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. The reason for this ing (see FIGURE 1) time-height graph of is that the air at cloud-base levels heats up diurnal wind variations at Batavia. The more slowly than the air at the surface following points are worthy of note. (from which it derives its warmth by the slow process of upward diffusion of heat 1. The sea-breeze first makes its appearance and moisture) and continues to rise for some (between 0900 and 1000), not at the ground- time after the temperature of the air at the level, where it is retarded by surface fric- surface has fallen sufficiently to reverse the tion, but at an altitude of about 300 ft. day-time sea-air temperature gradient. Ow- g. Thanks to the same surface friction the ing to the stabilization of the lapse rate most intense development of the breeze is brought about in this way, the surface cool- located about 600 feet up. ing is not readily communicated aloft, with 3. At first the vertical development of the the result that a gradient for a sea-breeze sea-breeze is rapid; later, after it has at that level is maintained till a compara- reached a height of approximately 3,000 tively late hour. feet, it is much more slow. 5. The complementary offshore counter- 4. Aloft the breeze persists long after it has current aloft is twice as deep—more than died away at the surface; indeed, whereas 6,000 feet thick—as the sea-breeze, only it is in force at the surface from approxi- about half as strong, and reaches its maxi- mately 1100 to 1800, at 4,000 feet (near mum force at 1700 at an elevation of 6,500 the top of the sea-breeze) it only com- feet. mences at 2000 and lasts (though only as 6. The over-all diurnal perturbation of the

FIGURE 1

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC FIGURE 2. Note.—It will be noticed that only in February and November does the sea breeze (represented by the mean daily maximum wind velocity recorded at Semarang) ex- ceed the force of wind over the open sea.

atmosphere extends up to at least 11,000 ity is recorded during the hours of daylight feet.7 (characteristic of land stations), and not 7. The land-breeze is very shallow (gener- during the night-time (when truly oceanic lo- ally not more than 600 feet thick), very calities record their wind maximum). weak (not often above 2 knots) and spas- It would, however, be fallacious to sup- modic (most noticeable round about 2400 pose that the Batavia "model" is standard and 0800). Weak as it is, however, from throughout the N.E.I. On the contrary, 0730 to 1000 it is overlain by an onshore land-and-sea breezes represent such a deli- counter-current. It lasts considerably longer cate balance between atmospheric and geo- 8 than the sea-breeze. graphical conditions (which vary widely 8. An important point (not covered by the from island to island, and even within a graph) is that the horizontal field of the given island) that no uniformity, either of sea-breeze varies widely, but is known to size, duration or frequency can be looked influence the diurnal wind rhythm of islands for. Almost all that can be sai*d about the at least 50 miles north of Batavia, and of area as a whole is, first, that conditions do sea, areas even more remote. Thus at Dis- not promote very strong sea-breezes:—these covery Eastbank Lighthouse, which is ap- require steep land-sea temperature gradients proximately 100 miles from the nearest part which are not feasible in the wet tropics of Java and is on a reef so small as to be in- where a large part of the incoming insolation capable of producing convectional effects, it must always be used to dry out the ground, is significant that the maximum wind veloc- instead of warming up the air; and, second,

7 A sea-breeze circulation of somewhat similar dimen- that the sea-breeze (whether unaided or in sions is commonly observed along the British E. Africa association with the regional wind) seldom coast. 8 The reason for this lies in the fact already noted, exceeds the force of the wind over the sur- viz. that the air temperature over the land is, on the 0 average, lower than the air temperature over the sea, rounding seas. This is well brought out and that, because of this, any sea-breeze which forms in the graphs (see FIGURE 2). is bound to last less than 12 hours. In actual fact a sea-breeze seldom lasts more than 9 hours (at Batavia from 1030 to 1810 on the average), while a land- 9 Applying this rule to the requirements of an assault breeze of sorts may be in evidence for 14 hours or so. landing on open beaches, the meteorological officer will

