WINDS FOR THE OLYMPICS

By ALAN WATTS Colchester, Essex

HIS is Olympic Games year and it is very probable that whoever walks T off with the coveted Gold, Silver and Bronze Medals for the yachting events will be able to ascribe part of their success to their ability to react correctly to some of the wind shifts which they met around the courses.

It might be too much to say that micro-wind changes control the winning of yacht races, and yet often whether you win or lose depends on getting a favourable shift just when you are about to lay a mark or stand across the finishing line.

Wind shifts of a few seconds duration cannot be ignored and neither can any shift of longer duration. The correct reaction must in every case be made when the shift appears, but how much better it would be to have enough basic knowledge of micrometeorology to divine the incidence of a favourable shift before one’s competitors. Just one such prediction can make the difference between success and failure.

It therefore becomes of major importance to acquaint, as far as practicable, our top helmsmen with as many helpful facts on micro-wind structure as possible. We, as a nation, pride ourselves on the sea being in our blood and yet it is rather rare for Britain to produce a Gold Medallist in the yachting events. In the single-handed class it is almost unheard of. Charles Currey achieved a silver medal a t the 1952 games, but had to use his skill as a seaman to re-work the poor sail he was given on his Finn in order to do it. Stewart Morris is one well-known name among the select brethren of Gold Medal winners but we have to go back to 1948 to find him winning a Gold in the Swallow class. Keith Musto and his crew were practically robbed of their Gold Medal a t the last Olympics in Enoshima, Japan, by a wind shift. They had to make do with a Silver.

No one is going to say dogmatically that a deeper understanding of micro- or meso-scale wind structure would have made any difference-but there always remains the possibility that it could. Olympic medals are won on a mixture of knowledge, skill, luck and that particular form of dedicated aggression to which all top-flight sportsmen are addicted. And in the sphere of knowledge -and luck-comes the wind.

At present the study of micro-wind changes as applied to yachting is in its infancy. Helmsmen talk about them. Race Committees required by Olympic rules to lay a starting line which will provide a I&nautical mile beat to windward would dearly love to know if the wind will shift appreciably just before the fleet is nosing up to theline. What isoften not realized, however, is just how difficult it is to be precise about any meteorological element. You have to have been a professional forecaster before such facts of life are fully rammed home by bitter experience. I do not think we should be deterred by

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the difficulties. Even the limited advances we have made in the elucidation of useful repeating patterns in unstable airstreams, the knowledge of sea-breeze- front phenomena, even the better understanding of the formation of thermals have all on occasions helped to knock off the seconds and even to effect a win.

On the meso-scale it is the changes over coastal waters when sea breezes are setting in against offshore winds which are of prime importance. We now know a great deal about sea-breeze fronts and their formation, their speed of advance inland and the sort of wind shifts to expect as they pass. There is still a lot of detail to be filled in, but once the front is a few miles inland on a good sea-breeze day then we can confidently expect a certain chain of events to attend its passage. This knowledge is important to estuary and inland water sailors. Unfortunately we know next to nothing about the speed of advance seaward of the corresponding anti-front or how long it may linger over the immediate coastal strip where Olympic-style courses are usually set. This is a major and unsolved problem.

A related micro-scale problem concerns the sometimes quite catastropllic wind shifts which appear under sea-breeze fronts when the offshore wind and the sea breeze are nearly evenly matched. From work done on anemograms at Thorney Island we know that a spectrum of erratic interchange exists between blocks of offshore wind and blocks of sea breeze. A clue to what promotes one block or the other comes from the knowledge that shower clouds, and thunderstorms particularly, can destroy the sea breeze under their leading downdraughts and promote it in the corresponding updraughts in their rear. I t may therefore be possible to anticipate such exceptional shifts which accompany the active sea-breeze front when it is over the sailing area.

