Chap. I] CHARACTERS OF THE TROPICAL CLIMATE 313 



ii. HEAT. 



The mean annual atmospheric temperature varies between 20° and 28 , 

 and is very steady when compared with that of higher latitudes ; even the 

 differences between the annual highest and lowest temperatures in the 

 equatorial district do not much exceed that of the diurnal variation of 

 temperature, and on the average are from io°-i3°C, but often much less, 

 even 5 . 



'The difference of temperatures between the hottest and coldest month near the 

 Equator lies between i° and 5°C, and does not exceed these figures, not even in 

 the interior of continents— at Lado in Central Africa, 5 N., it is 4-8° C. ; at Iquitos, 

 37 S., it is 2-4° C. ; at Equatorville on the Congo it is 1-2° C. But even towards the 

 limits of the tropics, and in the extremest climates met with within the tropics, the 

 annual variation of temperature hardly exceeds 13°. At Calcutta it is 10-3° ; at Hong 

 Kong, 13-4°; at Vera Cruz, 6-5°; at Havana, 5-8°; at St. Louis in Senegal, 9-0°; at Rio 

 de Janeiro, 6-5° ; at Kuka in Bornu, 12a ; at Khartum, 12-9° C. The annual variation 

 of temperature is therefore at many places less than the diurnal variation, the limits 

 of which we may perhaps assume to be 5 and 13 , for example at Equatorville, 8°; in 

 Batavia, 6-5°, and during August, 77 ; at Chinchosho in Loango, annual variation, 

 6-4°, but in July, 7-3° ; at Kuka in the dry season, 11-4° ; at Lado, difference 2h.-7h., 

 annual variation 77 , in the dry season n-i°; at Bakel in West Africa, 12-4° C. 1 ' 



Only at a few points, quite close to the limits of the zone, for example 

 in Southern China, is zero or even a somewhat lower temperature actually 

 reached now and then. The average maxima usually vary between 30° 

 and 3j° C. and remain below the extremes observed in extra-tropical 

 districts. 



Meteorological reports unfortunately only exceptionally give data re- 

 garding temperatures due to direct insolation, although this at least equals 

 atmospheric temperature in its importance to organic life 2 . Corresponding 

 to the position of the sun at the zenith or at a short distance from it, 

 the intensity of insolation during a definite period, for example in an hour, 

 is greater in the tropics than in higher latitudes, and must exert a corre- 

 spondingly greater heating effect. In fact, Pechuel-Losche at Chinchosho 

 found the soil heated very often to 75 , frequently to 8o°, and once 

 even to 82°C. Haberlandt, on the other hand, at Buitenzorg during the 

 wet season observed with a solar radiation-thermometer temperatures merely 

 like those usual at Gratz, namely 55°-5o"-7° C. at noon. The relatively slight 

 effect of insolation in this case is apparently a consequence of the large 

 quantity of water-vapour in the air. In continental districts much higher 

 temperatures prevail, at least during the dry season. The cooling of 

 vegetation by nocturnal radiation is certainly considerable during the dry 



1 Hann, op. cit., Bd. II, p. 12. 



2 Considering the danger of sunstroke in Cisgangetic India and other tropical conti- 

 nental districts. 