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC The result is that whereas off sub-tropical of air across them to prevent a density con- desert coasts (where ideal breeze-forming trast from building up,10 and also because conditions frequently obtain) the sea-breeze their annular shape means that the breezes component to the prevailing wind is often set up on the opposite sides of the atoll are as much as 20 to 25 knots—up to 35 knots likely to cancel one another out. Except on rare occasions—in the equatorial low- when the topography is unusually hilly (see lands of S.E. Asia it is exceptional to find below) and the prevailing wind so light that a sea-breeze contributing as much as a the heated air is able to accumulate in situ 10-12 knot component to the prevailing (i.e., is not wafted out to sea as soon as it wind. Exactly how large this component is begins to rise), land-and-sea breezes are in- depends upon a number of variables—geo- appreciable over islands less than approxi- graphical, seasonal and day-to-day. We mately 10 miles across.11 will consider the geographical factor first. It is probably true to say that there is no upper limit to the size of a land-mass ca- GEOGRAPHICAL VARIATIONS pable of producing a sea-breeze, although general considerations would lead one to 1. Size of Land Mass.—Theoretically suppose that when the land-mass is large there is no lower limit to the size of an enough—and topographically equipped—to island capable of producing land-and-sea retain day and night a heat surplus over breezes:—the veriest atoll could invoke the the surrounding sea, the sea-breeze would requisite circulation, provided the atmos- become transformed into a monsoon. The- phere was at rest, and a sufficiently large oretically this could take place when and land-sea temperature gradient could be sus- where the air brought in by the sea-breeze tained. In point of fact few atolls do pro- was unable to penetrate to the heart of the duce their own local circulations, because land area during the course of a single day, it requires only a very slight general drift

10 At Addu Atoll (Indian Ocean) sea-breeze^ are only not be far wrong in assuming that, in this area of light felt during the intermonsoonal months when the winds or moderate winds, the wind on any given day over are light and variable. the open sea (say 50-100 miles from land) is seldom 11 Ponape (Carolines) which is on the borderline for likely to be exceeded on the nearest coast, lee or size (area: 145 sq. miles), has sea-breezes during the windward, even during the epoch of the sea-breeze. At months July-October only; the prevailing Easterly other times of the day and night the wind inshore is Trade wind is then weaker (less than 3 knots at likely to be less than in the offing, and aircraft carriers 0600) than at other times of the year. As soon as in search of wind are more likely to find it by steaming the 0600 wind speed rises above 3 knots, the diurnal seawards than landwards. variation disappears. (See FIGURE 3.)

