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VARIABILITY VS. UNIFORMITY IN THE

TROPICS

By Professor STEPHEN SARGENT VISHER

INDIANA UNIVERSITY

T is commonly stated that tropical climates are extremely uni

Iform. This is only partly true. They indeed have compara

tively slight seasonal variations in temperature and in the length of day and night, and large areas have rather steady winds much of the time. But continual emphasis upon the uniformity of tropical climates is misleading because there are important variations in temperature and wind, while the rainfall of the lower latitudes appears to be more variable on the average than the rainfall of higher latitudes. There likewise appears to be more variation in storminess and in rapid change of air pressure than in higher latitudes.

Recent field investigations (financed by the Bishop Museum of Honolulu and Yale and Indiana Universities) in Hawaii, Fiji, tropical Australia, the East Indies, the Philippines and tropical China, and examination of official meteorological records concerning these areas and others have disclosed interesting evidences of tropical variability and have convinced me that the conventional statements, based on averages, are sufficiently misleading to make it worth while to emphasize the climatic variability occurring in the tropics.

The small seasonal contrasts in temperature which are characteristic of the tropics are perhaps the chief reason why the impression has been gained that tropical climates are uniform in other respects. Another reason for this common belief is the fact that the climatic data concerning the tropics are chiefly available in the form of averages. Averages by themselves are very misleading and should be supplemented as soon as possible by statements as to extremes, and as to normal extent of departure from means.

Although the average seasonal range in temperature is indeed small in low latitudes as compared with middle latitudes, there is an appreciable seasonal contrast in latitudes more than 10° or 15° from the equator. Indeed some parts of the tropics have about as great a seasonal range of temperature as certain especially uniform parts of higher latitudes. This is illustrated when the average

differences in mean temperature between the three warmest months and the three coolest of the following pairs of seaport cities are compared. Some of the cities in the right-hand column are not within the tropics according to the narrowest limitation of that zone. However, all are within the belt dominated by the Trades during most of the year, which is the belt commonly considered as tropical.

Even in regard to extremes of temperatures, some cities in fairly low latitudes have ranges which approach those of the less variable parts of the relatively high latitudes. This is illustrated by the following table showing the difference between the highest and lowest temperatures ever officially recorded at certain pairs of seaport cities up to a recent year.1

That there are appreciable seasonal contrasts in temperature in lower latitudes is not surprising when the seasonal variation in insolation is considered. In spite of the fact that the sun shines vertically somewhere between the two tropics every day in the year, there is a great change in angle of incidence. Few people realize that when the sun is vertically over the northern tropic

1 References to sources of data are given in a longer, more technical article on "Variability of Tropical Climates" to be published in The Geographical Journal (London).

(Cancer), it shines upon the southern tropic (Capricorn) less nearly vertically by 4 degrees than upon the Arctic Circle. The latitude of New York receives much more heat from the sun on June 21 than does the equator, for not only is the sun six degrees more nearly vertical than at the equator, but moreover the days are almost four hours longer.

Although on the average tropical regions show less contrast m seasonal change of temperature than do middle latitudes, the reverse is true in respect to daily range. The night has been called the winter of the tropics. The daily range is considerable in all lower latitudes, although it is less in the more humid regions than in the more arid. On the average it is distinctly greater than the normal range in higher latitudes. This is due to two chief influences: Day and night are more nearly equal in length, and hence there is a closer balance between the duration of the heating and cooling periods than occurs in higher latitudes, where the nights are too short in summer for marked cooling and the days are too short in winter for effective heating. The other great cause is the higher average temperature, since the escape of heat varies as the fourth power of the absolute temperature. This means that normally there is much greater cooling per nocturnal hour wherever the day-time temperature is high than where it is low. A third reason why the diurnal range averages greater in low latitudes (below 30°) is that a larger proportion of the area is arid or semi-arid than is the case in middle latitudes.

In the tropics the nights often become so cool that considerable discomfort results. Even in an insular climate like that of Suva, Fiji (latitude 18°S.), in spite of the wind blowing off the sea, and a rainfall of over 100 inches fairly evenly distributed throughout the year, it commonly becomes so cool at night that the sensitive residents wear wraps if they walk out late in the evening. Indeed, even the heavy army overcoats are frequently worn with comfort at night and in the early morning during the cooler season. In drier parts of the tropics, the nights become much cooler than in a humid locality like Suva. On the dry western sides of the Fiji Islands, for example, temperatures below 40°F. have been recorded near latitude 16° close to sea level, and in dry continental areas frost is not unknown near sea level, as for example within 20° from the equator in Australia and Africa.