Even if we solve some of the problems of predicting micro-wind changes which are met just offshore there will still be the problem of communication. What is studied and discovered has to be related to the problems of two distinct classes of recipient, namely helmsman and race officers. The instruction that can be given to helmsmen depends on the average crews’ knowledge of micro- meteorological principles. This is often meagre and only a deeper basic knowledge will allow crews to live with the ever present problem of being wrong so that sometimes one can be very, very right. What makes matters worse is that Olympic crews often have to sail out early, remain on the racing area for much of the day, and then sail back. The provision of a forecast in the morning covering such an extended period must lead to a low confidence. It would be low even for relatively large-scale variations. To provide forecasts of micro- or meso-scale changes over this period is more often than not going to be impossible unless the area is very well known to whoever provides the forecasts.

Committee boats are more fortunate in that they may be in touch with the shore by radio and so have immediate access to forecasts of wind changes if these can be foretold. Fig. xu shows where the Dinghy and Big courses are laid in the Olympic Trials area of Poole Bay and these are typical offshore positions for most Olympic courses wherever they are set. They are very akin to the positions to be occupied by the courses at Acapulco in the state of

I0

n - .- I E)

3

0

In

c

c

c

Fig. ~ a . The general situation of the Olympic trials courses at Poole Bay

Guerrero, Mexico, this coming October for the Olympic Games proper (Fig. ~ b ) . When courses are set just offshore like this the provision of accurate

information on wind shifts in the hours before the starting guns go is bedevilled by two factors. The first is that the starting time, of 10-12 local time, is when major changes due to sea-breeze forces are most likely to occur, and the second is that winds in the immediate offshore area most suitable for courses and competitors cannot normally be predicted by extrapolation of observations from an established and nearby shore station. In particular, referring to Poole Bay, the actual wind speed and direction a t Hurn Airport some 4 miles inland from the beaches of Bournemouth and 6 miles from the southern extremi- ties of the courses is, on days of light to moderate wind in summer, as likely to be different from that over the courses as it is to be the same. To lay down criteria of wind speed outside which we could say that Hum and Poole Bay had the same wind would exclude all the winds below about 15 kt, the very ones which are important. Thus a major problem exists with regard to providing help in laying the starting lines.

However as Poole Bay is being developed as a centre for Olympic Trials in this country the Royal Yachting Association has authorized some investi-

* 2554' 1 -1474' t 5

. A

-3

-1 = U

3 0

- c

z -0

-1

-2

-3

Fig. ~ b . The general situation of the Olympic course at Acapulco

gations in the area of Poole Bay, to see if any useful help can be given to Com- mittee boats and helmsmen on how the wind may change during the time preceding the starts and also during the races. To this end a preliminary investigation into winds in the area of Poole has been undertaken.

THE WINDS OF POOLE

Unfortunately the sea-breeze regime is weaker in the Poole area than at places further east such as Thorney Island where earlier sea-breeze studies have been made (Watts 1955). The nearest station which provides wind statistics is Hum, some 4 miles inland from the nearest point of the Poole Bay coast.

The normal sea-breeze direction at Hum is 160", and the winds were divided into quadrants or octants about the 160"-340" direction. Using the experience gained in the Thorney Island investigation the times when sea- breezes arrived at Hum (i.e. the hour when the wind changed to the southerly quadrant) were plotted against the mean surface-wind speed in the hour preceding 1000 BST. The wind at this hour we will call the morning wind and it is chosen because Hurn can in the practical case give their mean wind, and a forecast can be based on it before the fleet sets out for the race areas.

I2

14

12

lo

V kt 8

6

4

c

C

16

14

12

10

Vkt

6

4

2

O

K

X

X

X

X

CANNOT OCCUR

K xx --

IF SEA BREEZE IS TO - OCCUR IT WILL HAVE

OCCURRED

I 1

3 11 12 13 I4 15 16 17 18 19 20 8.57:

Fig. za.

U X

K)CX

KX

WILL HAVE OCCURRED

I

1 11 12 13 14 15 16 17 18 I9 20 B S.T.

Fig. zb.

Fig. z. Morning wind (V) plotted against time of onset of sea breeze at Hum Airport (t) with various morning wind directions. za Northerly quadrant. 2b. North-easterly octant. zc. Easterly octant. 2d. Westerly quadrant. Horizontal lines indicate hour during which wind came from the particular quadrant or octant. An X indicates 'cloudy' i.e. sea breeze would not occur

16.

