436 APPENDIX. 



(3) Air Tc7nperatures within the Crowns. — The mean 

 temperature of the air in the tree tops, after correcting for 

 elevation above ground, is rather higher than over open 

 fields. The effect of tree tops does not appreciably depend 

 upon the height of the station above ground. The effect 

 upon the minima is generally greater than on the maxima, 

 the total effect being a warming one. A tree-top station is in 

 general intermediate, as to temperature, between a station 

 near the ground in the forest and one in the open field. 



Evergreen forests show less difference between the temper- 

 ature in the crown and below, and altogether more uni- 

 formity in temperature changes throughout the year than 

 deciduous growth. 



The vertical gradient for temperature within the forest on 

 the average of all stations and all kinds of forest trees is 

 large, varying from 0.61° F. per 100 feet in April to 2.50° F. 

 in July. 



A reversal of the vertical gradient, namely, a higher temper- 

 ature above than below, occurs in the wood, especially in the 

 summer time. It also occurs in the open air regularly at 

 night, and may be three or four times as large as that just 

 mentioned. In general, the action of the forest tends to pro- 

 duce a vertical distribution of temperature like that over snow 

 or level fields on clear nights. 



(4) Air Temperature above the Crowns. — The tempera- 

 ture, at considerable heights above the forest, appears to be 

 slightly affected by the forest, and more so with evergreens 

 than with deciduous growth. The vertical gradients of 

 temperature within 30 feet above the tops of the trees are all 

 reversed throughout the leafy season ; the gradients are also 

 greater above the tree crown than below, at least during the 

 clear sky and calm air. The wind affects the temperature 

 under and within the crowns, but makes little difference above 

 them. The surface of the forest crown appears meteorologi- 

 cally much like the surface of the meadow or cornfield. It is 

 as if the soil surface has been raised to the height of the trees. 



