THE PRESENTATION OF VISIBILITY OBSERVATIONS 31

551.501.6: 551.591.3

T H E PRESENTATION OF VISIBILITY OBSERVATIONS

VISIBILITY CHARACTERISTIC CURVES AND A \'ISIBILITY IXDES

By R. M. POULTER

[ifanuscript received November 11, 1935-read December 10, 19S01

SUMMARY

The range of distance embraced by a visibility number used a t meteorological stations varies roughly as the lower limit of the range, so that one occasion in a year of visibility 3(500 m.-1 km.) is twenty times as significant as one occasion of visibility 7 ( 10-20 km.) , and so on.

Accordingly, frequency curves characteristic of six types of meteorological stations in the British Isles, judged by the surround- ing country, have been drawn by plotting the number of occasions per kilometre (n/d) covered by each visibility number.

The 54 curves refer to 7 h., 13 h., and 18 h. for summer, winter, and the year for stations grouped in the following categories :-

, 9 ,, ,, with sea fogs. ,, polluted air, rural. ,, polluted air. 9 , ,, ,, urban.

Coastal-clean air. Inland-clean air.

and show marked seasonal and diurnal variations.

I . The interpretation of visibility observations may be said to have suffered because these observations were introduced as a requisite of meteorology in war ; a few observations had been made before 1914, but systematic records were introduced and multiplied rapidly during the Great W a r ; post war meteorology has recognised their value and they are now firmly established in the routine of observing stations with little change in the method of making the observations.

I t was decided that the visibility was to be expressed by one figure of the telegraphic code for weather observations so that the scale ranging from the short distances of 50 m. or less in thick fog to the distances of 50 km. and upwards in clear air had to be expressed in 10 steps. The observations therefore are not made in miles or km., but approximately in a logarithmic scale in the form D=d x 2*xV, where V is the figure recorded and telegraphed and d = 5 o metres. The visibility is entered as V when objects are \,isible a t a distance D = d x 2.1v but are not visible at d x z * I V + ~ ; the range of distance to which any one value of I.' applies is also approximately d x 2*xV, i.e., each step is roughly equal to the sum of all the smaller steps, but there are irregularities.

The actual scale in use is:- . O X 2 3 4 5 6 7 8 9

Objectsvisibleat - 5om. 200 500 i k m . 2 4 10 20 5okm.and over But notat . 50m. 200 500 i k m . z 4 10 20 50 -

32 THE PRESENTATION OF VISIBILITY OBSERVATIONS

The difficulty of presenting visibility observations in a sum- marised form which exhibits the characteristics of a district o r even a station is evident.*

2. Obviously frequency tables of visibility give certain definite data, such as for instance that 43 per cent of the occasions have visibility between 4 and 10 km., and this percentage can be com- pared with others in the same column for other hours and seasons. But comparison between columns is restricted, for the distances covered by the various columns are in the ratios

0.05 0'15 0.3 0'5 I 2 6 10 30 - and if it happened that in a series of observations there was one Occurrence of visibility terminating in each km. of distance from the observer the summary would be

[Added later.

Columns 0-3 I I 2 6 1 0 3 0 -

The following figures from the Monthly Weather Reporf a re totals of the visibility column in Table IV for the years 1933-35 :-

0 1 2 3 4 5 6 7 8 9 T o t a l British Isles . 279 1842 2073 3579 7389 16284 52733 65005 72770 8119 230.073

Dividing these figures by 7.4 we obtain:- 38 250 280 480 990 2180 7100 8720 9780 -

which can be compared with the following distances in metres in each visibility number :-

and show how nearly n,ld is constant for visibilities 2 to 7. I f , however, the first and last pages of Table IV containing

Scotland, Ireland, Scilly and Guernsey are added separately and then pages 2 and 3 containing mainly industrial England, the following quite dissimilar sets of figures totalling practically the same are obtained:-

0 1 2 3 4 5 6 7 8 o T o t a 1 Scotland,

Ireland, etc. 28 576 703 ~1018 1930 4210 19336 34100 47977 6354 116,232 England. . 251 1266 I370 2561 5459 12074 33397 30903' 24793 1765 113.8411

