold air or m ounta ins.V ery often w hen w arm air flows over cold sea or lan d, it cools fro m belo w andthe coolingspreads upw ard s; the re lativehum id ityincreas es and if it appro aches 100 per cent, condensation will startin the form o ffog orm ist. As th ecooling spread s upw ards thefogwill rise som etim es to severalhundre d fe et. W hen th ere is no w ind,only that part of the air which is in contactw ith the ground will becoole d and th us notfog but only dew or frost willform .Theprincip al form s of condensation are

Free drops fog, clo ud particle, ra in -dro p.Dew condensed w ater.Frost actually dew form ed on surfaces w hose te m peratu re isbelo w freezingpoint.Gla ze a coating of clear, sm ooth ice on th e ground and trees.Snow soft fro zenrain.Hail form ed only in th unders to rm s.

A t very lo w tem pera tu res w ater m ay be transform ed from vapourto ice w ithout pas sing th ro ugh th e sta te of liquid water, and viceversa.P R E C I P I T A T I O N . W hen th e pro cess of condensation is carried farenough, th e w ate r-drops or snow crystals m ay becom e so larg eand

heavy that they fall from the cloud and, if th ey do not evaporate ,m ay reach the surface o f the earth.RADIATION

As sta te d already, the sun is the only sourceo f heat energyreceivedby th e earth's surface and by th e atm osphere. This energy is transm ittedby m eans ofradia tionwhichis in th eform of ele ctro m agneticwavesusuallycalle dlight and heat waves. Part ofth e sun 's radia tio nconsists o f vis ib le light, and th ere st of heat . A body can be w arm edby the absorp tion of either light orheat. A bodyw hich has a tendency to absorb heat energy faste r th an it radiate s it, will becom ew arm . The hotte r th e body the m ore in tense is its radiation.Because th e surface ofth eearth is a good radiator itis also agoodabsorb er of incom ing radiatio n, but as the atm osphere is a poor


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METEOROLOGY 13th e wind, there w il l be very little temperature variation on thewater surface.

CLOUDSClouds are the most useful visual indication ofth e condition s of th eatm osphere. They are very important to the soaring pilot becausethey not only indicate current conditions, but their types, movementsand sequence of change ofform also pro vide a good index to th enature of th ecoming w eather.A s already stated, clouds are formed by the cooling of masses ofdamp air caused by its upward motion. This water-vapour may betransformed in to water or ice. The process of transformation is asfollows.D ry unsaturated air flowing over oceans, lakes, rivers and dampground, vaporizes th ewater. Veryoften this water-vapour moves upin th e air with up-currents, andas it does so its temperature decreases .It can move horizontally or vertically, and on its way up may meetconditions which accelerate condensation. The grade of condensation can be seen by the form ofclouds. For example,heavy cumulusclouds mean extended condensation, very light or low clouds meanmild condensation, and stratus clouds, stabilization of condensation.The difference between clouds of various forms is due to theascending motion of th e air. The various motions are(a ) Large scale convection.(b ) T urbulent motion.(c) Uphill currents.(d) Currents of warm air moving upwards over a wedge of coldair.Convection currentsproduce c onditions which are very unstablestrong up-currents and gusts. When the wind increases with heightthe clouds formed by convectio n currents sometimes take the formof long parallel lines equally spaced, each w ith a flat base. Theseare one type ofcumulus cloud (see below).Turbulent motion produces low-level clouds in the form of strato-cumulus. These can form only when th e air on th e surface of theground is humid and when turbulence is active, usually when th eground is cold.A ir carried by a strongwin d against a rangeof hills or moun tainsis forced to rise and, if sufficiently damp, a long bank of m ore orless continuous cloud forms on or near the hig h ground .W hen warm air f lows over a w edge of cold air, th e cloud formedhas no definite structure. A s the ascent continues, the amount ofcondensed water-vapour increases, and the larger drops fall as rain.


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1 4 THEORY ANDTECHNIQUEOF SOARINGNear the base of a sloping surface of cold air the cloud is callednimbo-stratus. H ig her up th e slope the cloud is m ore uniform, butm uch thinner, and is called alto -stratus.From the soaring poin t of view clouds m ay be classified as lifting

30000

20OOQ

10000

FIG. 13. CLOUD S A N D T H E I R C H A R A C T E R I S T I C Sclo uds, non-liftin g clouds, and clouds which have not yet beenexplored.Lifting Clouds

C U M U L U S . This cloud is very important to the soaring pilo t andit will be necessary to give a detailed description of it . C um ulusclo uds are formed on days when there is enough m oisture in th e airto lead to condensation, when the cold air extends to great heightsandthe sun is shining. T herm al currents are then form ed due to the


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M ETEOROLOGY 15w arming up of the ground surface. A ir which begins to rise in sucha thermal current is w armer than the surrounding air b ut becomescooler as it moves upwards. Then if the temperature reaches dewpoint, condensation will take place and cloud will form.The main factor characterizing cumulus development is th einstability of th e air above th e cloud base. If there is an inversio nabove th e cloud base (temperature increasing with height) th ecumulus cloud w il l be very flat, and w il l very soon disappear.Should the air above th e cloud base be very unstable, cumulus

F IG . 1 4 . F O R M A T I O N O F C U M U L U S C L O U Dcloud may build up to a considerable height. If th e air below th ecloud base is stable, a milky vapour w ill form instead of cumulus,and on calm hot days this may cover the sky.Apart from vertical movement within th e cumulus, a slow rotationmay be found. This rotational movement is started by thermalup-currents and is in an anti-clockwise direction in th e northernhemisphere, when the cloud is viewed from above. The air motionswithin this cloud are great, and are reflected in th e active bulgesforming th ecloud itself. When thiscloud is growing, a greatdeal ofcondensation takes place, with th e result that a large amount ofheat is released. This additional release of heat adds to the air'sinstability and produces further cloud development.If a temperature inversion is being formed, then thermal up-currents having a te mperature equal to th e temperature of th esurrounding air cannot move upwards. Their movement is checkedand they m ay even disappear completely. Cumulus clouds may forman inversion when, during unstable conditions of th e atmosphere,large amounts of hot air are being carried upwards by means ofup-currents . If this inversion is only in the form of a very thin layer,the vigorous cloud form ation may penetrate it . In such a case th e


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16 THEORY AND TECHNIQUE OF SOARINGcloud m ay produce to w ers and even thunderstorms. During theday, as the te m perature of air increases , th e base of cumulus cloudswill rise to higher altitudes.T he motion of cumulus cloud may be div ided into threecategories(a ) M otion due to it s formation.(b ) Motion due to w ind.(c) Rotation.The m ain factor of interest to the soaring pilot is the degree ofinstability of th e air above the base of the cum ulus, as this controlsthe development of th e cloud. If there is an inversion above th e