14

12

10

V kt 8

6

4

2

0

CANNOT OCCUR

WILL HAVE OCCURRED

x

x

X

1

10 11 12 13 14 15 16 17 18 19 20 B S T

Fig. zc.

MOST WINDS SWING GRADUALLY TO Vkt SEA BREEZE DIRECTION

8

04 I

10 11 12 13 14 15 16 17 18 19 20 B ST

Fig. zd.

The same kind of coiridors of occurrence (Fig. 2) were found as a t Thorney but the data were much less precise and detailed than in the latter case, where the actual anemograms were studied. In spite of having to use wind statistics which were limited to the hourly means for the preceding hour, some important facts emerged when all the winds for May and June 1962 to 1966 inclusive were assessed.

I. Morning wind frorn the Northerly Qzladrant (295"-025") The sea-breeze is difficult to forecast when the wind is from this quadrant,

i.e. directly opposing the sea-breeze onset. It must be noted here that the sea-breeze generating forces at Hum are weaker than they are further east. Wallington (1959) shows from his sea-breeze frontal positions that it is only just possible to detect the main frontal lines as far west as Hurn and that the rate of advance inland is relatively slow. Even so Fig. za shows that on good sea-breeze days i.e. less than half cover of cloud during the forenoon, then a definite upper limit can be set on the morning wind speed which will allow a sea-breeze. For example northerly winds of 8 k t will find the sea- breeze arriving at Hum before midday, and when 10 kt by 1400. Even a 13-kt wind can be reversed as late as 1600-1700. However it is difficult to make any other definite statements about the time of onset a t Hum with winds in this quadrant.

2 . The North-easterly Octant (025°-0700) This is a much better behaved direction for sea-breeze onset there being

definite areas on the V-t diagram in which the sea-breeze cannot occur or will have already occurred (Fig. zb).

3. The Easterly Octant (o7oo-rr5") While easterlies which transform into sea-breezes are rarer than north-

easterlies a t Hum, they are equally well-behaved and enable one to say when a sea-breeze will not occur due to wind speed (Fig. Z C ) .

4. The South-easterly Octant (1rg"-160") With an initial direction so close to the normal sea-breeze direction it is

not surprising to find an almost universal mode of steady swing through to beyond 160" at an average rate of swing of

S =64/ V deg per hr The natural tendency for a sea-breeze to veer with the day tends to keep the swing going for most of the high-sun hours. It appears that it is never com- pleted at Hum before 110+1200.

j. The Westerly Quadrant (205"-295") Without recourse to all the weather maps for the 128 days with morning

westerlies it is difficult to say which swings towards south and south-east were due to normal backing ahead of encroaching lows and how many to sea-breeze- or both. However, when trying to provide information for sailing events it is the general mode that is important as a guide no matter what the cause may be. Thus while a mere 10 of the 128 days could be positively identified as sea-breeze days with the wind swinging suddenly into the southern quadrant (Fig. zd), 51 days showed the steady swing from west towards south at an average rate of

S=roo/V deg per hr

It is obviously impossible to say that a wind with a given direction and speed at 1000 hr will become a wind with another direction and speed by any particular time. Yet to a Committee boat trying to orientate a starting line the knowledge that the wind is likely to back some 10 deg in the hour between the decision to lay the line and the starting gun is important.

Equally the helmsman who is expecting the wind to back fairly steadily during the race will err on that side of the mean course to the windward mark towards which the wind is expected to shift. If beating into a backing south- westerly he will make his tacks have a bias towards the port-hand side of the direct course to the windward mark, assuming the latter to lie in the eye of the mean wind. This is because if the wind backs it Will make him more and more free to lay off for the mark on a fast port-tack close-reach towards the end of the beat.

