TEMPERATUKE IN WOODS, GLADES, AND PLAINS. 83 



sun's rays under these favorable circumstaTices pour on the surface ^^^ 

 times as much heat as is required for evaporation. If the maxi- 

 mum heat of sunshine is reduced by one-half for latitude, by seven- 

 eighths because of night and of low morning and afternoon tempera- 

 tures, and again by one-half because of cloudy weather, making its av- 

 erage value one-thirty-second that given above, yet there is more than 

 enough over each square foot of forest to effect the transpiration. 



It is only where the transpiration has been very active during the day 

 and contiiuies into the night that its cooling could be possibly injurious, 

 but as the temperature cools the transpiration itself is checked, and 

 besides the moisture which it has poured into the atmosphere serves as 

 a screen to prevent rapid radiation from the soil, and its condensation 

 returns some of the heat that had been taken up. There might, however, 

 occur a combination of circumstances, very rapid transi)iration during the 

 day absorbing the sun's heat, a clear night, calm air and a low temi)er- 

 ature from other sources coming on Avith the evening, when the trans- 

 piration might cause the temperature to fall below freezing. This could 

 hardly happen, however, without a meteorological change toward cold, 

 and this change must come on toward evening, for otherwise it would 

 itself check the transpiration. 



The combination would be more likely to occur over herbaceous 

 growths, especially over grass and cereal crops, than over forests. 

 Their transpiration, it appears from the table (pnge — ), is decidedly 

 greater than that of the forests, and they make rapid advances much 

 earlier in the spring when such low temperatures are otherwise possi- 

 ble. 



TEMPERATURES IN^ WOODS, GliADES, AlVD PLAINS. 



The foliage of the trees reflects a considerable part of the solar rays 

 which reach it, and this heat is reflected in all directions. That part 

 which passes toward the sky is probably lost, and plays only a small 

 l)art in warming the air. The part which is reflected longitudinally or 

 at downward angles has a very favorable path for absorption by the 

 air, more favorable in fact than that of the noonday direct rays of the 

 sun. The absorption of this heat should occur, for the most part, in 

 the vicinity of the forest. The temperature around the forest ought, 

 therefore, to reach a somewhat higher maximum on sunny summer days 

 than that of the air at some distance from the forest. 



On the other hand, the surface at the stations near the forest is ex- 

 posed to nocturnal radiation of heat to the sky about as freely as is the 

 station at a distance, and, as the former is often in the lee of the forest, 

 its air is generally more stagnant. Tliis ])romotes cooling of objects at 

 the surface of the earth, and as the station near the forest is as much 

 exi)osed to celestial radiation and the relative stagnancy of the air 

 favors this cooling, and also favors the communication of it to the ad- 

 jacent air, the temperature at such stations should often fall lower than 