F IG . 15. DISSOLV ING O F C U M U L U S C L O U Dcloud, th e vertical velocity of the thermal up-current will diminish.A s cumulus cloud can only rise as far as it has been forced by suchan up-current, it wil l flatten when it meets the inversion and m ayform strato-cumulus. If there is instability above th e base of th ecloud, the temperature of th e surrounding air will decrease fasterthan th e upw ard moving air in the thermal up-current, and th e cloudmay build up to high altitudes .During it s formation th e cumulus cloud has fluffy edgesand when it is dissolving it has hard and ragged ed ges . Dissolvingcumulus is also characterized by its flat top. T he time a cumuluscloud takes to dissolve depends entirely on the size of the cloud,and although a large cloud may take hours dissolving, the life ofan average cumulus is fairly short, fifteen to twenty m inutes from it sbirth to its disappearance. Nearly all clouds which form and growrapidly , disappear with similar rapidity . T he life of cumulusdepends on heat supplied to th e cloud by thermal up-currents. A slong as there is such a supply th ecloud is in th e stage of development,but if this supply ceases th e cloud w ill start to disappear. The


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METEOROLOGY 17dissolution may start from the to p of th e cloud downwards, orfrom th e bottom upwards.The formation of cumulus cloud depends entirely on thermalup-currents, and during early morning when th e up-currents areweak, only sm all clouds will be formed. The time of maximumintensity of thermal up-currents is between 1 and 2 p.m. and duringthat time th e maximum growth of cumulus may be expected. Insummer, however, a second peak period between 4 and 5 p.m. maybe observed. Naturally the base of cumulus will be higher in summerth an in winter, as it is strictly associated withair temperature.When th e supply of heat during the growth of a cumulus cloud is stopped, mechanically or thermally , for a short period and thenresumed, a second cloud may form and, due to the wind, follow th eprevious one. In such a w ay cloud streets are formed.It must be admitted that it is often difficult to see ifa cloud isforming or breaking up, and cumulus cloud looks very differentfrom the air than from the ground. However, with some practiceand a knowledge ofthe fundamental principles of cloud formation,this difficulty can be overcome, and the soaring pilot will learn torely on his own judgment.

Characteristics. Average altitude 4000 ft, high altitude 12,000 ft,lo w altitude 1000 ft; approximate altitudeof to p of cloudaverage7500ft, highest 12 ,000-15 ,0 0 0 ft.C U M U L O - N I M B U S . Cumulus cloud quite often grows to enormoussize and if it contains water-drops or snow it is still called cumulus,but if it develops above freezing level it is classified as cumulonimbus. The main conditions necessary for this developmentare thatthere must be a large surface area over which there is an unlimitedsupply of moist air, a very unstable atmosphere and weak wind,or none at all. When the developm ent takes place the water dropsbecome supercooled and as the vertical currents in cumulo-nimbusare very strong, some parts of it may be pushed to very high levelsand the water-drops turn into ice. The cloud then takes the formof an anvil . The cooling at high levels will lead to condensation,which liberates heat and allows convection to continue to a heightof perhaps 40 , 000 ft.Cumulo-nimbus generally appears as a heavy mass . It may developfar above the ground, and during its horizontal travel it will accumulate moist, cold air which sweeps over ground warmed by the sun.If the up-currents in front of th e cumulo-nimbus are very strong,the suction of the moist air may be very great, and as a result a rollof cloud may form which will travel along with the moving cumulonimbus. This roll may extend for miles , providing excellent soaring


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18 THEORY AND TECHNIQUE OF SOARINGconditions. Strong up-currents may also be found in th e fronta lpart of th e cumulo-nimbus. The middle part, however, will be aregion of severe turbulence.Cum ulo-nimbus is associated mainly with very hot weather whenthere is no wind blowing, and with cold fronts where moist, coldair moves over warm ground. It often produces a thunderstorm.Soaring pilots m ust remember that in cumulo-nimbus cloud ic ingconditions are very heavy. There is seldom any precipitation whilethe cloud is developing, but th e rain usually starts when th e cloudbegins to disintegrate. W hen this happens the rain from thefront of th e moving cloud is usually very light, increasing towardsth e rear.Characteristics. Average altitude 5000ft, high altitude 8000ft,low altitude 1000 ft; approximate altitude of to p ofcloudaverage18,000 ft, highest 42,000 ft; approxim ate thicknessaverage7 0 0 0 ft,greatest 30,000 ft.

N IM B U S . The formation of this cloud takes place, as a rule, in awarm front, when warm air moves along the edge of cold air.After the passing of cirro-stratus and a lto-stratu s cloud, a dark heavycloud can be seen from which a steady rain falls. This is nimbuscloud.The soaring pilot m ay find weak but steady up-currents in thiscloud. They will not be so vig orous and concentrated as in cumulusor cumulo-nimbus, but with very carefu l manoeuvring it may bepossible to soar and evento gain height. This weak lift m ay be foundunder th e entire cloud base.Characteristics. Approximate altitude 4000ft; thickness 2000-4000 ft.

A L T O - C U M U L U S . This cloud fo rms just below the cirrus level, th emain difference being that alto-cumulus is formed from wate rpartic le s instead of ice. These water particles are heated from tw osources: directly from th e su n and indirectly from the earth.During th e day both surfaces of the cloud will have th e tendency toevaporate, but during the night only the lower surface will evaporateas there is radiation from the earth and the air may move upwards.The upper part of the cloud will cool, however, and th e air maybegin to move downwards. This vertical movement of air may lastlong into the night, until the cloud disappears completely.Alto-cum ulus produces waves and bands, due to th e flow of onelayer of air over another of different temperature, density andhumidity. W aves produced in such a way m ay have great lengthand amplitude. Their birth is very similar to thebirth ofsea waves.The air motion leads to differences in temperature, and as th e top


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1 9of th e w ave is colder th e w ater-vapour m ay condense into clo ud.The lo w er part, being warm er, m ay rem ain clear. The verticalvelocity found in such w aves m ay be of3-4 ft/sec. The w aves canbe affected by th ewind, and if the w ind above th e w aves is strongerthan below th ey w ill lie w ith th e w ind. Ifit is not very strong thew aves m ay lie across the w ind.A s th e conditions w ithin alto-cumulus arevery unstable, thunderstorm s often develop from them .Characteristics. A verage altitude 13,000 ft, high alti tude 36 ,000ft, lo w a ltitude 5000 ft; average thickness 1000 ft.

F IG . 16 . W A V E S P R O D U C E D I N A L T O - C U M U L U S C L O U D SS T R A T O - C U M U L U S . A heavier and lower variety of alto-cum ulus,strato-cum ulus as a ru le is in th e form of dark w alls or waveswhich m ay lie very close to each other. It appears when upw ard-m oving a ir is in a sta te of condensation stopped at a certain

level due to inversion. It is m ost frequent in w inte r in anti- cyclonic conditions w hen strato-cumulu s m ay cover the w hole sky,w ith occasional gaps here and there through w hich th e sun m aybe visible.Characteristics. Average a ltitude 7000 ft , hig h alt itude 24,000 ft,lo w altitude 2000ft; approxim ate th ickness 1200 ft.N I M B O - S T R A T U S . This is a ty pical fronta l cloud and is form ed bythe over- and under-running of a ir m asses of different te m pera turesand densities. It fo rm s a lo w layer of dark grey colour, which isnearly uniform , and usually brings contin uous ra in and snow. It isassociated w ith storm y conditions over an extensiv e area. Strongvertical currents m ay be found under this clo ud.Characteristic s. A verage alti tude 300 0 ft , hig h a ltitude 6 0 00ft ,lo w a ltitude 500ft; approxim ate thickness 40 00ft .