FIGURE 3

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC and so allowed a core of undisturbed heated mountain range. Here the breeze sets in air to build up, sufficient to cause the mean about 2000 and lasts until 0900 and is suf- daily temperature to rise above that of the ficiently strong (force 2 or more) to be ob- surounding sea. In this way a surplus of servable 25 miles off the coast. Both land- heat would accumulate, i.e., an area of rela- and-sea breezes tend to be guided by con- tively low pressure would form. This would tours so that the flow of air is canalized encourage the formation of a permanent along the valleys running more or less in the sea-breeze effect, or at least of a perma- general direction of the wind. The greater nent onshore component to the regional the stability of the air, the more pronounced wind. is this canalizing effect:—for this reason Nature of terrain. land-breezes which form in conditions of (a) Topography.—This is imporant, temperature inversion, i.e., extreme stability, both in relation to the strength and direc- follow the contours more markedly than tion of local breezes. Since the air is heated sea-breezes. and cooled most where it is near the ground, (&) Vegetation.—The character of the the air over a heated slope is normally vegetative cover becomes important in areas warmer (over a cooled slope, normally where forces working for and against sea- cooler) than air at the same level over breezes are well-balanced. Since bare rock lower-lying ground. Such differences do heats up more quickly than any other sur- not permit of equilibrium and convectional face, it follows that dry( i.e., desert or semi- circulations are set up giving an upward desert) coasts and vegetated coasts in the (= anabatic) component over a heated dry season will, ceteris paribus, develop slope, and downward (= katabatic) over a stronger sea-breezes than those well-supplied cooled slope. This circulation may be ex- with vegetation, for in this latter case much plained by assuming that the horizontal air of the incoming insolation will be absorbed movement towards the mountain arises from in drying out the air instead of in increas- the fact that the relatively light warm air ing its temperature. In this connection it that rests on the mountain slope is squeezed is not without interest that one of the out and upwards along the mountain side strongest and steadiest sea-breezes in the by the air of the free atmosphere acting as N.E.I, occurs in the Paloe Valley of West- a denser liquid. From this it has been de- ern Celebes which has the lowest rainfall duced—and there is no dearth of support- and the least luxuriant vegetation of any ing evidence—that land-and-sea breezes are place in the Archipelago. While the likely to be stronger and steadier where the strength of this particular breeze no doubt coast is backed by gently-rising valleys owes something to the configuration of the and hills, especially when they have a valley (which is deep-sided and hence favor- sunny aspect,12 than by flat ground. Thus able to up-valley air flow in the day-time) sea-breezes are much better developed along it is primarily connected with the fact that, the hilly west coast of Sumatra than along being treeless, and so fairly dry, the valley the low-lying east coast southeast of Palem- slopes heat up rapidly and thus promote con- bang. They are also more than twice as vection more readily than the neighboring forested lands. steady at Padang (hilly hinterland) as at Singkel (low-lying)—both on the west coast. 3. Proximity of Other Seat Sources.— Similarly, the strongest land-breezes experi- Frequently in the N.E.I, land- and sea-breeze enced along the north coast of Java are effects are only weakly developed when from general consideration (e.g., absence of found in the vicinity of Cheribon, Tegal and strong monsoons, presence of hilly terrain) Pemalang which are backed by the Slamat they might be expected to flourish. In such cases it will often be found that the heat 12 There is little or no data about the effect of aspect on the strength and timing of the sea-breeze. From sources provided by other, neighboring is- first principles, however, it is reasonable to assume that lands are responsible. Thus in the northern the breezes will be stronger, and start up earlier (other things being equal) along slopes facing towards the sun half of the Straits of Malacca, the land- than on those facing away from it.

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC and sea-breeze effects are quite strong:—but depending on the differential pulling in the southern half, where the Straits are strength of the two source regions. Thus much narrower (less than 40 miles), they when the Snellius Expedition was observing are negligible. The reason, apparently, is in the Celebes Sea, approximately 140 miles twofold:—the proximity of Malaya and Su- from the E-W trending coast at Celebes and matra prevents strong sea-air temperature approximately 200 miles from the N-S trend- gradients from arising, while such gradients ing coast of Borneo, the following diurnal as do exist do not give rise to sea-breezes wind variation was reported: —

Wind 0800 1000 1200 1400 1600 1800 2000 2200 2400 0200 0400 0600

1.* 210° 218° 203° 206° 214° 219° 172° 192° 195° 203° 228° 207° 2.* 16 15 13 13 11 11 12 124 14 13 14 144 3.* WSW W'S ESE NE N NW'N E E'S SE'E SE'E NW'W SSW