The classic Olympic course is a triangular one with an initial beat to wind- ward of at least I) n m i which sorts out the competitors and ensures that they do not all arrive at once at the windward mark (Fig. 3). There is then a broad reach on starboard tack, a gybe at the wing mark followed by another broad reach on port tack to the leeward mark near the starting line. There is then another beat, a dead run and a final beat to the finishing line which the Committee boat has been laying in the interim.

The basic course cannot be altered once it is laid and the fleet has started and so the bias of the wind shifts can be highly important if the classic attri- butes are not to be swept aside by the vagaries of a shifting wind.

Unfortunately, as has already been pointed out, an analysis of the meso- scale changes attendant on the establishment of a sea-breeze regime at Hurn is not necessarily applicable to Poole Bay. We know already that all winds less than 12-15 kt may show some divergences and it is one of the objects of the R.Y.A.'s investigation to find at least some criteria by which a closer approximation to the wind field over Poole Bay can be achieved.

Some progress was made during the recent summer when an Olympic training regatta was imminent at Poole. John Bonham-Christie, Rear Commo- dore of the Poole Bay Olympic Sailing Association, volunteered to patrol the area of the courses in his motor yacht Diligmce during the forenoons of likely

W y , i w l n d Moan

diroctlm

et a 2nd Rwnd

I Ino

Fig. 3. Three different rounds of an Olympic-style course

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Fig. 4. Caletilla and Caleta beaches are typical of the modern and developing environs of The hotel in the middle foreground, behind the island, is now completed

The rough buildings in the right foreground are now the The direction of view is as shown in Fig. ~b

sea-breeze days with offshore winds. His notes on the wind speeds and direct- ions, while too few to make a basis for forecasting, confirmed earlier ideas about the general behaviour of the sea-breeze convergence zone; i.e. that a calm area exists over the coastal waters within a couple of miles of the coast and this area moves slowly out to sea as the sea-breeze front moves more rapidly in over the beaches and their hinterland. It is the timing of the move- ment of the seaward limits of this calm area which is a major stumbling block to adequate forecasting for the events. Obviously the courses could not be in a worse position on sea breeze days when components of the morning wind are off the land, because if the latter is Force 3 or less it is destined to become fitful and possibly zero. The question is whether the sea breeze will have picked up across the starting lines before the scheduled starting time, and if it just has done so how long may it be before it has crept out the 14 miles to the windward (sic) mark?

Such considerations will be equally valid at Acapulco, where the Olympics proper are due to be held between 13 and 24 October this year.

.4capulco. and stands 10 storeys high. site of the Hotel Bocachica.

THE WINDS OF ACAPULCO Both Acapulco and Poole have a land-locked harbour (Fig. 4 and 5 )

Acapulco is indeed the but Poole’s is shallow and Acapulco’s relatively deep.

Fig. 5. The horseshoe-shaped cloud edge following the curve of Poole and Bournemouth Taken from the Royal Motor Yacht Club jetty at Sandbanks beaches 31 May 1967.

looking towards Bournemouth (Fig. IU.) at 1200 BST

best deep-water anchorage on the Mexican west coast and for centuries has been a stopping-off place for both piracy and more legitimate trade. It has, like Poole, a long history, but while Poole may have changed greatly in the past 20 years it has not had to absorb anything like Acapulco’s rise to fame as a tourist attraction. Reading the U.S. Navy’s sailing instructions of 1947, Acapulco was then ‘a town of unattractive appearance with narrow paved streets and houses of adobe construction with tiled roofs overlooking Santa Lucia Bay.’ In 1947 it had a population of 25,000 Indians and others of negroid stock and a 10,000 a year influx of tourists. Twenty years later there are over 300 registered hotels accommodating some 80-90,ooo visitors a t any one time. Over the year more than half a million people come to sample the uneasy balance between sophistication and the old-world Mexico to which pockets of the environs of Acapulco still manage to cling.

Meteorologically Poole has a sea-breeze regime that tends to be weak while Acapulco has one that is strong. Proof that the sea breeze is a pretty certain wind at Acapulco is found in the ventilation cut (Fig. ~ b ) which has been made to cool down the old town whose average relative humidity hovers between 80 and 90 per cent. Yet a review of the winds off the coast where the courses are to be laid shows some interesting variations from an entirely sea-breeze-dominated regime.

