Rains : Effect of Woodlands Stated. 17 



to supply its place. An upward current is thus formed, and the air 

 rising and cooling finally comes to the dew point, and the moisture 

 becomes visible as cloud. A column of smoke from a burning 

 clearing will sometimes thus form a cloud, and may cause rain. 



68. A country interspersed with groves of trees, presents contrasts 

 in heating tendencies favorable to the formation of these upward 

 currents of the air. Broad areas of cloud passing over a great ex- 

 tent of country, and usually accompanied by a low barometer, are 

 due to more general causes, but always show a reduction of temper- 

 ature in the region where they form. 



69. Rains, are caused by a condensation of moisture from cooling 

 below the dew point. The success of forest growth depends largely 

 upon their amount and their seasonable distribution throughout the 

 year, and especially their occurrence when vegetation is most active, 

 and when the new layer of wood for the season is forming. 



70. A rain guage on the ground will collect in a year more rain 

 than one on the roof of a house, and the latter more than one on a 

 high tower. Currents of air may tend to cause this difference. in 

 part, but it seems to show that the rain-drops gather in size as they 

 descend. 



71. On the contrary, in a dry time, we sometimes see filaments 

 of rain descending from a cloud, which dry up and disappear in the 

 warm air below without reaching the ground. When such clouds 

 pass over large bodies of woodland, where the temperature is cooler 

 and the air more moist, these filaments extend down and afford a 

 shower of rain, but dry up again as they come to the warm air of 

 the fields beyond. Applying these principles of humidity and tem- 

 perature to Forestry, let us consider what effect a woodland can have 

 upon them : 



72. In the growing season, there is a vast amount of evaporation 

 going on from the foliage of trees, the moisture being derived from 

 the soil. This evaporation is a cooling process, and as it both in- 

 creases the amount of moisture, while it reduces the temperature, 

 it tends to bring the air to the dew point ; in other words, to the 

 condition favoring the formation of dews and rain. In the nicely 

 balanced state of the atmosphere that we often find in summer, this 

 change may sometimes be the turning point that decides between 

 rain and drouth. 



