HAP. n CHARACTERS OF THE TROPICAL CLIMATE 213 



ii. HEAT. 



The mean annual atiiiosp/tcric tcntpcraiure varies between 20" and 28°, 

 k1 is very steady when compared with that of higher latitudes ; even the 

 fferences between the annual liighest and lowest temperatures in the 

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

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

 en J . 



•The difference of temperatures between the hottest and coldest montli near the 

 quator lies between 1° and 5°C., and does not exceed these figures, not even in 

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



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

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

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

 ang, 13-4'' ; at Vera Cruz, 6-5 " ; at Havana, 5-8"; at St. Louis in Senegal, 9.0° ; at Rio 



Janeiro, 6-5° ; at Kuka in Bornu, 12-1° ; at Khartum, 12.9" C. The annual variation 

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

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

 tavia, 6.5", and during August, -j-f ; at Chinchosho in Loango, annual variation, 



, but in July, ■j-^'' ; at Kuka in the dry season, 11-4'' ; at Lado, difference 2h.-7h., 

 nual variation 77', in the dry season ii-i°; at Bakel in West Africa, 12-4'' C." 



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

 Southern China, is zei'D or even a somewhat lower temperature actually 

 iched now and then. The average maxima usually vary between ■:5o" 

 d 35° C. and remain below the extremes observed in extra-tropical 

 tricts. 



Meteorological reports unfortunately only exceptionally give data re- 

 ding temperatures due to direct insolation, although this at least ec^uals 

 nospheric temperature in its importance to organic life-. Corresponding 

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



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

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

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

 nd the soil heated very often to 75°, frequently to 80°, and once 

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



season observed with a solar radiation-thermometer temperatures merely 

 : those usual at Gratz, namely 55°-56-7° C. at noon. The relatively slight 

 ■ct of insolation in this case is apparently a consequence of the large 



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

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

 etation by nocturnal radiation is certainly considerable during the dry 



Hann, op. cit., Bd. H, p. 12. 



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



al districts. 