If now the percentage in any column is divided by the distance (in kilometres) covered by that column a new index of visibility frequency is obtained which enables comparison between columns to be made in addition to comparisons already possible. Thus, if a visibility frequency summary is treated by multiplying the figures in the respective columns by the following factors :-

a set of visibility frequency indices is obtained which can be plotted Dr. Whipple in opening a discussion before the Royal Meteorological Society

in December, 1921 (Q.].R.Meteor. Soc., 48, 1922, p , 86-89) used medium range of visibility and also curves of totaled frequencies s M i n g from 30 km. and working downwards to fog. C. S. Dunt in a paper read before the National Smoke Abatement Society in May, 1935, in London, favoured a similar form of curve of totalled fre- quencies starting with fog and working upwards to good visibilit).: frm these he is able to read off the distance at which an object can be seen on, say, 75 per cent of occasions and this figure is used as a criterion of visibility at the station.

50 150 300 500 1000 2000 6000 1oo00 3oooo -

2 0 2013 1013 t I fr 4 1 / 1 0 1/30 -

THE PRESENTATION OF vrsimrm OBSERVATIONS 33

t o form a visibility characteristic curve for the station at the particular season.

This treatment brings o u t the remarkable variations which occur in thick fog frequency between station and station, and from hour t o hour and season to season at one station. Examples are given in Fig. I . Fig. 2 gives for comparison curves based on the

120- \ \

I I

CO,tl&y c----c Setlly 0-0 lnvcrncss .. ... a(9.m) K-w *r

Fxo. 2.-Ordinam visibility frequencies for typical rtations in winter at 7 a.m.

actual untreated frequencies. In plotting these visibility charac- teristic curves logarithmic paper should be used, but little practical inaccuracy arises if instead of using the distances in kilometres on a logarithmic scale the visibility code numbers are plotted a s a linear scale.

3. TO make the best use of this method in illustrating the variations of fog and visibility throughout the British Isles, the

34 THE PRESENTA'rION OF VISIBILITY OBSERVATIONS

stations for which 10 years' visibility observations are available were divided in consultation with hfr. C. S. Durst into five groups described by the following terms [added l u t e r , in which '' polluted " refers to the optical and not to the hygienic properties of the air] :-

Coastal-clean air.

Inland-clean air. ,, -polluted air.

,, -polluted air-rural. 7 , 7 7 ,, -urban.

a 1 I 1 2 1 3 1 4 1 5 1 6 I 7 1 6 1 9

Vl5lBlLITY

I t ' ? ? 5p 4 4 I Yn 4 DislQncc 4' I t

u*r ... . . COASTAL STPTION~ - C l e m Air. Wvltr ---

c Year -

THE PRESENTATION OF VISIBILITY OBSERVATIONS 35

COASTAL STATIONS. INLAND STATIONS.

Calshot. Upper Heyford. Pembroke. Cranwell. Scilly. Cardington.

Clean Air-Sea Fogs (Ccs). " Polluted " Air-Rural ( I p r ) .

Wisley. Ross. " Polluted " Air (Cp) .

Scarborough. Spurn Head. " Polluted " Air-Urban ( lpu) . Ciomer. Yarmouth. Felixstowe. St. Leonards. Brighton.

Kensington. Kew. Greenwich. Stroud Green. Durham. Birmingham.

'2 .5 1 km. 2 10 20 50 I 1 1 1 1 1 a- _.....

COASTAL %,4TlOp'K c Cknn kr - %a 6 9 3 Wmtcr --- YPar -

~ israncc '?

FIQ. 4.

Percentage visibility frequencies for these groups of similar stations were then computed for 7 a.m. (or g a.m.), I p.m. and

36 THE, PRESENTATION OF VISIBILITY OBSERVATIONS

6 p.m. for the seasons winter (Dec.-Feb.) and summer (June-Aug.), and for the year, making nine sets of observations for each of the six groups. Each of the 54 sets was then converted into sets of visibility frequency indices by the appropriate factors ; 51 curves were then drawn and are shown in Figs. 3-8.