4.* 5 3 1 1 3 3 7i 3 3 1 5 11

* 1. Observed Wind Direction. 2. Observed Wind Velocity (knots). 3. Vector Difference between actual and computed Monsoon direction (i.e. land or sea breeze component). 4. Velocity of land or sea breeze component (knots). of appropriate strength, owing to the oppos- The most conspicuous features of this di- ing pull of the heat sources (see FIGURE 4). urnal variation of the land-sea breeze com- ponent are the north component, i.e., onshore with regard to Celebes, occurring at 1600, and the much stronger east component, i.e., onshore with regard to Borneo, occurring at 2000. If we are correct in arguing as above, that sea breezes are inaugurated on the coast and work steadily seawards, then FIGURE 4 these changes of wind component can be When two heat sources are close enough explained as due to the difference in the together for the two sea-breeze " spheres'' distance of the ship from the two coasts, to overlap, and one of the sources is much i.e., from the two heat sources, one of which larger than the other, it is likely that the (Borneo), by virtue of its greater size, sea-breeze produced by the smaller source exercises a much stronger pull than the will be " rubbed out" by the breeze pro- other, as exemplified in the greater strength duced by the larger region. This regularly of the east wind component (7% knots) happens in the Solent off Cowes where the as against 3 knots for the north wind sea-breeze is offshore with relation to the component.13 Isle of Wight. In the tropics it is probably A similar effect on a smaller scale is pro- safe to say that small islands lying within duced by the islands of Billiton and Borneo 20 miles of a major land area will not nor- on the winds of the Discovery Eastbank mally have land- and sea-breeze systems of Lighthouse during the inter-monsoonal their own, and that small islands within 50 month of April when a southerly wind at miles of such areas are only likely to have 0600 backs to SSE by 1200 (because it then very feeble systems. feels the "pull" of Billiton 50 miles to Where the heat sources are so arranged, NW). By 1800 it has veered to SSW (be- as for instance when coasts are at right cause the Borneo sea breeze has, by that angles to each other, that they do not pull the air in diametrically opposed direc- 13 It is noteworthy, however, that in spite of its greater strength, the Borneo sea breeze works seawards tions, a sea-breeze of variable strength and at a rate no faster than the Celebes breeze, viz. 200 miles in approximately 7 hours, as against 140 miles direction is often produced, the variations in approximately 5 hours.

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC hour, worked seawards far enough to make tween the two is gusty and is often dan- itself felt at Eastbank). gerous to the inattentive seaman. 4. Prevailing Wind.—Where pressure Other ways in which a strong offshore gradients are tight, turbulence normally monsoon can affect the sea-breeze are:— prevents strong thermal pressure differences 1. By causing it to make a belated appear- from arising. Where the pressure gradient ance, for the sea-breeze has first to neutralize is for onshore winds, the horizontal tem- the gradient wind force:—it is only when perature gradient (sea-land) becomes flat- the monsoon is no longer able to match the tened out, the pressure difference due to thermal gradient that the warmer, lighter surface heating (although of the same mag- offshore wind is rolled back by the sea-breeze nitude probably) is spread over a wider "front."14 zone and accelerations due thereto are conse- 2. By causing the onset of the sea-breeze to quently small. At the same time the added be more sudden than usual. The longer the loss of momentum of inflow due to friction onset is delayed the greater does the sea-air (dependent on the square of the velocity) temperature contrast become:—this is be- is much greater when a wind, say, of 10 lieved to make for a more violent inflow than knots increases to 15 knots, than when a under normal conditions. 5-knot wind increases to 10 knots. Where 3. By reducing the inland penetration of the pressure gradient is for offshore winds, the sea-breeze. Thus along the N. Java the stronger the wind the smaller will be coast during the E'ly monsoon, the regional the horizontal temperature difference and wind is occasionally too strong to allow the the greater the turbulence tending to sweep sea-breeze to penetrate inland as far as Ba- the air outwards with the upper current, tavia (5 miles from coast), but not quite and also the greater the retardation re- strong enough to prevent it from being felt quired to reverse the gradient wind motion. at the coast itself. Clearly, then, there will be some maximum 4. By diminishing the maximum speed of speed beyond which a sea-breeze cannot as- the sea-breeze component, both over the land sert itself and since, as we have seen, an and over the sea:—this follows from 1 and increasing wind increases the difficulty of 3 above. producing the sea-breeze (in at least three 5. By causing a greater irregularity in sea- different ways), the demarcation between breeze occurrence. Irregularities are par- speeds permitting and prohibiting a diurnal ticularly liable to occur round about the wind-reversal is likely to be a sharp one. epoch of change-over from sea- to land- As a general rule for the tropics, it can breezes, and vice versa. be stated that where there is an offshore With onshore winds sea-breeze effects wind of more than 15 knots, diurnal wind (e.g., deflection and marked increase in reversal will only take place if there is an speed during the forenoon) will often be exceptionally favorable conjunction of all observed at speeds above 15 knots, but the other contributory circumstances. Just how additive effect becomes relatively small if delicate this equipoise can be is well demon- the undisturbed (i.e., regional) wind has a strated during the months of June-August speed greater than the pure sea-breeze along the S.W. coast of Celebes (Makassar effect. Strait). Here the Broe~boe (=E'ly Mon- As there are no coastal parts of the soon slightly accelerated by its passage N.E.I, and adjacent areas where the mean through the mountain back-bone) frequently wind exceeds these critical velocities, it fol- dominates the situation inshore, but is su- lows that sea-breeze effects should seldom perseded by onshore wind 4-5 miles out to be ruled out on general grounds. sea, where apparently the sea-breeze (i.e., thermal) component is finally able to over- 14 During the height of the E'ly (actually SE'ly, i.e., offshore) monsoon, the sea-breeze at Batavia sets come the resistance offered by the monsoon in nearly 2 hours later than in March when the mon- (i.e., pressure gradient). The "front" be- soon is onshore, viz. 1130 instead of 0930. Upon occasions it fails to appear before 1330.

