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Tropical Land and Sea Breezes* (With Special Reference to the East Indies)

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Tropical Land and Sea Breezes* (With Special Reference to the East Indies) Tropical Land and Sea Breezes* (With Special Reference to the East Indies) GEORGE H. T. KIMBLE t AND COLLABORATORS HROUGHOUT THE COASTAL WATERS of Nor unfortunately has it proved possible to T the S.E. Asia and S.W. Pacific Com- produce a "formula" for forecasting land mands, departures from the general and sea breezes. Where rules of thumb are monsoon current are more prominent than given in the text, they are only to be re- the monsoons themselves. This is a conse- garded as first approximations of purely quence of two things:— local validity, since no two coasts react in exactly the same way, or to the same de- 1. The ill-defined pressure-field, especially gree, to land and sea-breeze stimuli. It within 10° of the equator (and consequent follows then that, whenever possible, guid- feebleness of the general air circulation). ance should be sought (either from writ- 2. The strong development of local winds ten sources or local inhabitants) before owing their existence to the intense insola- forecasts of coastal wind conditions are tion and mountainous character of the issued. country. MECHANISM OF THE SEA-BREEZE In normal times these departures are of lit- tle more than academic interest except to From an operational point of view the the natives who, throughout the Netherlands sea-breeze is more important than the land- East Indies, are accustomed to regulate breeze, both in respect of its strength and their fishing activities by the diurnal rhythm sphere of influence. In broad outline, the of land and sea breezes,1 and in whose af- motivation of the sea-breeze is simple. It fairs local mountain winds play a sufficiently forms in quiet atmospheric conditions when disturbing role to acquire special names air over the land heats up rapidly and ex- (e.g., Broeboe, Wam~brau, Bohorolc). In pands at an appreciably faster rate than the present emergency, however, the signifi- the air over the adjacent water; the pres- cance of these departures can hardly be over- sure difference produced aloft in this way emphasized, for the initial success of am- induces, first, a very slight flow of air from phibious operations hinges very largely on land to sea; next, this leads to a reduction the meteorological officer's ability to fore- in surface pressure over the land which, in cast the beach conditions (sea and swell) its turn, causes a compensatory air flow from which are likely to arise from a given com- sea to land at the surface. This suggests bination of seasonal (= monsoon) and di- the formation of a circulatory flow of air, urnal (= land and sea-breeze) wind factors. but in middle latitudes (e.g., British Isles) for one reason or another (e.g., strong gra- It is primarily with a view to helping me- dients, weak insolation) such a circulation teorological officers appraise and forecast would appear to be exceptional. In low lati- local variations of the diurnal wind factor tudes, however, it is apparently quite com- in the above areas that this memorandum mon ; thus pilot balloons in Netherlands East has been prepared. Owing to our ignorance Indies are frequently driven out to sea on of the precise structure of land and sea- the upper return current only to be blown breezes it has not proved possible to deal back to the coast at sea-level by the sea- with the subject on a rigidly physical basis. breeze. This has happened to three pilot * Originally prepared as a British Naval Meteorologi- balloon ascents (1200, 1330, 1600) in one cal Branch Memorandum (126/44), under supervision of the Director, Naval Meteorological Service, Ad- day before now—a phenomenon which pre- miralty, who has kindly given permission to publish supposes, incidentally, that the down-current it here.—Ed. t Commdr. Kimble is now Chairman, Dept. of was greater than the free lift of the balloon. Geography, McGill University, Montreal, Canada. 