(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 in 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.

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

30°) and give large and important regions a sharp seasonal contrast in wind directions. Between the winter and the summer monsoons, there commonly is a spell some weeks in length when the winds are irregular and often light. After they become steady in direction they often fluctuate notably in velocity from day to day as well as between day and night.

(5.) Although winds due to cyclonic disturbances do not occur so frequently within the tropics as in higher latitudes, they are significant. The "boxing of the compass," during which the wind comes from every direction in turn, occurs many times a year in most parts of the tropics, while occasionally cyclonic gales or even violent hurricanes occur. Official Japanese daily weather maps and annual summaries of storm tracks show an average of over fifty tropical cyclonic disturbances a year in east longitudes 115°145°, while a study of the Australian daily weather maps for 20 years shows an average of over 30 a year in similar longitudes south of the equator. Thus in less than one seventh of the circumference of the earth there are over 80 cyclonic disturbances in an average year. This is, however, the stormiest sector.

Mention should be made of thunder squalls which are, on the average, more violent in low latitudes than in higher latitudes and more frequent. In addition, several regions in subtropical latitudes experience tornadoes or similar storms. Thus it is evident that there is considerable variation in respect to winds in the tropics.

Variations in rainfall have perhaps even greater significance than variations in temperature or wind. The indications are that in respect to dependability of precipitation, the lower latitudes are notably less fortunate than are middle latitudes. In order to compare the variability of rainfall in the tropical half of the globe with that of the higher latitudes, I have inspected the official records for many cities in both zones. The selection was impartial, being determined solely by whether or not the data were available. The comparison is between the greatest and least annual precipitation officially recorded before a recent year. The length of the record varies, but in general it is shorter in low than in higher latitudes and hence tends to lessen the apparent range in lower latitudes. Tables 3 and 4 give the figures to the nearest one tenth of an inch. It will be noticed that the maximum amount of rainfall received in a year was less than twice the minimum for Chicago, Christiania, Edinburgh, Ottawa, Paris, Pekin and Tokio, and only a trifle more than twice the minimum in the case of Amsterdam, Berlin, Berne, London, New York, Petrograd, St. Louis, Vienna and Wellington, N. Z. Very few middle or high latitude cities appear to have ex

perienced three times as much precipitation in their wettest year as in their driest. Madrid, Washington, D. C., and Vladivostock are exceptions as are some cities in southern Europe, while Hobart, Tasmania, Buenos Aires, Rome and San Francisco are notable for having received about four times as much. However, many geographers class Buenos Aires, Rome and San Francisco as subtropical. Furthermore, Madrid and Vladivostock have an average rainfall of less than 20 inches, and thus are more subject to large percentage changes than is the case where the normal rainfall is larger.

Turning now to the lower latitudes: Among 20 scattered cities selected impartially, in no case was the officially recorded rainfall of the wettest year less than twice that of the driest. Only in Caicutta and Caracas did the ratio fall as low as 214. In Johannesburg it was 22 times as great, and in Durban, Hongkong and New Orleans it was 234. In Colombo and Honolulu it was about 3; in Bombay, Buenos Aires and Manila each about 32; in Madras 42; in Brisbane and Singapore 5; and in Rio de Janeiro 13.4. All these cities have a normal rainfall of 30 inches or over, and the mean for the group of cities is 55.6 inches in contrast with a mean of