i CLIMATE AND ASPECTS OF THE EQUATORIAL ZONE 229 



The drops of rain rapidly increase in size while falling through 

 the saturated atmosphere ; and during this process as well as 

 by the formation of dew, the heat which retained the water 

 in the gaseous form, and was insensible while doing so, is 

 liberated, and thus helps to keep up the high temperature of 

 the air. This production of heat is almost always going on. 

 In fine weather the nights are always dewy, and the diagram 

 on the preceding page, showing the mean monthly rainfall at 

 Batavia and Greenwich, proves that this source of increased 

 temperature is present during every month in the year, since 

 the lowest monthly fall at the former place is almost equal to 

 the highest monthly fall at the latter. 



It may perhaps be objected that evaporation must absorb 

 as much heat as is afterwards liberated by condensation, and 

 this is true ; but as evaporation and condensation occur usually 

 at different times and in different places, the equalising effect 

 is still very important. Evaporation' occurs chiefly during 

 the hottest sunshine, when it tends to moderate the extreme 

 heat, while condensation takes place chiefly at night in the 

 form of dew and rain, when the liberated heat helps to make 

 up for the loss of the direct rays of the sun. Again the most 

 copious condensation both of dew and rain is greatly influ- 

 enced by vegetation and especially by forests, and also by the 

 presence of hills and mountains, and is therefore greater on 

 land than on the ocean, while evaporation is much greater on 

 the ocean, both on account of the less amount of cloudy 

 weather and because the air is more constantly in motion. 

 This is particularly the case throughout that large portion of 

 the tropical and subtropical zones where the trade-winds con- 

 stantly blow, as the evaporation must there be enormous 

 while the quantity of rain is very small. It follows, then, 

 that on the equatorial land-surface there will be a consider- 

 able balance of condensation over evaporation, which must 

 tend to the general raising of the temperature, and, owing to 

 the condensation being principally at night, not less power- 

 fully to its equalisation. 



General Features of the Equatorial Climate 



The various causes now enumerated are sufficient to enable 

 us to understand how the great characteristic features of the 