These curves are pictures of atmospheric obscurity of any sort which reduces visibility; a straight line through o indicates n o obscurity sufficient to reduce visibility below figure 9 (50 km.;, IOO per cent of occasions of visibility 4 (I km.) woutd give a scale

'I ,G., / \

/ \

p-': ' . . .. . . . . . . . . ..... ........... ... .........__ _...

*.-< ......... .. -*-

I /=-I,

1 FIG. 5.

THE PRESENTATION OF VISIBILITY OBSERVATIONS 37

value IOO, and IOO per cent of dense fog (less than 50 yards) would give a scale value 2,000.

Fig. g gives for comparison frequency curves of relative humidity a t Kew and Valentia, which show the small seasonal variations a t Valentia, apart from an increase of humidity on summer mornings, and the large seasonal variations a t midday a t Kew.

*;J \ kc.r 1 4 I 0 lo SO * %or -

1 1 I t '2

I Dlsfbfice '?' lNLPN D STATlONs - Clean bk UAk --

FIO. 6.

4. EXAMINATION OF TEE CURVES. (i) Coastal 8tatiOn8-ckan air.

and little diurnal or seasonal variation.

(ii). Coastal Stations-clean air-sea fogs. These curves show two pronounced maxima at visibility I and

at 6 to 7; there is little seasonal variation, but during the day the fog frequency diminishes towards I p.m., and increases again towards 6 p.m.

The lack of seasonal variation distinguishes these two sets of curves from all the rest; the distinction is emphasis4 by the increase of sea fog during summer a t 7 a.m.

These are simple curves showing a maximum a t visibility 7

Visibility o is rare.

38

(jji) Coastal Btationa-" polluted " air. and 6 in

the figures for winter and the year'; but in the summer curves t h e fog hump almost disappears throughout the day, and in winter there is an appreciable frequency of visibility 0.

(iv) Znland Stations-clean air.

and 7. not occur.

THE PRESENTATION OF VISIBILITY OBSERVATIONS

These curves show definite maxima at visibility

In the winter and year curves maxima exist at visibility I Visibility o does In summer there is one maximum at 8.

VISIBILITY

1 4 INLAND STATIONS - Pollured Ar - RURAL.

I

- 10

- 16

-12

I I fi/ \ \

\ I I '. I \

\

I I

fk

\ \

THE PRESENTATION OF VISIBILITY OBSERVATIONS 39

FIQ. 8.

40 THE PRESENTATION OF VISIBILl1'V OBSERVATIONS

(v) Inland Stations-" polluted " air-rural.

and 6 in winter and throughout the year. the fog tendency is diminished.

bility o does not occur.

In the mornings there are pronounced maxima at visibility I At I p.m. and 6 p.m.

Visi- In summer there is one maximum at visibility 6 to 7.

FIG. %-Humidity fregaencier.

THE PRESENTATION OF VISIBILITY OBSERVATIONS 41

(vi) Inland Stations-" polluted '' air-urban. These Curves show the greatest seasonal variation ; in Summer

there is a maximum which varies from visibility 5-6 in the morning to 8 in the evening.

In winter there are maxima at visibility I and 3 to 4, and in the morning these tend t o form one maximum a t visibility I . In the winter visibility o is common and the year's figures show an appreciable proportion. (vii) Gene+aZ Conclusions.

It will be observed that as the fog hump becomes more important it moves t o the left and at the same time the main hump moves to the left. Therefore by adding together the values of n/d for each visibility number a total is obtained which characterises a station at the hour or season under consideration. For the whole of the British Isles all the year round the figure would be 27; for a coastal station with clean air the figure is between 8 and 12, while for Greenwich a t g a.m. in winter it is 170. Sea fogs nt Scilly bring the value up to 47 for 7 a.m. in the summer.

The year's figures for Kew are:- 7h. 13h. 18h.