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC Owing to the fact that the unaided land- during the course of 12 months a given breeze is always weaker than the sea- stretch of coast will experience a range of breeze—seldom being more than force 2— wind velocities and directions comparable clearly it does not require very much of an to those experienced instantaneously onshore monsoon to nullify its effect alto- throughout the whole area. These seasonal gether. By and large, the land-breeze ceases variations are well brought out in the fol- to be perceptible when the onshore monsoon lowing table which, for want of space, may is force 3 or more. When it is weaker than be taken as representative of the Tcind this, a certain amount of unsteadiness in- (though not necessarily of the degree) of shore may be observed, the land-breeze seasonal wind change wrhich occurs through- being in force for part of the night and the out the E. Indies between the latitudes of monsoon for the rest. Thus at Balikpapan 10°N and 10°S. (E. coast of Borneo), the thermograph 2. Variations in Opportunity for Surface often records a marked lowering of the tem- Heating.—Sometimes the incidence of sea- perature during the early evening (due to breezes shows a greater seasonal variation land-breeze effect), followed round about than variations in wind velocity and direc- midnight by an equally marked rise of tem- tion would lead one to expect. Investiga- perature (due to the setting in again of tion suggests that such variations are gener- the monsoon). At other times, the land- ally related to seasonal variations in rain breezes are in evidence throughout the whole and sunshine amount which, between them, of one night, and the monsoon throughout largely determine the state of the ground the whole of the next. and the rate of surface heating and cooling. This connection is well brought out at Ba- SEASONAL VARIATIONS tavia: the graph (see FIGURE 5) of the Within a homogeneous source-region, there diurnal variation of strength of the N.- are frequently conspicous variations in the component (i.e., onshore wind component) intensity and constancy of both land- and shows much the same phase as the graph of sea-breeze, as between the dry and wet sea- mean daily range of temperature which is, son, or between one monsoon and another. of course, a rough measure of the amount The chief seasonal control factors are: — of insolation received. It will be seen that 1. Variations in Strength and Direction the sea-breeze effect is strongest when the of Prevailing Wind.—Variations in time range is greatest, and weakest when the (i.e., season) look much the same as the range is smallest. Comparison with the variations in place we have just discussed. mean monthly rainfall graph elicits the fact In view of the seasonal migration of the that the maximum sea-breeze effect coin- wind belts across this region, it follows that cides with the end of the dry season (that

Monthly Variations in Wind Direction and Force At/Near Poeloe Bras (Off NYW. Sumatra)

J. F. M. A. M. J. J. A. s. 0. N. D.

1 % Steadiness of Wind Direction (day and night)15 83 76 50 22 56 66 74 72 68 62 22 55 2. Mean Monthly Wind Force (Beaufort) 3.9 3.3 2.7 2.8 3.4 3.9 4.1 3.8 3.3 3.3 3.4 3.2 3. Mean Diurnal Variation of Wind Direction 9° 9° 23° 11° 6° 5° 3° 7° 2° 2° 10° 16°