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20 THEORY AND TECHNIQUE OF SOARING

Non-lifting CloudsC I R R U S . This cloud is associated with large movements of airmasses and appears at great heights. It is a detached cloud with afeather-like structure, whitish in colour. It is composed of icecrystals w ith extrem ely low tem peratures, ( 30F to 40F). Itcan generally be seen about 1000 miles ahead of a cyclone, and ifapproaching from th e south-west in th e northern hemisphere, is anindication of an approaching depression.A s cirrus cloud is associated with strong and changing winds, itwill probably produce no lift suitable for soaring.Characteristics. A verage altitude 35,000 ft, low altitude 13,000 ft,hig h altitude 90,000 ft.A L T O - S T R A T U S . The grey or grey-blue veil ofalto-stratus usuallyappears at th e same heightas alto-cumulus. It is composed of smallw ater-drops w ith occasional ice crystals, and th e sun and th e moonare usually hidden by it. It is often found in a cyclone.Characteristics. A verage altitude 15,000ft, low altitude 6000ft,hig h altitude 40,000 ft; approximate thickness 1700 ft .S T R A T U S . This is th e cloud which appears at th e lowest altitudes

and m ay envelop highground completely. It resembles fog but doesnot rest on th e ground. It is generally very wet.Characteristics. Average alt itude 2500ft , hig h altitude 4000 ft.

THERMAL CURRENTSThe distribution of wate r over th e surface ofth e earth and in th eearth is th e most important factor in th e birth of thermals. A varietyof physical effects combine to make water much more conservativeof heat than land, and slower to warm up and cool down. This facthas a m oderating influence on temperature to aconsiderable distanceinland. G eological formations and th eirresultant soil types are alsoimportant factors in the determination of up-currents. Darkcoloured soils and surfaces absorb m ore of th e sun's heatthan lightercoloured ones, andare generallywarmer by day, causing the adjacentair to be warmer also . Dry soils such as sand have a low specificheat responding rapidly to temperaturechanges, while wet soils suchas clay retain m oisture and are th erefore conservative of heat andcold. Foundations of chalk and limestone evaporate water fairlyquickly, after which th ey warm up rapidly and are usually goodsources of the birth of therm al up-currents. Forests and woods alsoaffect temperature, particularly th e maxim um temperature, whichtheymoderate bycasting shade, offering a largesurface for radiation,absorbing heat in th e process of evaporation from th e fo liage, and


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M E T E O R O L O G Y 2 1the production of fog, mist and cloud which ward off the directrays of the sun.The sun's rays coming through the troposphere and clear atmosphere lose about 20-30 per cent of their heat energy. The intensityof the earth's radiation depends mainly on temperature, and thehotter the earth the m ore radiation there will be. For strong thermalcurrents the temperature difference must be very large over a bigrange of altitude, or the temperature difference between a thermalcurrent and the atmosphere must be maintained to help to build upstrong vertical m ovem ent.A mass of air which is warmer than the surrounding atmospherem ay be caused to rise mechanically or thermally, and it will moveupwards with increasing speed until it dies out where the atmosphereis stable. When the mass of air has moved upwards, more air willflow in to take its place. This will also become heated and after awhile form a new therm al current in the same place. The tim e takenin the formation of a thermal current varies considerably, and m aybe from five minutes to periods in excess of an hour. A mild steadywind is a help in the building up of a thermal current, especiallywhen it forms on a hill slope, but a high wind prevents its formation.

As a rule thermal currents are most frequent during calm summerdays, when they occur over roads, sand dunes, ploughed fields,wheat fields, beaches and rocks. Isolated hills, especially short orconical ones, have warmer sides than the adjacent atmosphere at thesame level and act like chimneys producing up-currents.For the formation of a thermal current the ground should bewarmer than the air, which happens more often during spring andsummer than in winter. Clear, sunny days are usually associatedwith cold polar air, and stronger up-currents are always found ina cold front than in a warm front. The maximum vertical velocityof thermal up-currents is surprisingly great, 1000-1500 ft/min, andthey m ay even exceed these speeds.After several hours of flying experience, the soaring pilot willfind that he can locate the thermal up-currents quite easily; sometimes when the air is dusty it is also possible to see the rotation ofthe rising air.

THUNDERSTORMSThe thunderstorm most frequently occurs in early spring and earlysummer. For a thunderstorm to develop there must be sufficientmoisture at an altitude of from 5000-10,000 ft, or an unstablecondition of the atmosphere up to about 10,000 ft. Thunderstormsmost usually occur in regions where there is strong surface heating,

3(A.i6i)


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22 THEORY AND TECHNIQUE OF SOARINGespecially in regions of light winds, or when one layerof air overrunsanother ofa lower temperature, producing convection, or when asaturated layer is underrun and uplifted by a denser layer. Theperiod of a thunderstorm is very irregular, and its frequency andintensity depend upon th ehum idity of th e air and th e rapidity of th elocal vertical convection.A t th e time when th e earth reaches its maximum heat, usuallybetween 1 p.m . and 2 p.m., the condition of th e atmosphere is m ost

FIG. 17 . T H E A I R -M A S S T H U N D E R S T O R M

favourable to th e formation of a thunderstorm because verticalconvection of the air over land reaches its greatest altitudes andproduces th e heaviest condensation and clouds.There are three main types of thunderstorm, air mass, frontaland orographic .A I R - M A S S T H U N D E R S T O R M S . These occur in regions ofhigh temperature and uniform pressure which are usually associate d with lightwinds and small turbulence. The land in these regions is verystrongly heated, causing intense convection. Thunderstorms in these regions ta ke place after tw o or three days of very warmweather when th e air near the ground is so intensely heated that


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M ETEOROLOGY 23convection reaches very hig h altitudes. The speed ofm ovement ofth ese thunderstorms is approxim ately 1 5-2 0 m .p .h ., their area is 8-4 0 m iles in diam eter, and their base m ay be as low as 1 000 ft.The m ost characteristic fe atureofthis ty peof thunderstorm is thatit takes place only durin g th e day.F R O N T A L T H U N D E R S T O R M S . If convection is caused not by landheatin g, but b y cold air m oving under w arm air, th e te m peraturelapse rate and th e vertical convection it self will be gre ater, and this m ay bring hig h-level thunderstorm s. They are caused w hen th ew arm air in it s c lim b over the slow ly m oving cold air, releases it spotentia l in stability, w hich is follow ed by vio lent convection w ithin

FIG . 18 . U P- A N D D O W N - C U R R E N T S I N C U M U L U S C L O U D , W H I C H isDEVELOPING IN TO A FRONTAL THUNDERSTORM

the cum ulu s clo ud w hich has form ed due to the adiabatic cooling of upward-m oving w arm air.B ecause of hig her temperatures, higher m oisture content, and agreater degree ofpotential instability within th eair m asses, thunderstorm s predom inate during th e warm seasons of the year.O R O G R A P H I C T H U N D E R S T O R M S . These th understorm s ta ke placew henthe air is fo rc ed upa m ountain slope ,and are the heaviesttype,extending to high altitudes. The average height is 15 ,00 0 - 2 0 ,0 0 0 ft.T H U N D E R S T O R M - W I N D S . Half an hour before a thunderstormreachesa giv en place, the w ind th ere beginsto die dow nand to changeits direction. A s a rule the w in d is from the south or south-west,which is alw ays acro ssth epath of th e com in g storm . W hen th e stormis very near, say halfamileaw ay ,thew ind b low svery gently to w ardsthe storm , and w hen th e rain of the storm approaches, th e windblow s in vio lent gusts with th e direction of th e com ing storm.These suddengusts of win d last only for a w hile, and m ovew iththethunderstorm . D irectly in frontof the rain there is w arm airwhichis fo rced upw ards w ith great velocity b y th e incom ing cold air.