Since Poole is on the 5rst parallel and Acapulco is on the 15th, the geo- strophic control in the latter case is about 1/3 of what it is at Poole and winds tend to blow more directly from high to low pressure. There were, however, some distinctive basic morning winds off Acapulco in October 1966 when the

I8

Fig. 6 . The general situation and climatology of Acapulco

Mexican hydrographic vessel A rcturus made hourly records while anchored in the area of the courses.

Analysis of these records-which are the only extensive and reliable ones which have come to hand at the time of writing-allow their division into the following classes of 'morning wind' (i.e. the mean wind direction around 1000 local time). I. Easterly mornings. Easterly winds are usually due to storms at sea when they reach 12 or more knots, and to highs over the Bay of Campeche when they are light. If a hurricane should track close to Acapulco in October 1968 then it will engender strong easterlies. There were 14 occurrences of easterlies in October 1966, and on five of these occasions the winds could be considered to be strong. On all but one of the occasions of 10 kt or less a t 1000 the wind veered through south to west during the day, whereas winds which were 12 kt or more at 1000 remained easterly all day. There was the usual tendency to succumb to the sea-breeze forces later when the morning wind was stronger. Thus winds less than 2-3 knots c" t 1000 veered SW (true sea-breeze direction) before 1200, while winds of 3-4 kt veered SE between 1100 and 1200. Winds of 5-6 kt at 1000 remained easterly until after 1400 but all winds of less than 10 kt at 1500 were westerly or calm by 1800.

Thus it seems that the limit of easterly component of wind speed which the sea-breeze forces cannot overcome is between 10 and 12 kt. 2. South-westerly mornings. These are winds from the general sea-breeze direction and they all veered during the day when they occurred as morning winds. However, an interesting feature was that whatever wind direction had been achieved by 1100 was maintained until 1400, but winds of less than 5 kt at 1500 had veered again by 1600. With winds greater than 5 kt the

Strong easterlies appear to remain easterly all day.

direction at 1500 was maintained until between 1600 and 1700, when a veer again occurred, Thus the mode of the sea breeze to veer with the day is maintained with these winds. 3. Westerly mornings. Westerly winds are often due to lows off the Tampico- Vera Cruz coast (Gulf of Mexico) and are aided by the sea breeze. When they are weak they may be due to highs at sea. There were 10 westerly mornings when the winds maintained their moo-hr direction until after 1300, but all winds less than 9-11 kt between 1400 and 1500 veered to the NW. There was always the tendency to veer, and stronger winds which decreased during the day veered NW in the late afternoon. 4. North-westerly mornings. When the north-westerly is strong then it is due to Northers or Tehuantepecers which are storms tracking through the pass of the Isthmus of Tehuantepec (Fig. 6). The sea-breeze effect is felt on north- westerlies of less than 10 kt in that they back to west with the day. If they are just a couple of knots then the sea-breeze effect is sudden and the wind swings to SW, but not before 1200 in the cases studied.

Winds in the SE and NE octants were largely non-existent in October 1966 and study of the Venezuelan weather maps-the only ones as yet available for the area-suggest no definite pressure pattern which would account for their absence. Of course winds from SE occurred, but they were during the veering phases of morning easterlies.

The above analysis is open to criticism, but one had to clutch at every straw of information when attempting some attack on the problem of the wind regime of a place so poorly served with meteorological data as Acapulco. One relatively certain thing that can be done is to draw a rough topographic map and see what stable surface-hugging winds would do if forced to flow across it following streamlines. The obvious directions to choose are those of the major morning winds plus a general wind off the land.

Strong stabilization would be expected to occur near the surface in warm offshore winds, for while the water is relatively warm along this coast the tide is inappreciable and so warm water from the beaches cannot be swept out to sea on the ebb as happens for example along the south and southeast coasts of England. Thereiore the diurnal variation in land temperature is not at all reflected in the coastal water temperature as is the case, say, in Chichester Harbour (Watts 1955). Thus in general the sea will be cooler than the land during the day.