15 The figures signify the % no. of occasions on which the wind blows from a given compass point.

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC FIGURE 5. Monthly variations in mean maximum daily range of temperature, and mean daily strength of north (= sea breeze) component of wind at Batavia, with mean monthly rainfall superimposed. is when the paddy fields are dry and, so, 1. Changes in the Strength of the Pre- when the earth's surface responds most vailing Wind.—While synoptic charts are readily to the sun's rays), and minimum unlikely to be of much assistance (unless effect coincides with the height of the rainy barometric readings of several stations, in season.16 close proximity to one another, can be relied upon), variations in the behavior of wind DAY-TO-DAY VARIATIONS and pressure at a reliable up-wind report- ing station may be of considerable use in In spite of the over-riding uniformity of giving advance notice of the arrival of a weather conditions (more conspicuous in surge of sufficient strength to affect the the dry season than wet), there is, over the diurnal rhythm of wind at the coast in whole area, a day-to-day variation in many question. of the elements, notably in the strength 2. Changes in Lapse Bate, i.e., in Air- and steadiness of the sea-breeze. These Mass Properties.—Clearly these must be variations arise in much the same way as important, since the disturbance of the seasonal changes, the only difference being pressure equilibrium will be greater (at that day-to-day variations are on a more moderate altitudes) with stable than with restricted scale than the others. unstable air, provided the same amount of Useful clues to the occurrence of such heat is available to warm up the air-mass variations in land-and-sea breezes are pro- in each case:—in other words, the heat is vided by the following indications: — spread through a deeper layer of unstable, than of stable air, and so does not produce 16 But like all other land-and-sea breeze rules, this one is of only local validity. Thus, at Benkoelen at any given level such a large thermal (S.W. Sumatra) the sea-breeze is best developed in the pressure gradient. On the other hand, con- rainy season. The reason for this seems to be that the rain (and cloud) arrives too late in the day to inter- densation is more likely to occur in unstable fere with the diurnal range of temperature which is only one degree or so smaller then than during the dry air, which means more heat is available to season:—actually the sunshine figures show there is give rise to pressure differences in unstable very little seasonal difference in the expectation of sun- shine at a given hour of the day up to 1500 hours by air than in stable air. In general, observ- which time, of course, the sea-breeze has already begun to decline. ers seem to think that unstable air is more

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC likely to induce sea-breezes than stable air.1T air so humid that the sky becomes overcast, In the S.W. Pacific and S.E. Asia regions or nearly so, at an early hour, the conse- day-to-day differences in lapse rate are quent retardation in the upward march of small by extra-tropical standards and can- temperature over the land will have a not usually be sensed without local upper marked inhibiting effect on the sea-breeze air data. We say "local'' advisedly, be- mechanism. So, too, will any other factor cause in moving slowly across the island- tending to curtail surface heating:—thus dotted waters of the N.E.I., air-streams a heavy nocturnal rainfall (a characteristic have ample opportunity to adapt themselves feature of many lee-shores) will cool the to the changing surface conditions; with air below normal and, at the same time, the result that their structure is much more saturate the soil, thereby delaying and dim- granular than is the case with the faster- inishing the day-time rise of air tempera- moving airstreams of higher latitudes. The ture. This will be particularly noticeable rate at which cumulus clouds build up—and when, as is customary in such cases, the their time of starting—will often act as a early morning sky is overcast with thick guide to the likely strength of the day-time stratiform clouds. On the other hand, a breeze. bright sunny morning following on a dry— 3. Variations in Opportunity for Surface but preferably not a "breathless" night Heating.—(See also under SEASONAL VARI- (for this makes for low minimum tempera- ATIONS). If convection is so strong and the tures)—is a good predisposing sea-breeze factor. Just how important the march of 17 In mid-latitudes the sea-breeze is generally stronger air temperature is in relation to the much and more reliable on days when convection clouds form more conservative sea temperature is clear than on days when they do not.

FIGURE 6a. (Compare with FIGURE 6b.)

