TROPICAL CLIMATES AND BIOLOGY — CARTER 431 



rising higher by the humidity, cloudiness, and other conditions. Ex- 

 tremes of heat are characteristic not of the equatorial regions but of 

 the deserts in subtropical latitudes. 



Though in fact the sun passes north and south of the vertical every- 

 where within the Tropics, the double seasonal change is practically 

 restricted to latitudes within 10° of the Equator. Farther from the 

 Equator than this, the sun does not pass far enough from the vertical 

 at the summer solstice to produce a noticeable effect. But the range 

 of seasonal temperature change increases as we pass away from the 

 Equator and may be as high as 8° C. at the Tropics. It is, however, 

 still less than in our latitude, where it may be as much as 15° C. 



Ramfall is more important than the temperature in determining 

 the differences between tropical environments. Equatorial regions 

 are in general characterized by fairly high rainfall, because they lie 

 where, between the north and south trade winds, rising currents of 

 air are cooled and their moisture precipitated — the region known at 

 sea as the doldrums, also a region of relatively high rainfall. As the 

 sun passes north and south, this area of high rainfall follows it, and 

 near the Equator the rainfall decreases. The result is that, although 

 the seasonal change on the Equator is small in temperature, in rainfall 

 it is considerable. It is unusual in an equatorial climate for any 

 month to be entirely without rain, but the difference between the driest 

 and wettest months may be great. Baker and Harrisson [1] ^ com- 

 pared tropical climates in this respect and find that the rain of the 

 wettest month is more than 2.5 times that of the driest in all but 3 

 percent. The farther we go from the Equator the difference increases, 

 the dry season following the sun with a lag of 1 to 2 months. Total 

 ramfall also becomes less, especially beyond 15° N. and S. where the 

 subtropical dry belt is approached. 



The character of the rain as well as its amount is very important 

 in controlling environmental conditions in the Tropics. All over the 

 Tropics cyclones with large variations of atmospheric pressure, such 

 as we know here, do not normally occur. The rain is almost always 

 convectional — owing to upward movement of currents of air — but its 

 frequency is very different from one region to another. In the rain 

 forests there is often rain on almost every day — there are on the aver- 

 age 249 rainy days a year at Para near the mouth of the Amazon — 

 and the rain usually falls as a storm of an hour or two's length, often 

 with thunder. In some deserts the intervals between storms may be 

 more than a year on the average. 



It is the frequency of rain more than any other feature of the cli- 

 mate that controls the nature of environments in the Tropics. Where 

 rain is frequent, the environment is unable to dry between storms, 



■ Figures In brackets refer to list of references at end of article. 

 625325—62 29 