Sea temperature readings in the area of the courses taken every 2 hr during the 8 days (8-15 October) of the 1965 Acapulco regatta gave a mean of 30-5f o.5OC with one occasion of 32°C which fell during the day to 31°C. A current of about 0.7ho.1 kt from between W and NW was the constant companion of the 8 days. From the wind point of view there were seven westerly and one SSW'ly mornings and so this was a more stable period than any encountered in the whole of October 1966. However, all these westerlies backed into the SW or SSW by the late forenoon or early afternoon, while the SSW morning wind veered to west by 1200 hr and stayed there. Obviously the major

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'2:; , . . . . . . . . . . . . . . . . . . . . . I-:.': b\ . . . . . . . . . . &&L . . . . . . . . , . . . I . . ' . ' . ' . ' . ' . . . ..

KEY A Star and Fiying Outmmn ckass B Finn class

c 55Mutra and Dragon clau

Land abovo 3Ook

Fig. 7. Likely streamlines for stable surface winds from four significant directions at Acapulco. 7a. Sea-breeze direction (SW). 7b. Wind off the land. 7c. Wind from N W (west is very similar). 7d. Wind from the east

21

influence is that of the sea breeze and Fig. 7 shows the possible streamlines which might be expected to follow the curves shown. The other three figures are similar constructions for wind off the land, north-westerlies and easterlies.

The general incidence of light winds with a superimposed sea-breeze regime in 1965, 1966 and also during the recent 1967 regatta, when Britain sent a team, might appear to auger well for the coning October. There is, however, one ever-present threat to the tranquility of Acapulco. The numbers of tropical cyclones moving north off the west coast of Mexico in the years - Total numkr - Number rnovlng inland

Fig. 8. Numbers of tropical cyclones off west coast of Mexico in the years 1947-61

1947-61 are plotted in Fig. 8 together with the number moving inland. The total number reaches its peak in September-the traditional hurricane month- but the number which move in over the Mexican coast comes to maximum in October itself. Most of these go ashore to the north of Acapulco but Hurri- cane Tara went ashore between Acapulco and Zihuantanejo (some 140 miles up the coast) on 12 November 1961 with the loss of 500 lives. The October hurricane tracks for 1947-61 (Rosendal1963) show that Acapulco is most likely to remain free from any such disaster as that mentioned above, but the circu- lation of a hurricane tracking past the State of Guerrero could conceivably blow out the yachting events for a couple of days should it unfortunately coincide with the period 13-24 October.

These Mexican west-coast humcanes tend to be less severe than those of the Gulf of Mexico, but they can achieve 100-125 kt winds and while the range of hurricane force winds (64+ kt) is relatively short it does not need anything like that speed to whip up the sea off Acapulco and make it impossible to race.

Thus while the forecast for Acapulco this October is of winds less than Force 4 which are always being modified by sea-breeze influences, there re- mains the possibility of a strong blow which will require the use of some of the spare race days which have been set aside for that eventuality.

REFERENCES ROSENDAL, H.E. 1963 Mexican west coast tropical cyclones, 1947-61. Weather-

wise WALLINGTON, C. E. The structure of sea-breeze fronts as revealed by gliding

flights. WATTS, A. J. 1955 The sea-breeze at Thorney Island. Met. Mag., 84,

1959 Weather, 14 (9). pp. 263-70

PP. 42-8

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See Page 39 CONVECTIVE CLOUD AT T H E INTERTROPICAL CONVERGENCE ZONE

Photographs by J . Tunstall

Photograph by A. F. Bunker

Fig. 4 of 'Problems of a monsoon ocean' (see page 28). Rough sea and scattered low Taken from the Woods Hole Oceanographic clouds off Somalia on 30 August 1964.

Institution research aircraft flying at 4 .7 km

b

AN EFFECT O F POLARIZED SKY LIGHT a. vertically polarized component of sky light: b. glass; c. ice feather; d. pool of melt water; e . wooden window frame; j. window sill (see page 39)