1 Off N. coast of Java their sailing vessels start out What is by no means so simple to under- at night on the land-breeze and return about noon with the sea-breeze. stand is how the sea-breeze builds outwards Unauthenticated | Downloaded 09/23/21 10:48 AM UTC horizontally. The initial flow at the surface stantially, because the flow of heat upwards is, of course, mechanical—heavy air taking in the ground below the surface is too slow the place of the rising lighter air: in the to replace that which is lost by radiation. N.E.I, these up-currents are of the order Consequently, whereas the sea surface sel- of 150 feet/min. The subsequent flow would dom cools more than y2° to 1°F during the appear to be partly a dynamical consequence course of a tropical night, the land surface, of this inflow and partly a result of the under ideal radiation conditions,3 can cool fact that as the air over the sea descends as much as 15° to 20°F. Although such from aloft (which it does, vide pibals at conditions do not obtain in the Far East, Batavia, at approximately one-third the thp cooling is quite likely to be carried to speed of the up-current) it warms up adi- the point where it brings the air temperature abatically and so becomes warmer, i.e., less farther below the sea temperature by night dense, level for level than the air further out than it rises above it by day:—i.e., even in to sea. This (cooler) air moves in under the wet tropics the nocturnal (= reversed) the adiabatically warmed air and so travels sea-air temperature gradient will usually be landwards into the ambit of the circulatory steeper than the day-time gradient.4 Where flow. Thus aided and abetted, the sea- this density condition occurs in a favorable breeze gradually extends seawards—and topographic setting,5 a gravitational offshore continues to do so as long as the air over breeze is easily set in motion. As the sharp- the land is warmer, and so lighter, than the est gradients will be found over the sea, be- sea surface air. Agreeing with this view tween heavy cooled air which has drifted off of the growth mechanism are the two facts: the land and that which has been continu- first, that the time of onset of the sea-breeze ously over the sea, the '1 active'' region (i.e., becomes progressively later as distance from where main density difference is concen- the coast increases,2 and second, that out to trated) is entirely over the sea and the maxi- sea the breeze often sets in as a sudden gust, mum development of the land-breeze should, reminiscent of a miniature cold front. On theoretically at least, be located there rather this hypothesis, therefore, there are two gen- than over the land. The landward extension erating, or " active,'' regions in the sea- of the breeze is, on this view, merely the breeze circulation—the main one lying over dynamical consequence of the activity over that strip of land near the coast where tem- the sea. As there is only little differential perature is rising by direct heating (and so cooling over the land itself, the pressure producing a thermal pressure gradient), and difference required to move air across the a subsidiary one over the sea where subsi- land must be provided by the thermal effect dence is working in a small way to bring of subsidence. In these circumstances the about a somewhat similar state of affairs. air-flow may be unobservable (owing to sur- face friction) at anemometer levels, when MECHANISM OF THE LAND-BREEZE there is quite a definite drift in the free air above.6 The cause of the land-breeze is the differ- ence between the effect of nocturnal cooling A CHARACTERISTIC LAND-AND-SEA BREEZE (by radiation) over land and sea. Whereas the sea cools down only very slowly (as soon The most thoroughly analyzed land-and- as the surface layer cools, it sinks and mixes sea breeze in the whole area is the one at with warmer water beneath, thus offsetting 3 Viz. practically cloudless skies, a dry atmosphere, a dry terrain and long nights. the surface fall of temperature), the tem- 4 Throughout the N.E.I, the mean difference between perature of the land falls quickly and sub- the air temperature over the sea and over the coast is between 3°-4°F, the air temperature over the land be- ing the lower of the two; that is, there as a natural 2 Thus, at Edam, approximately SO m. off the N. predisposition for land-breezes, rather than sea-breezes, Java coast, it sets in about 60 mins. later than at the to form. coast itself. In the Celebes Sea, 140 m. from the 5 For example, where the coast is backed by a valley nearest land, a notable wind deflection^ (due, we may- leading from high ground, for this tends to encourage, suppose, to the outgrowth of a breeze originating on the even as it canalizes, the gravity flow of cool air. N. coast of Celebes) has been observed to occur at ® For a fuller discussion of land-and-sea breeze mech- 1600, i.e., some 6 hours after it might be expected to anism, see Synoptic Division Technical Memorandum, have begun at the coast. (See also p. 102, fl 8.) Met. Office (London), No. 58. Unauthenticated | Downloaded 09/23/21 10:48 AM UTC Batavia. We can get an idea of its struc- an almost imperceptible drift of under 2 ture and main features from the accompany- knots) until midnight.
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