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2 4 THEORY AND TECHNIQUE OF SOARINGW hen a thunderstorm is approaching there is usually a fall inbarometic pressure, sometimes beginning several hours beforehand.The temperature when th e thunderstorm arrives is rather high, butfalls rapidly w ith th e first gust of wind, sometimes to as low as 1 4 F .There are a number of risks involved in flying in a thunderstorm ,principal among which are th e violently bum py conditions and rapidchanges in th e direction of th e air movements, which produceunequal loads on th e wings of the sailplane, and are strong enough

F I G . 19 . O R O G R A P H I C T H U N D E R S T O R Mto break them. Another risk is that ofbeing struck by lightn ing.V io lent vertical currents lift drops of rain with considerable speed,causing them to break up and take on a positivecharge of electricitywhile th e surroundin g air takes a negative charge. The chargedwater-drops, being heavier, collect in th e lo w er part of th e cloud,while th e negatively charged air forms the top. These conditionsgive rise to th e discharge of electrical energy between the differentparts of th e cloud, or between the cloud and th e earth. When asailplane enters an electrically-charged region, it may concentrateth e charge, raise it above sparking poin t or, due to conductivity,short circuit it . A s a sailplane is built mainly of wood, it s electricalresistance is very high, and the discharge may cause it tocatch fire.

AIR MASSESWhen air of th e same temperature and humid ity covers a large areaof land it is called an air mass. This area may extend to hundreds ofsquare miles. If the underlying surface is uniform and if the aircurrents are favourable, the air mass will tend to becom e uniform in


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METEOROLOGY 25a horizontal direction. The areaswhere suchairmassesare form edare called air mass sources. The masses are know n as tropic al orpolar.

C H A R A C T E R I S T I C S O F T R O P I C A L AIR . W arm , moist, stable clouds,w ith tem peratu re above su rface tempera tu re .Maritime very moist, low clouds; fog.Continenta l dry, high clouds.C H A R A C T E R I S T I C S O F P O L A R A IR . Cold , dry near surface, unstable ,therm al currents , broken cumulus clouds.Maritime clouds.Continentalvery cold and dry.Because air travels fromhigh-pressu reto low-pressure areas, therewill be, in th enorthern hemisphere, north winds blowing from theequator and south winds from thenorth pole. The direction of thesewinds is also affectedbytherotation oftheearth. The earth ro tatesfrom west to east and th e air near the equator mov es eastwards.Furth er from th e equator the air will have less eastw ard velocity.The air which moves north in the north ern hem isphere, owing toits greater eastw ard velocity, becomes a south-west wind, whereasthe air mov ing southw ard is mov ing into a region which has a

greater eastw ard velocity, so that th e north wind becomes a northeastwind. A s thetw otypes ofair mass are very vital to thesoaringpilot, since many fl ights are affected by them, a more detailedexplanation is given below.The w arm tropical air mass is stable as a rule, and th e moisturecontent is high, particularly in the low er layers. W hen th is masstrav els tow ards a colder region, its temperature will be higher th anthe surface temperature over which it travels. As this w arm masscools from below, the temperature drop will extend over a la rgearea, and mayreach dewpoint, in whichcase fog will form.The maritim etropicalmass usually flowsfrom th e M editerraneanin summer andfromthe tropical A tlantic in winter. In this air massthere is generally a stable lapse rate , or an inversion in the low erlayers, slight turbulence, steady winds, poor visibility, and highrelativehumidity. Stratus clouds, drizzle, mist, fog and dew may beexpected. W henatropical mass invades a warmcontinent in summer,instability rapid ly develops. In winter, however, the stabilityremains constant and deep layers of fog may cover large areas ofcontinent.The continenta l tropic al mass com es mainly from North -eastEuropein summer, andfrom NorthAfricain winter. This air massis much drier th an maritime air, andju st av erage re lative humiditymay be expected. V isibility within this mass will be fair.


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26 THEORY AND TECHNIQUE OF SOARINGW ithin the polarair m ass w e find stable stratif ic ation, low specifichumidity and low tem perature. If this m as s m oves tow ard sw arm erregions, th e tem perature of th e low er layers w ill be less than th esurface tem perature over which it travels . The polar air heatedfrom below will develop therm al in stability, and th e resultingconvective currents, after reaching the level of condensation, willform cumulus clouds and som etimes thund erstorm s. This m ass ofair is generally associated w ith turbulence in low er levels, dry

F I G . 20. G E N E R A L D I A G R A M O F W A R M F R O N T

Cold Front

F IG . 21 . G E N E R A L D I A G R A M O F C O L D F R O N Tad iabatic lapse rate, and good vis ibility. If it travels over w ater itwill pic k up m oisture which, by m eans of convective currents, is brought up to hig her le ve ls . The re sult w ill be th e form ation ofcumulo-nimbus clouds.FrontsW hen a m ass of tropical air m oves towards a m ass of polar air ,the linebetweenthese tw o m as ses is called a warm front, as thew armair is displacing th e cold air on th e surface of the earth . C onverselyth e line w hich separates a m ass of polar air advancing tow ardstropical air is called a cold front.A stationary front is that along w hich one mass does not replace


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METEOROLOGY 27the other. A n occlu ded fron t is th e fro nt result in g from a coldfron t overtak in g a w arm front. To characterize th e fron t it isnecessary to use the cloud system as it is the only visual in d ic atio n of the condit ions producing a front.(a) Warmfron t. H ere the tropical air , having a m uch sm alle rdensity th an the polar air, has a te ndency to ascend along the frontal surface and to cool adiabatically . As it possesses som e hum idityth is air will condense and a cloud syste m develop. A generalpictu re o f a w arm front w ith ty pic al c lo ud fo rm ation is given in Fig. 22 . Itcan be seen that the w arm and m ois t air m oves slo wly upw ards and produces clouds 100-300 m iles wid e and as m uch as

F IG . 22. T H E W A R M F R O N T

1100 m iles long. T he height of the clouds is betw een 6000 and 20,000 ft. T he upw ard -m ovin g air cools and is usually associa ted w ithaperiodof com ple te clo udle ssness at first , then cirrus and cirro-stratus fo rm s, and lo wer stratus and fog.F rom the ground it is quite easy to re cogniz e an approachingw arm front as shown in Fig. 22 . G enerally a w arm fron t w ill com efrom the south-w est in E ngland and w ill be ind ic ated by a decre aseof pressure and by thickenin g of clouds, particularly w ith alto-stratus clouds whic h bring rain in sum m er and snow in winte r.U sually the clo uds and pre cipitation will re m ain until the w arm aircovers the gro und com plete ly and the cool air becom es so shallow that it w il l not li ft the w arm air sufficiently to fo rm clouds. In thisw arm fron t the m ovem ent of the w arm air is so slow that there isnot sufficient energy to build up-curren ts whic h w ill be stro ngenoughforsoaring. This type of sto rm is calle da stable w arm front.

