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 