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC FIGURE 6b. Note that the sea breeze lasts longer at Padang than at Menado (8*4 hrs. against 6). This is related to the greater strength of the opposing regional wind at the latter place—a fact that explains why the land-sea air temperature excess at the moment of the sea-breeze onset is larger at Menado than at Padang and also why the sea breeze at Menado is neutralized even before the land temperature has fallen below the air temperature over the sea. from the following (see FIGURE 6). When When the prevailing wind has an onshore the prevailing wind is offshore the sea-breeze component, this rule does not apply:—then is unlikely to set in until the air temperature the forenoon shift of wind direction and over the land has risen between 1°-2°F increase of speed, signfying the onset of the above the air temperature over the sea.18 sea-breeze effect, may occur much earlier— This normally occurs about 30 minutes to 1 as early as 0830 at Menado. In such circum- hour after the air temperature deficit has stances, the main factor governing diurnal been made good. Thus at Padang the air wind change would appear to be turbulence temperature over the land normally rises which is responsible for the working down above the air temperature over the sea by of the stronger upper wind to the surface 0930; at Medan by 0945 and at Menado by 0915:—the corresponding normal times of over the land has fallen below the temperature over the 19 sea:—this is partly due to the fact that the sea-breeze onset of sea-breeze are 1020, 1045 and 1030. mechanism takes time to "run down" after the con- ditions which gave rise to it have stopped. FIGURE 18 According to Snellius Expedition results, coastal air no. 8 suggests that the continuance of the sea-breeze temperatures are unlikely to rise, on the average, more is also partly due to the fact that the pressure over than 2°F above temperature of air over sea. the sea remains higher than the pressure over the land 19 It should be mentioned that the sea-breeze con- (Batavia)—i.e., the pressure gradient is in favor of an tinues to blow for an hour or so after the temperature onshore wind—until approximately 1930.

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC (which in itself will occasion a slight shift when the land-breeze begins (see FIGURE of wind direction). 7); (b) relative humidity:—this character- istically shows a quite considerable rise with LAND-SEA BREEZE AND LOCAL WEATHER the onset of the sea air; and (c) cloud Land and sea-breezes are a cause as well and rainfall. When the sea-breeze is op- as an effect of diurnal variations in weather posing the regional wind, i.e., when the elements, for, inter alia, they re-act on the latter has an offshore component, the con- march of (a) air temperature:—the ther- vergence that takes place at the landward mograph curve frequently—though not in- limit of the breeze is productive of heavier variably—shows a flattening off after the cloud and more rainfall than that which is breeze has set in, and a sharp falling off found within the ambit of the sea-breeze

FIGURE 7a. Daily March of Temperature at Manggar (E. Coast Billiton). The ther- mal effects produced by land- and sea-breezes are well exemplified in this autographic record. (1) On 24 and 25th, the sea breeze was well-marked, and was responsible for the flattening off of the temperature curve after its onset. Onshore winds prevailed throughout the night of 24-25th and 25-26th; hence the small diurnal variation. (2) The much larger amplitude of the record on the 27th, 28th and 29th was largely the result of the land breeze ousting the onshore wind during the night. (3) On the 26th, the diurnal rhythm was badly disturbed by a rainstorm occurring round about 1000. The cooling of the air by the rain and subse- quent overcast sky account for the fact that the temperature never recovered from the early set-back.

FIGURE 7b. Diurnal March of Temperature and Wind Velocity at Makassar (S. W. Celebes) in August. It is to be noted that the sea breeze eases off before the temperature has reached its maximum; this is because the main sea-breeze control is the difference be- tween the temperature of the air over the land and the temperature of the air over the sea, rather than the actual temperature of the air over the land. This difference is at its maxi- mum ca. 1400; thereafter, although the air (over the land) temperature continues to rise, the air (over the sea) temperature rises somewhat faster.