In the unstable w arm fro n t, w ell ahead of it , cirrus and stra tusc loud w ill be seen . Insid e th e front the air is w arm , turbulent andvery unstable. The soarin g pilo t m ay find strong up-curren ts and even scattere d th unders torm s. These thunderstorm s are ofa m oregentle character than the sto rm s fo und in cold fronts. Icing


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28 THEORY AND TECHNIQUE OF SOARINGconditions m ay be encoun tered w hilst flying in a w arm front as the clouds within it often consist of supercoole d w ater and the t emperature is betw een 15F and 32F .W hen aw arm frontapproaches L ondon from Cornw all , as shownin Fig. 2 3, th e first indicationof the approaching storm in Cornw allw ill be north-east m oving cirrus and cirro-stratus clo ud; then rainw ill cover th e area. The indication that a w arm front is passing Cornw all w ill be a decre ase of rain, a rise of pressure , a change ofw ind (in the cold reg ion there w as north w ind w hich changes to

F IG . 23. W AR M F R O N T M O V I N G N O R T H -EAST F IG . 24. W AR M FR O N T P A S S I N G T H EM I D L AN D Ssouth-w est) , and an increase in tem perature. Due to convectiveinstability, clouds w il l form and th understorm s m ay occur in th isfron t, and th e sam e fron t passing th e M idlandsw ill produce overcastskie s and heavy rain .(b ) Coldfront. A tropicalm oist air m ass originating over reg ionsofconsiderable heat and m oisture becom es very moist up to highlevels . W henvery dry polar air m oves in the form of a w edge in toa m ass ofwarm air, the w arm airis pushed upw ards. Thiscauses itto cool adiabatically, genera lly to suchan exte n t that saturation an dcloudform ation result . As a rule th ese clouds are cum ulus, and areactually form ed by convection or instability w ith in the air m assalong the cold fron t.W hen the headof th e cold front m oves in to th e area of w arm air,


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30 THEORY AND TECHNIQUE OF SOARINGd o w n -cur re n ts p re vai l , w hich m ay cause the structural fa ilureof the sailp lane .(c ) Occludedfront. A s the sp eed of trav e l of the c o ld frontis fa s tit m ay o v e rtake the s low -m ovi n g w arm fro n t. W he n the tw o fro n ts

F IG . 2 6 . F A S T - M O V IN G C O L D F R O N T C A T C H E S U P W IT H S L O W - M O V IN GW A R M F R O N T A N D F O R M S O C C L U D E D F R O N T

meet , the a re a of w arm a ir w hich is be tw e e n the m w ill b e squa sh e d a n d w ill m o v e up w ard s . T he w arm r eg io n is the n sa id to b e o ccluded .W hen this h a ppe n s the m isty d rizzle -b e lt c harac te r ist ic of the w armre g ion w ill b e e l iminate d .Co ld front occlus ion o ccurs w he n the a i r in the re a r of a co ld

F IG . 2 7. C O L D FR O N T OCCLUS ION T he cold air d isplaces the cool air , pushin g it up . This cool a ir w il l mov e along th e cold f ront .

frontis co ld e r a n d the re fore d e n s e r than that in ad v a n ce ofit. T hi sco ld e r a ir w il l disp lace a ll o the r a ir d urin g the c o ld front t r a ve l,p ushin g it up , an d the co o l a ir o n the he ad of the frontw ill startmo v ing up a lon g the s lop e ofthe co ld f ron t. T he w e ather a s so c ia te dw ith this will b e s im ilar to w arm front w e a t he r . Pre c ipi tat io n w ill


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METEOROLOGY 31extend back in to the area of colder air , and heavy low clouds m ayfo rm w ithin this colder air m ass.In a w arm fron t occlu sion the air behind th e cold fron t is w arm erand less dense than thatin advance of it. The cold fron t will startclimbing along the colder air beneath th e warm front. This type ofocclu sion is very com m on. The w eather w il l be of th e cold fron ttype.(d) W arm front approaching m ou ntains. W hen the warm airm oves to w ards a m ounta in range it w il l be lifted up by the slope ofth e mountains. This w ill cause very intense convection over the

FIG . 28 . W A R M F R O N T O C C L U S I O Nm ountains and precipitation w il l take pla ce, first in th e fo rm oflight ra in, th en steadily growing in in tensity. Severe th understo rm sm ay develop .Aw arm fron t usually breaks up over a m ounta in ra nge, andpartofthe w arm airm ay descend alo ng the lee slope w hile th e otherpartm ay m ove upw ards. Heavy cum ulo-n im bus clo ud m ay develo pover th e peak of the m ountain and re m ain there for a long tim e,eventually changing in to stratus. A s much of th e m oisture contentis lo st through precipitation , lasting som etim es several hours, th ecloud m aydisappear and thew eather im proveuntil th e undercuttingcold air arr ives.Thepart ofthe w arm air whic h has b een lif te d over th e peak ofthe m ounta in m oves up alo ng th e cold air and m ay cause vigorousconvection andhig h th understorm s.(e) C oldfrontapproaching m ou nta ins. T his condition is of greatimportance to the soaring pilot because heavyup-currents are oftenassocia te d w ith it, extending to very hig h levels. Cold frontcharacte ristics change in accordance w ith the terrain over w hich itpasses. M ounta in sw ill slow dow n th espeed oftravel of acold fron t.W hen the cold air is lifted over a m ountain range, precipitation


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FG.2

WARMAIRMOVINGTWARDSAMOUNTAINRANGE

FG.3

DEEPMETOFWARMFONTTUNDESRM

OVERMOUNTAINRANGE

FG.3

BREKINGUPOFWARMFRONTOVERMOUNTAINRANGE

FG.3WARMAIRLF

upBYMOUNTAINRANGE

CAUSSHIGHTUNDESRMS


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FG.33.THEMOUNTAINRANGESOWSDOWNTHESED

OFTRAVELOFCOLDFRONT

ColdAr

FG.35.COLDFONTPASNGMOUNTAINRANGE

FG.3COLDAIRLIFEDUPBYMOUNTAINRANGEPRODUCES

INSABILITYWHICHMAYEXTENDTOVERYHIGHLEVEL

ColdAir

FG.3COLDAIRPASNGTHEMOUNTAINRANGEBEOME

WARMER,CLEARERANDSABLE


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34 THEORY AND TECHNIQUE OF SOARINGw ill occur, mainly in the form of ra in, but som etim es as snow,especially over the h igher ridges. W hen the air starts descend ingalong the lee side of the mountain it is heated adiabatically,and at the bottom of the mounta in it w ill have a m uch highertemperature than originally, and a m uch lo w er specific humidi ty.O n the w ay dow n the slo pe the air w ill becom e clear, w arm and stable .

