EXTREMES OF HEAT AND COLD. 61 



cooler leaves and twigs. In cooling- it becomes heavier 

 and falls toward the ground. A rising current of 

 warmer air is formed to supply its place, and so the 

 colder air flows off along the surface into the open 

 country and causes local breezes. At night the air 

 currents are reversed. The air in the forest is then 

 warmer than the air outside, because the cover checks 

 the radiation of heat, and so the colder air moves from 

 the open country toward the woods. In these ways 

 the influence of the forest is felt at a distance. 



The amount of this cooling of the air has been meas- 

 ured in certain places. It is naturally found to be 

 greatest in summer: while in winter and at night the 

 air in the tree tops is a little warmer than in the open. 

 It is important to add that the cooling effect of the forest 

 is greater than the average in the mountains, and less 

 in the plains. 



EXTREMES OF HEAT AXD COLD. 



The extremes of heat and cold are moderated by the 

 forest. Observations on this point have been made, for 

 example, in Bavaria and Wiirttemberg. They showed 

 that the lowest temperature of everv day in the 3'ear 

 was higher, on an average, by nearh' 2- in the forest, 

 while the highest temperature was lower b}' nearlv 4:^. 

 The greatest heat of the da}' in the summer was Tt^ 

 less in the forest than outside. Prussian observations 

 showed that for ten A^ears the greatest heat of the day 

 in July was, on an average, nearly 6^ lower in the 

 forest, and the greatest cold of the night in January 

 nearly 3^ less than outside. It should not be forgotten 

 that the latitude, the elevation, and the exposure had a 