Another type of marked cooling in the tropics is the sudden drop, often as much as 6° or 8°F., which occurs in thunder-storms, which are very frequent in many parts of the tropics, far commoner on the average than in higher latitudes. Sometimes, as when hail falls in quantity, the temperature-drop is much greater.

Hail storms are not very rare in some tropical localities. For example, ten hail storms were reported in ten years in latitudes 13° to 16° S., near sea level in the Northern territory of Australia. Three hail storms occurred in Panama (latitude 9°) in a twelve year period.

Cold snaps of still other types occur within the tropics. For instance, cold winds sometimes sweep down from higher latitudes and bring low temperatures surprisingly near the equator. Zero temperatures have been officially recorded in subtropical Northern Florida (lat. 30° N.), and a temperature of only 10° above zero F. has occurred on the Gulf Coast of Mexico in latitude 251⁄2° N. Central coastal Queensland is subject to "severe frosts" during four months in the year within 21° of the equator, while freezing temperatures have occurred even in the day time in southeastern Asia in latitude 22° at sea level. Still farther south, on the China Sea near Manila, latitude 15°, for example, northerly gales in winter occasionally make overcoats welcome even in the day time. Similar cold snaps occur in the cooler months in other tropical localities such as the Hawaiian Islands, Jamaica and Fiji. Indeed, remarkable as it may seem, the Weather Bureau reports a snow flurry practically at sea level at Mahukona, Hawaii (lat. 20° 11′), lasting ten minutes on December 29, 1921. Perhaps even more surprising is the great cooling reported as not rare in winter on the coast of Venezuela, in latitude 10° N. Director Ugueto, of the Cajigal Observatory, announces that gales from the north off the sea occasionally bring temperatures of 45° F. or even less, in the day time, lasting a number of days. They are not associated with thunderstorms, for the sky is clear at the time.

Because of the sensitiveness of the residents of the tropics to low temperatures, chills and colds often develop and sometimes pneumonia. Many observers have been impressed by the abundance of coughs and catarrh in the tropics. They may be more common there than in Canada. Indeed there is considerable truth in the saying that "cold causes more suffering in the tropics than in polar or subpolar regions."

Now as to the winds: Five chief sorts of variation within the tropics merit attention: (1.) Even when the direction is fairly constant, there is a marked diurnal variation in velocity. Calm nights are the rule in trade wind deserts and nearly calm nights are common elsewhere on the land except upon exposed elevations. Likewise at sea, while the diurnal range is less than on land, it is notable. For example, Tetens reports a diurnal range of over 50 per cent. in the velocity of the wind at Samoa. In higher latitudes, while the wind frequently dies down at nightfall, and normally

weakens, windy nights are by no means uncommon, and very frequently the wind is stronger by night than by day. In the tropics, windy nights occur on lowlands only during the passage of rather rare severe cyclonic storms. Moreover, disturbances of an intensity which would give strong nocturnal winds in middle and high latitudes cause only moderate winds at night at low elevations in the tropics. This is due to the influence of the comparatively great decrease in vertical convection at night in low latitudes caused by the greater cooling of the surface air than of the overlying free air. It is for this reason also that even relatively steep barometric gradients in monsoonal regions permit a marked dying down of the surface winds at night.

(2.) Seasonal as well as diurnal variations in the velocity of the trades are common. "Half Gales' are characteristic of Fiji, the New Hebrides and many other South Pacific groups in their spring months, and even "whole gales" are frequent during the northeast "monsoons" in the China Sea during winter. On the other hand, in other months calms or light breezes are the rule when the doldrums migrate past, as they do twice each year with the seasonal change in the altitude of the sun. Along the margins of the tropics calms likewise occur when the extra-tropical belt of high pressure migrates equator-ward in the cooler season.

(3.) There is a radical seasonal change in the direction of the Trades when they cross the equator; those crossing from the north change from east-northeast winds to northwest, due to the deflective effects of the earth's rotation. Consequently many places near the equator have easterly winds much of the year; calms while the doldrums are migrating past, and westerly winds when the doldrums are situated in higher latitudes on their side of the equator. Then, as the sun returns equator-ward, calms and easterlies recur.

(4.) Another evidence of tropical variability is that land and sea breezes are more characteristic of the lower latitudes than of the higher. This is because the contrast in the temperature of land and water averages greater in low latitudes. Indeed while in middle and high latitudes sea breezes are rare except during the hottest weeks, in many parts of the lower latitudes they occur almost every day in the year, and give a wind régime which is very different from the constant easterly trades supposedly characteristic of the tropics. The monsoons are a special type of land and sea breezes, since they blow towards the land for many consecutive weeks during summer, and in the opposite direction in winter. While produced by temperature contrasts of extra-tropical regions, the monsoons are most strongly felt in tropical latitudes (below