CYCLONEIt has been stated before thatover the earth's surface there is a continuous circulation of air from one source to another, and at thebeginning the various m ovements ow e their characteri stics to thesourc e over w hich they have orig inated . During progress over landand sea these characteristics w ill chang e, and if the air ta kes a longtime to tra vel over a certain ty pe of area, it w ill assum e localcharacteristics. M ovem ent of the air is alw ays from high pressureto low , but thepath of travel w ill not follow a straig h t line. D ifferences of pressure are d ue to the differences in temperature w hichare m et over theearth's surface.The rotation of the earth has a g re at effect on the directio n of aircirc ulation , and in the northern hemisphere the air is constantly deflected to the east. In are as of low pre ssure there will be atendency for air to flow outw ard s in the form of a spiral, and inareas ofhigh pressure , inwards.Anotherfactorw hichaffects air circulatio n is the fric tion betw eenm oving air and the earth's surface.Areas oflow pressure , w here cold polar air is m ixing w ith warmtropical air, are called cyclones or depre ssions. These areas are veryextensiv e and usually have a d iameter of about 1000 m iles. Thed irection of m ovement ofa cyclone is generally from w est to east,and the velocity of travel is g re ater in w inter than in sum m er. Theyare also more frequen t in w inter than in sum m er.The w inds w ithin thearea ofa depression will alwaysblo w tow ards it s centre. Should there be a cold easterly wind blow ing in th enorthern part of the British Isles and a w arm w esterly w in d inSouthern Eng land (see Fig . 37), th e warm air m aybe forced into theareaof cold air by therotationalm ovem ent ofthe earth or by frictio nbetw een the tw o air m as ses, and form a bulge. As gases of d iffere nttem pefatures and densities do not m ix w ell togeth er, turbulence willre sult along the line separating the tw o masses and giv e rise to w av esand bulg es.A s a cold air m ass travels with a m uch greater velocity than awarm mass, it will usuallyoverta ke the w arm air, pushing it upw ards.


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FIG . 37. FIRST S T A G E O F C Y C L O N E FIG. 38 . D UE T O R O T A T I O N A L M O V E M E N TD E V E L O P M E N T O F T H E E A R T H A N D F R I C T I O N B E T W E E N C O L DEasterly w in d in th e north, w esterly in so uth England. A N D W ARM M A S S , A BU L G E MAY FO R M

FIG . 39 . D E V E L O P M E N T O F B U L G ED otted area shows ra in . F IG . 40 . FULLY D E V E L O P E D C Y C L O N E


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36 THEORY AND TECHNIQUE OF SOARINGW hen th e cold air of a cyclone overtakes warm air, and sto ps thesupply of warm air from th e ground, th e area of warm air willdisappear and in stead of th e original cyclone there will be a hugebubble of relatively warm air at high level. The cyclone has beenoccluded. This occlusion usually starts in th e centre of a fu lly

Warm

Cool

FIG . 41 . W A R M M A S S O C C L U S I O N I N F U L L Y D E V E L O P E D C Y C L O N E

FIG. 42 . D I S T R I B U T I O N O F A IR M A S S E S A N D C L O U D S A L O N G A-B O F F IG . 4 1

developed cyclone and is shown in Fig. 41 . If th e air in front of th ecold mass happens to be even colder than th e m ass itself, th ere willbe a warm mass occlusion (see Fig. 42) . This occlusion seldom lastslonger th an one day.Ifth e air in front of th e cold m ass is w armer that that of th e mass,th ere will be a cold mass occlusion as shown in Fig. 43, characteristicsof which can be seen in Fig . 44 .During th e approach of a cyclone, cir rus clouds will be seen first,th en cirro-stratus, alto-stratus, nim bo-stratus and eventually rain.The pressure will fall steadily, visibility will be good unti l the raincomes. This will be followed by a very sudden temperature rise, th e


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METEOROLOGY 37rain changing to drizzle, mist or fog, and instead of the nim bo-stratus clouds, stratus and strato-cumulus clouds will form. W h e na warm front passes there w il l beno rain, but only drizzle .

Cool

Cold

W armFIG. 43. C O L D M A S S O C C L U S I O N IN F U L L Y D E V E L O P E D C Y C L O N E

Cirro-stratusV- - - - - , " 8 9

#_Alto-cum ulus

FIG. 4 4 . A IR M A S S E S A N D C L O U D S A L O N G C-D O F FIG. 4 3ANTICYCLONE

A reas of high pre ssure are called anticyclones and they comprise aregion ofhigh press ure w ith an area of highest pressure in th e ce ntre.The air w ithin an anticyclone f lows in a clockwise direction aro undth e centre.Tw o . types of anticyclone ca n be distinguished, cold and warm.The cold anticyclone form s around th e poles, where, due to snow -covered ground, th e surface conditions are uniform and th ere is ahuge are a of intense cold. This anticyclone produces northerlyor north-easterly wind in the British Isles. A cold anticyclonemoving south w ill be very dry, cold and stable w hile in the Arcticregion, but on its w ay ove r the sea some moisture will be transmittedand produce towering cumulus and cumulo-nimbus clouds. Strongand extensive up-currents reaching veryhigh altitudes are associatedw ith this type of anticy clone. Cold anticyclones form when th e cold


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METEOROLOGY 39air m oves south in the rear of a family ofdepressions, but they dono t last m ore than a few days.The w arm anticyclo ne develops from a cold anticyclone. Theair w ithin it is w arm and dry , and it moves very slowly , seld omexceeding 20 m .p.h., although the speed is hig her in w inter than insum m er. O nce form ed, th e warm anticyclone rem ains for aboutsix days. It form s over th e Azores Archipelago andstays therefo r aconsiderable tim e becom in g very w arm and m oist. Should this anticyclone m ove north , the surface air tra vels over progressivelycold erw ater andbecom es coole r, and stability willdom in ate it to aheight of abou t 2000 ft. B y th e tim e it reaches England it will bew arm and will be characterizedby hig hcloudsanda high percentageofm oisturewhich will have a tendency to precip itationat th e slightestprovocation.T he direction of travel of anticyclones is the sam e as that ofcyclones, usually from west to east in the north ern hem isphere.They genera llycarry good weath er w ith them .