Winter ... ... '14 76 91 Summer ... ... 25 7 7 Year ... ... 100 38 53

Index figures obtained in this way give a quick indication of the suitability of a site for an aerodrome as regards visibility.

The best visibility is found where great distance separates the locality from smoke producing agents and where, in addition, the locality is on the leeward side of high ground SO that salt and other impurities have been washed from the air by precipitation on the windward side.

Otherwise the best visibility all the year round is found on the N.W. coasts; but in summer afternoons and evenings the best visibility is inland, even in urban surroundings.

On winter mornings at Castlebay and Blacksod Point and on winter evenings at Tiree and Blacksod Point visibility hardly ever falls below I km. and seldom below 2 km. ; the corresponding curves show one maximum a t visibility 7 (10-20 km.). Typical figures are :- Visibility . . o I 2 3 4 5 6 7 8 9 Frequency % . o o 0 0 0.4 2'9 199 4 4 5 28'2 4 2 %/krn. (n/d) . 0 0 0 0 0 4 1'4 3'3 4 4 0'9 -

At 13 h. and 18 h. in summer at inland stations the same remarks apply and the mean figures for inland (clean, rural and urban) stations are :- Visibility . . o I 2 3 4 5 6 7 8 9 Frequency % . o o o o 0 2 2.0 11-2 206 60'2 5'7 %/krn. (n /d) . o o o o 0'2 1.0 1'9 2.1 2.0 -

At all other hours, seasons and localities, sea or land fogs, with high humidity, cause a second maximum in the curve, commonly at visibility I , so that the majority of curves have two maxima. I t is convenient t o refer to the maximum in the curves

42 THE PRESENTATION OF VISIBILITY OBSERVATIONS

due t o high frequency of fog with visibility I as the " moisture hump " and the main maximum due to the visibility of the norm:iI atmosphere containing salt and other particles as the " normal maximum." As the normal maximum varies from its extreme position at visibility 8 a t 13 h. in the summer at inland stations with clear air (Ic) on the one hand, to the other extreme of visibility 3 at urban stations on a winter morning, the increase of atmospheric impurity causing this change is also responsible for a corresponding increase from o to 28 in the frequency index (nld! for visibility o and I .

Thus we may say that the best visibility occurs in air relatively free from salt and in which there are no large water drops, and that the next best visibility occurs in air free from suspended water drops. Bad visibility occurs with salt and water drops present; the addition of smoke causes the worst visibility, so that conditions become suitable for a ring of dense fog round large cities where both moisture from outer suburbs and smoke from inside are plentiful ; but a t the centre visibility is not so much reduced because moisture is not sufficiently abundant over great areas of compara- tively warm, dry roads, pavements and brick buildings; nor is there usually such a marked inversion in the morning to hinder the dissipation of smoke particles.

THE PRESENTATION OF VISIBILITY OBSERVATIONS 43

5. DIURNAL VARIATIOKS OF URBAX FOG Hourly observations of visibility a t Croydon Aerodrome in

winter have been subjected to analysis on the same lines, and curves a rc shown in Fig. 10. In January from I a.m. to 10 a.m. there is a pronounced maximum at visibility I , but for the rest of the day the maximum is at visibility 0.

Taking the three months December to February, the morning hours I a.m. to 7 a.m. show maxima at visibility I and 5 while the rest of the day's observations show maxima a t o and 4, showing the effect of smoke or reflection from the fog (or both) after dawn.

I n February there are pronounced maxima at visibility o and 4 from 8 to 10 a.m. while before dawn the maxima are at visibility I and 5 .

6. Having classified stations according to their visibility and fog characteristics, it becomes very evident that the combination of frequencies for dissimilar stations can only give very misleading

1 0 1 I 1 2 I 3 1 4 1 5 1 6 1 7 1 8 Visibility Number

F I ~ . ll.-Summary of the varying positions of the maxima.

results and must be avoided. For it is possible to select a few stations in the British Isles so different in visibility and fog-forming tendency that when their frequencies for a long period are totalled up and combined the resulting characteristic curve is a straight line. I n fact if all the visibility figures appearing in the Afonthly Frequency Tables of ?he Uritiah Isles for a year are totalled, prac- tically a straight line is obtained in spite of, or rather, because of all the differing component causes illustrated in this paper, with their varying maxima which a re shown in Fig. 1 1 .

