31 
foliage. Moreover, as the evaporation increases, in the same degree the 
temperature is modified, since the greater the amount of water changed 
into vapor the greater the quantity of heat absorbed in the process. Re- 
ducing the temperature increases the relative humidity of the atmosphere. 
Hence, in two ways the atmospheric conditions are made more favorable 
for a copious and general rainfall at the approach of low barometric areas 
during the hot season. 
It has always been true, perhaps, that many thunder-storms and 
showers during the summer months, and particularly in July and August, 
give moisture to very limited areas. Careful observation during a nuin- 
ber of years has convinced the writer that as the forests have disappeared 
the average territory covered by our summer thunder-storms has been 
gradually and greatly decreasing. Repeatedly during the last few hot 
seasons, and especially during the one just past, the arrival of a low 
barometric area caused the formation of a few thunder-clouds, but these, 
instead of increasing in volume and advancing so as to cover a larger and 
larger region, soon dwindled and disappeared. The failure of the present 
deforested areas to add to the sum total of the general atmospheric mois- 
ture, as the heated conditions of the low barometric area approached, and 
also the failure in the formation of vapor in the given locality, both 
served to decrease the rainfall of the thunder-storm. This was due, first, 
to the lack of a local vapor supply to add to that brought in by the winds 
from a distance, and which is very necessary for the formation of clouds 
in the hot season. Again, when the supply of moisture may have been 
sufficient to form a thunder-cloud, its advance was into a highly heated 
diy region with its low relative humidity. The absence of any considerable 
local evaporation, and the resulting high temperature caused the re-evap- 
oration of the condensed moisture of the clouds and no precipitation fol- 
lowed. 
The weather conditions of the deforested areas during the hot months 
are more and more nearly approaching those of the hot arid regions of 
the west, where a thunder-cloud formed under favorable conditions very 
frequently disappears because of re-evaporation as it advances into a 
territory more highly heated, and of a lower relative humidity. 
In another way also the presence of forests tends to add to, and 
their absence to diminish, the precipitation of the summer months, and 
that is in causing secondary showers after the main storm is over. The 
enormous leaf surface, covered with moisture by the rain just passed. 