ICE FORMATIONO ne of the dangers of flying in clouds is prolongation of flight inic in g condit io ns, especially when thecloud is so th ick thatnotverym uch can be seen outside the cockpit. D uring winter, icing willm ost often be found in th e str atus clo uds of a cold front. Cle arice will form at tem peratures betw een 32F and 25F. If lo werte m peratures prevail, rim e ice will fo rm .Another com m on fo rm of icing takes place when flying in cold air through which rain falls, when th e tem perature of th is air isbelow freezingpoint.The alto-stratus clo uds of the warm frontare the m ost treacherous,particularly when th e tem perature ofthe cloud is betw een 32F and25F, as th e m ost severe clear ice accum ulatio n will form .The cum ulu s clo uds w ith sub-freezing levels which developwithin m oist air, will alsoproduce icing, owing to th e large am ountofm oisture accum ulating in clouds of th is ty pe. Thunders torm sappearing in w arm weather will also produce ic ing as a ru le.Ice fo rm s near the stagnation pointofth e wing profile, in fa ct anarrow region to either side of the stagnation point serves as th eprimary w ater-catchin g surface. W hen flying in icy rain the supercooled droplets approach the le ading edge, are deflected by the airstream w ith out being broken, and avoid th e wingentire ly . M ost ofth e sm all undeflected droplets str ike the leadin g edge in the vic inityof th e stagnation point. The la rger drops will not spring off at thetim eof im pactw ith the le adin gedge, but willsp lash, forming a very


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40 THEORY AND TECHNIQUE OF SOARINGthin water surface. A s the ra dia ting surface of the drople t hasincreased suddenly to many times its original siz e , very rapid radia tionof the heat contained in the water takes place. The new droplets striking the leading edge w ill freeze im mediately and som e ofthemwill fly ro und the w ing.During th e run along the wing surface the water continues tofreeze and evaporates on the remain ingpartof the w ing. This ice form ation is called cle ar ic e (see Fig. 49), but when the ice accumulates due to the presence of mois ture in small particles, it is called rime ice (see Fig. 51).

(a)

(b)

FIG. 50 . FIN A L S T A G E O F C LE A R IC EF O R M A T IO N O N

( . a ) roun d w ire.( t > ) stream line wire.(c) leadingedge ofw in g.

FIG. 5 1 . R IM E I C E F O R M A T I O N O N (a ) ro und wire.( & ) streamline wire,(c) wing pro file.

Ice form ation may also occur outside the cloud in completelyclear air, under certa in conditions. Flying at high altitudes at atem perature below 32F and rapidly descending, the sailplane maypass a layer of warm damp air, and as the surface of th e sailplanestructure is sti ll at a low tem perature, the air striking it may coolbelow its dew poin t.It should be remembered that the formationof ce takes plac e veryrapidly and all pilots should be familiar with icing conditions andknow how to avoid them.The ice formation w ill change the aerodynamic characte ris tic s ofa sailplane completely , and as ice bloc ks the static and pitot-headtubes, blind fly in g may be foundverydifficult. The most dangerousfactor, however, is that thetotal weightof the sailplane w ill increase


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METEOROLOGY 41and vibration due to unequal lo ading of wings and tail unit m ay beimposed on th e structu re , causing its failure.Ice m ay also form on th e leading edge of ailerons, elevator andrudder, and m ay block them completely, m aking the sailplaneuncontrollable .

SLOPE CURRENTSSoaring by m eansof slope currents is one of the eas iest methods, asit is alw ays possible to tell w here lif t areas m ay ex is t. In the ear lystages of soaring development these currents w ere used ex tensivelyand formed the only energy available for m otorless flight. Evento-day slope soaring forms the m ain part of soaring training andnearly all pilots go through a slope-soar ing course. Its importance,however, has been som ew hat neglected ow ing to the rapid progressof th erm al soaring.In the past a great deal of w ork has been done in estimating thelifting characteristics of hills and m ountains and the results aresumm arized in the following explanation.Slope currents form over mountain slopes w hen there is a horizonta l m ovement of air towards them . The m ountain slope deflectsthe air vertically from its initial horizontal movement, and alsoincreases its speedof travel.The strength or vertical velocity of slope currents depends uponw ind velocity, the height of th e m ountain, and meteorologicalconditions existing around the m ountain .

F L O W O F A IR O V E R M O U N T A I N S . W hen am ountain is not part ofa range butstands separately ina regionof ground, forming a cone,theair m oving tow ards it w ill flow ina horizontal and vertical plane .If the base of the mountain is small, theair willhavethe tendencyto pass it in a horizontal plane. Such isolated m ountains m ay produceliftareas extendingup to one-thirdof their height. The lift distribution, how ever, will depend entirely upon the shape and size of themountain and the velocity of the w ind.W hen a mountain is in the form of a continuous ridge, only asmall part of the air will flow around it in a horizontal plane, andm ost of it willhavethe tendencyto flow in avertical plane, over theridge. A m ountainw hose length is four times its height will producea uniform air flow in a vertical plane. O ver such a m ountain th elift areamay extend to a height four times th at of the m ountain .V ery steep slopes m ay produce a turbulent flow over th e ridge,and w hen soaring in such a region pilo ts should be particularlycareful not to fly on the lee side, on account of the prevalence ofvicious dow n-currents w hichm ay often exist in thelee areas.


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42 THEORY AND TECHNIQUE O F SOARINGThe best lift areas are usually found over the slope where thevertical component ofwind velocity caused by the deflection oftheslope is the greatest. The vertica l velocity is greatest near the slope

FIG. 52. MOUNTA IN S O F CON ICA L FIG. 53. THE ID E A L S H A P E O F A M OUNTA INS H A P E P R O D U C E S M A L L LIFTIN G The lift area extends to four times the heightofA R E A S the mountain.The only lift availab le m ay be found justbehind thetop of themountain.

FIG. 54. D I S T R I B U T I O N O F V E R T I C A L C O M P O N E N T O F WIN DV E L O C I T Y D U E T O T H E S L O P Eanddecreaseswith heightfromthe slope . Bowls andpockets form edinaslope increase the ra te of flowof air, and as the velocity offlowincreases also, the vertical component of velocity will pro vide stillstronger lift in such areas .The influence of the tempera ture lapse rate plays an important


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METEOROLOGY 4 3part in th e lifting characteristics of a mounta in. W henthe lapse rateis higher than th e dry adiabatic la pse ra te (5-4 F per 1000ft) th eair w hich has beendeflected by the slope of th e mountain and travels

FIG. 55. I N C R E A S E O F V E R T I C A L V E L O C I T Y D U E T O A B O W L

FIG. 56 . I N V E R S IO N F O R M E D O V E R T H E M O U N T A I N R E D U C E S LIFT A R E A C O N S I D E R A B L Y

upw ards may reach very great heigh ts . In such cases th e verticalarea of height ma y be increased to fiveor six timesthe normal height,which, it will be remembered, is four times th e height of themountain.If th e lapse rate is low er th an the dry adiabatic, and inversion


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44 THEORY AND TECHNIQUE OF SOARINGforms over th e mountain, the flow of air above th e mountain m aybe stopped. In such a case no lift will be found in spite of th e wind.When cold air covers th e valleyat th e base of th e slope, th e vertical

FIG. 57 . C O N D I T I O N P R O D U C I N G No LIFTIf cold ai r covers th e valley in front of th e mountain , in spite of

wind there may be no lift over th e ridge.flow of air m ay be greatly reduc ed. This happens very often duringautumn and winter mornings. Later in th e day, when th e cold airhas been moved from the valley by th e wind, th e soaring conditionsover th e slope may improve.