7. I n making use of figures for 9 of a visibility summary the following considerations must be borne in mind.

Of about p stations contributing \-isibility observations to the Monthly Weather Report only 1 5 have a visibility object at or near 50 km. ; a t other stations visibility 9 is estimated by the clarity of the atmosphere. Of the 15 stations only 10 have an object a t or near 2 0 km.

44 THE PRESENTATION OF VISIBILITY OBSERVATIONS

Therefore observations of visibility g must be used with caution, and this for a further reason. An object a t 50 km. can only be visible if the object or the observer is 600 feet above the intervening horizon, or i f both are 2 0 0 feet above the intervening horizon. Obviously this introduces a new factor, namely, that at stations having a visibility object at 50 km. a great portion of the path of the visual ray is above the lower layers of smoke and mist, and therefore these stations should have higher figures under visibility g than those stations whose figures appear in italics denoting esti- mated values.

Examination of the frequency tables in the Monthly Weather n8pol.t for 1934 shows that, on the average, stations with a 50 km. object observed it 31 times in the year, but stations without a n object estimated the visibility to be 50 km. or more on six occasions only.

CmlJ Polluter O-J \ ---P--- h w ---K--- hbnd P.LU(Rur0l) I & c k ~ ~ d -9,

Cmd.1 Clron - C- Ihbd

Cm*tol C1r.n (kr f.1) - 5- lnlml Ck4.r \ P I l u t d 1 ' ---u--,

(Orb-) I

FIO. 12.-Typical curvea for the year a t I am. using nl Jd.

8. ANOTHER NETUOD

\Vhile these curves exhibit well the fog and low visibilities, the occasions of visibility g are not provided for, but the curves have the advantages of easy and rational computation ; further, the area under the curve has a definite significance.

Another method has therefore been devised to provide as well for these high visibilities as for figures o and I . Instead of dividing n, by d,, divide by d,t; then instead of factors

2 0 20/3 1013 2 I 4 ?J 1 / 1 0 1/30 the following new set is obtained :-

4-5 2-6 1.8 1.4 I 0 7 0 4 0.3 o r 8 (0.1)

t o which the empirical value of 0.1 can be added for visibility figure 9.

T H E PRESENTATION OF VISIBILITY OBSERVATIONS 45

Figures for the year at 7 a.m. at all types of stat ions h a v e been plotted in Fig. 12.

9. I am indebted to the Director of t h e Meteorological Office for t h e use of the summarised visibility frequencies on which th i s paper is based.

DISCUSSION hlr. L. C. W. BONACINA drew attention to a list of health resorts.

including Cromer, under the heading “ polluted air,” and asked hlr. Poulter whether he could see his way to use inverted commas, or in some other way qualify the designation.

H e thought that whatever technical reason hfr. Poulter might have for calling the air at such places polluted, the medical officers and others, who pride themselves on the purity of the air at their respective resorts, would be highly indignant to read that they breathe polluted air.

Mr. Bonacina supported very strongly Mr. Poulter’s generalisation concerning the great seasonal contrast in visibility in urban districts. From long experience of residence in the high-lying suburb of Hampstead, he could say most emphatically that whereas it is extremely common at midsummer to see right across London to the Surrey Downs and into the rural heart of Essex, just as clearly as if there were no town, it is never possible in mid-winter to see across London and hardly ever possible even to see the dome of St. Paul’s half-way across. But he could say equally emphatically that the seasonal contrast from Hampstead in the opposite direction towards the country was not nearly so marked, for there are days even in winter when the spire of Harrow Church which is about twice as distant as St, Paul’s could be clearly seen, and also the North Mrddlesex hills with the Chiltern ridges beyond.