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC FIGURE 8. Diurnal Variation in Sea-Land Pressure Differences, Java. itself (see FIGURE 9). A similar state of the prevailing wind is onshore. Convergence affairs occurs at (or near) the coast during of a rather more oblique sort—resulting the night in those hilly regions where the from the fact that the direction of the mon- land-breeze opposes a weak monsoon (see soon, though offshore, differs from that of the land-breeze, as in FIGURE 10—would also seem to be responsible for the second- ary, nocturnal, maximum observed at some coastal places, e.g., at Menado (Northeast- ern Celebes) during July, August and Sep- tember when the mean direction of the southeasterly monsoon differs from that of the land-breeze, here well-developed with the aid of mountains, by about 45°. When the regional wind is inappreciable, over the land convergence occurs by day and di-

FIGURE 9 FIGURE 10

FIGURE 9). This helps to account for the vergence by night. This helps to accentuate 20 nocturnal rainfall maximum which many the normal diurnal variation of cloud places on or near the coast experience when amount resulting from ascending (day) and descending (night) air currents. Over 20 The conspicuous thunderstorm activity of many coastal places can also be attributed, partly at any rate, the sea, divergence and subsidence, i.e., to the same factor.

Unauthenticated | Downloaded 09/23/21 10:48 AM UTC clearance, occurs by day and convergence, distance inland, have a depth of 5,000 ft and i.e., cloud formation, by night, thus pro- a mean maximum speed equal to, but seldom ducing a reversal of the diurnal incidence exceeding (unless strongly reinforced by an of cloud over the land.21 The maximum and onshore monsoon) 15 knots. Normally, minimum cloud epochs are even more however, they extend only 15-20 miles sea- sharply defined over sea areas when the lat- wards and inland, have a depth of between ter are partly land-locked. Thus, in the 3-4,000 ft and a speed (unaffected by re- northern half of the Straits of Malacca the gional wind) of 8-10 knots. proximity of the two heat sources, while it Sea-breezes are weak, often non- discourages (as we have seen) the forma- existent : — (a) in the rainy season (which is usually the cloudiest season). (b) along coasts backed by lagoons, marshes, and paddy fields (when under water). (c) when the prevailing wind exceeds 15 knots. (d) when the coast is within 50 miles of FIGUKE 11 other heat sources. (e) when the land area in question is less tion of strong land- and sea-breezes, accen- than 10-15 miles across. tuates the day-time tendency for divergence over the sea, and the night-time tendency They are least steady in direction for convergence (see FIGURES 4 and 11). (though not necessarily weakest in force) during the intermonsoonal months when

SUMMARY there is no gradient wind to exercise a steering effect. 1. Sea-breezes.—Other things being equal, The usual duration of the sea-breeze in sea-breezes are strongest and steadiest in N.E.I, is from approximately 1030 to 1800. direction: — If it first has to neutralize an offshore pre- (a) in the dry season (which is usually vailing wind, it may set in as late as 1330, the sunniest season). but it will seldom be found blowing at the (&) in regions which have a light annual surface after 1930 (local time). Unaided, rainfall, or a very well-marked dry season. it does not usually exceed force 3 except be- (c) along coasts which are backed by tween the hours of 1200 and 1600. high ground or mountains (preferably not 2. Land-breezes.—The land-breeze is much forest-clad) and are free from large off- less predictable in its behavior. Under the lying islands. most favorable conditions it may start as (d) when there is a light onshore pre- early as 1900 and last to shortly after sun- vailing wind (less than force 3). rise, but unless it is implemented by an offshore general wind (trade or monsoon), Under the most favorable combination of or is blowing in the vicinity of a river circumstances encountered in the Far East valley flanked by high gronnd, it will not palpable sea-breezes may extend seawards exceed force 2, or extend more than 10 upwards of 50 miles, and about the same miles seawards, or be perceptible more than 500 ft above ground-level. 21 It is on account of this that the day-time "land- formed" clouds give the impression of drifting out to The quietest times of day coincide with sea during the evening. Although this may be the case at times, it may be taken as a general rule that the epoch of change-over from sea- to land- the clouds dissolve above the land and form afresh over the sea during the night. breeze, and vice versa.

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