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CHAPTER IIDIRECT SUN T H E R M A L UP-CURRENTS

T H E condition which is essential for the birth of any thermal up-current is that the particular mass of air must be warmer than theair immediately surrounding it. Without this condition there willbe no therm al up-current formation.If there is an area of dry, hot ground surrounded by w et and coldground, and if the sun shines uninterruptedly, the dry and hot surfacewill release heat and the layer of air which lies immediately abovethat surface will become warm. As this air gradually becomeshotter it will expand and decrease in density. The cold, wet airwill force its way towards the dry, hot surface in the form of awedge owing to its greater density, and will push up the warm, dryair, due to the local difference in pressure, / ? x d p - ^ . This pressuredifference is infinitely small but sufficient to start the vertical

movement of air. This movement is very slow, and at a certainheight h (see Fig. 59 ) the difference in pressures completely disappearsas they become equal, i.e. p dp v = p z .This movement of air in the layer which is very near the ground isindependent of the lapse rate within it. Should the adiabatic lapserate in this layer of air be smaller than the dry adiabatic, as itusually is, the air m ovement is mainly horizontal and if there is anyupward travel it is due to initial velocity, inertia and difference indensity. The height to which the warm, dry air m ay ascend whenthere is a lapse rate near the dry adiabatic, depends mainly on thevelocity of air travel from wet, cold areas to dry, hot areas, and thisdepends on the intensity of the warming-up of this air along itspath of travel.When the velocity of motion is sufficiently large and the thicknessof the layer of air near the ground with a lapse rate near the dryadiabatic is small, the travelling air m ay break through. As soon asthe lapse rate becomes greater th.an that of the surrounding air, thebubble will gain an upward velocity.

The above discussion is valid only for conditions of no wind.With a wind, however, there is horizontal movement of the wholemass of air, cold and hot. It often happens that the air becomeswarm over one stretch of ground, still warmer over the next, andcools again over the next, but the inflow of cold air under warm is45


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46 THEORY AND TECHNIQUE OF SOARINGcontinuous. This flow m ay be of sm all velocity , and is m ore inten-sive from both sides th an from the front or back.The stronger th e w ind, th egre ate r the turbulence and th ickness ofth e ground layer, and th e longer th e t ime needed to warm up thetravellingvolum e of air. This m eans that th eform ationofa th erm alis m ore difficult. This is in agreem ent with experience which show sthatth e stronger th e wind th e fewer are direct sun therm als .

Wet cold ground Dry hot surface Wet cold groundF IG . 58 . FIRST S T A G E O F F O R M A T I O N O F T H E R M A L U P - C UR R E N T

Warm airis beingpushedupwards by inflowing cold air.

Wet ground Dry surface Wet groundF IG . 59. D E V E L O P M E N T O F T H E R M A L U P - C U R R E N TA t heighth , pre ssures p dp , and p z becom e equal.

DEVELOPMENT OF A THERMAL BUBBLEW hen warm air, having bro ken through th e layer of air near th eground, enters air which possesse s a lower te m peratu re than itself,it will m ove upwards. Having previously travelled in variousdirections it w ill now be sucked in and begin its upward travel. Dueto th e activity of the therm al bubble there w ill be a horizonta lvelocity over a la rge ground area, which m ust re sult in verticalm ovement due to inertia . It m ust be remem bered, however, thatsuch adevelopment is only possibleif th ere is aconstant heat supplyfrom area A (see F ig. 60). Should there be a slight wind or a cloudbetween the sun and area A, th e heat re lease from th is are a willdecre ase and th e warm air break contact with its source.A s a rule such an individual m ass is known as a th erm al bubble,but it is far from having a perfect spherical shape. W hen such a


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DIRECT SUN THERMAL UP-CURRENTS 47thermal bubble is liberated the cold air f lowing in from all sides willfill th e gap left by th e travelling air, and w ill receiv e a verticalcomponent due to suction and its own velocity. It may happenthat th e sucked-in cold air w il l form a continuation of the thermalbubble, and increase it s length considerably.A thermal bubble may contain some moisture, and w hen it stemperatu re reaches dew point, th e water-vapour will condense and

W e t ground D ry surface W et groundF IG . 60 . F IR ST S T A G E O F F O R M A T I O N O F T H E R M A L B U B B L E

Cold air

Dry surfaceF I G . 6 1 . TH E C O L D A IR F I L L ST H E G A P L E F T B Y M O V I N G T H E R

M A L B U B B L E

Dry surfaceF I G . 62 . T H E I N - G O I N G C O L DA IR F O R M S C O N T I N U A T I O N O F

T H E R M A L B U B B L E

a cloud will form . Under such a cloud the bubble of warm air mayextend to very low altitudes. Naturally such a "long" bubble willdecrease its length in a short time.Immediately under the cloud th e li fe of the therm al up-currentmay lasta long time due to th e big vertical velocity contained in th ecloud and the large inertia forces superimposed on th e surroundingmasses of cold air. This explains w hy , during a day with little wind,


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48 THEORY AND TECHNIQUE OF SOARINGa pilo tmay succeed in staying under abig cumulus cloud for a longtim ewhileother pilo ts try ingtoget under th e samecloudare forced to land.

F IG . 63 . U N D E R T H E C L O U D T H E T H E R M A L B U B B L E M A YE X T E N D T O V E R Y L O W A L T I T U D E S

F IG . 64 . N E A R T H E G R O U N D T H E V E R T IC A L V E L O C IT Y M A Y B E Q U I T E L A R G E ; I T D E C R E A S E S W IT H I N C R E A S IN G A L T I T U D ETHE LIFE OF A THERMAL BUBBLEThe upward tra vel of a th erm al bubble is due to the difference intemperatu re between itand the surrounding air, or in oth er word s,to the different densities. During its vertic al travel the bubble also


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DIRECT SUN THERMAL UP-CURRENTS 49expands, causing a decrease in vertical velocity . Near the groundth e vertical velocity may be quite large and depends upon the temperature difference between th e bubble and th e surrounding air.Velocity near the ground may be 1 0 ft/sec but at higher altitudes itmay drop to 2 ft/sec or disappear completely.Upward Velocities in a T hermal Bubble

The most economical method of launching for the majority ofschools and clubs is by winch or auto-tow. This kind of launchingseldom provides more than 1000 ft of height, so it may be of someinte rest to dis cuss a few factors whichgive a more complete picture of thebehaviour of thermal bubbles at suchlow altitudes.Much valuable work has been donein this sphere in Germany and Poland,and th e following figures are basedon the results obtained from thesetw o countries. Taking normal conditions favourable to thermal bubbleformation, i. e . ground heated upduring th e morning, light cumulusclouds, lapse rate greater than dryadiabatic, slight wind, then Fig. 65can be prepared.From Fig. 65 it can be seen thatat 1000 ft height the most common upward velocities existing in athermal bubble will be in a range of 3-5 ft/sec (40 per cent of thetotal). From the soaring point of view these velocities are small butthey are sufficien t to enable experienced pilots to stay in the air andeven gain height. The

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Theory and Technique of Soaring