Mr. Poulter had referred to the disappearance to a large extent of ground water fog from central London. This was a fact, but i t only dated from the second decade of the present century. H e doubted whether the change was also true of hlanchester, Birmingham and other industrial towns, as they were much smaller than London and more open to fog-drift from the suburbs.

hlr. E. G. BILHAM congratulated Mr. Poulter on approaching this question of visibility from a new standpoint, and said he thought he had brought out a number of interesting points. The most striking point about the curves, as drawn by hlr. Poulter, was the presence in nearly all of them of a prominent hump a t visibility I , and he thought this gave rather a misleading impression of the frequency of occurrence of dense fog. For visibility I the divisor d in the variable n l d was very small, only 0-15 km., consequently a relatively small change in the numerator would make a considerable difference in the ordinate. I t would be useful if hlr. Poulter would indicate what the value of n was, in a few cases.

Dr. F. J. W. WHIPPLE, President, said he hoped that the table which hlr. Poulter had put on the board would be reproduced when the paper was published. In that table hlr. Poulter had set out the frequencies of different visibilities for the British Isles as a whole. The method adopted in the paper was practically to compare the frequency of each visibility at a given station with the averages for the whole country. He himself had found Mr. Poulter’s diagram easily readable with that idea in mind, but unfortunately the interpretation broke down in the case of visibility g , which was the most interesting of all. Owing to his original generalis-

46 TIIE PRESENTATION OF VISIBILITS OBSERVATIONS

ation thus failing, hlr. Poulter had not been able to give an idea of which stations enjoyed very high visibility, although he had referred to Inverness, where it was possible for about half the time to see nearly 30 miles. I t was very disappointing that the diagram did not show a hump a t visibility 9 for such stations.

hlr. R. hl. POULTER (in reply) said he was very conscious of the defect mentioned by Dr. b’hipple and it was on this account that he had added a t the end of the paper an alternative method which could be made to show something for visibility 9. H e regarded the curves, how- ever, as concerned with obscurity rather than visibility. With regard to bfr. Bilham’s remarks, he would be very glad to give him the figures he wanted though he believed most of these were already contained in the paper.

I t must be remembered that if a station experiences visibility I in 3 days in the year, it means that I per cent of the observations occur in a distance of 0.15 krn. while visibility numbers I to 8 cover 50 km. with generally less than IOO per cent, so that it is fitting for I per cent of visibility I to have a hump in these diagrams up to a scale value of 6.; per cent. In all these cases visibility numbers o and z have values fitting the curves drawn ; visibility I does not stand out disproportionately. J f there were no physical reason for the fog hump, a smooth curve would give a value of 0.1 per cent for visibility I instead of, say, Calshot’s suninier morning figure of z per cent.

5jf.521.9 On the emission of the sky during twilight and during the eclipse

totality of 1936, June 19: preliminary note by‘D. J. Eropkin

\ (Communicated by Prof. S. CHAPhfAN, F.R.S., July, 1936)

During the last two years N. A. Kosirev and D. J. Eropkin have obtained the following results on the emission of the night sky, the polar aurora and the zodiacal light, with a luminous spectrograph I : 1-5.

The green line 5577A is only half as intense one hour before dawn as a t midnight. The intensity of this line in sunlit polar aurora: also diminishes in the same proportion. T h e intensity of the light of the twilight sky increases towards the red (Ilford hyper-panchromatic plates were used).

During the eclipse totality of 1936, June 19, a t Rustanaij, sky spectra with the same spectrograph were obtained, with an exposure of two minutes. The spectra arc unfortunately very intense. They show emission in ’the red region. The red emission, both during the eclipse and in the twilight sky, may be attributable to neutral oxygen atoms 01. and be connected with the decrease in the green light 5577.4 caused by sunlight, that is to say, with instability of 01 atoms in the state 1.5, when sun-illumined; perhaps the atoms are raised first to the IP, state, thence descending to the ED, state whence, by transition to the ground state * p red light is emitted. If so, the sky spectrum would resemble nebular spectra, in which the green line 5577A is weak and the red doublet is intense.