FOREST INFLUENCES. 309 



DIFFERENCE OF METEOROLOGICAL CONDITIONS WITHIN AND WITHOUT THE FOREST. 



(1) Soil temperatures. — The general influence of the forest on soil temperatures is a cooling 

 one, due to the shade and to the longer retention of moisture in the forest floor as well as in the 

 forest air ? which must be evaporated before the ground can be warmed. As a consequence, the 

 extremes of high and low temperature within the forest soil occur much later than in the open, 

 and both extremes are reduced, but the extreme summer temperatures much more than the winter 

 temperatures. (Pp. 40-46, Bui. 7.) 



The difference between evergreen and deciduous forests, which almost vanishes in the winter 

 time, is in favor of the deciduous as a cooling element in summer and autumn, while during 

 spiing the soil is cooler under evergreens. The effect increases naturally with the age and height 

 of the trees. (Pp. 46-50, Bui. 7.) 



(2) Air temperatures under the crowns. — The annual range of air temperature is smaller in the 

 foresf than in the open ; the effect upon the minimum temperature (i. e., the effect in winter) is 

 less than on the maximum temperature (i.e., the effect in summer.) The combined effect is a 

 cooling one. The range of temperature is more affected than the average absolute temperature, 

 or, in other words, the moderating influence is greater than the cooling effect. (Pp. 51-53, Bui. 7.) 



The monthly minima for middle latitudes are uniformly reduced during the year, and the 

 monthly maxima are much more reduced during the summer than during the winter. On the 

 average the forest is cooler than the open country in summer, but about the same in winter, with 

 a slight warming effect in spring. (Pp. 53-58, Bui. 7.) 



The difference between the mean monthly air temperatures in the woods and in the open 

 varies with the kind of forest much more than is the case for soil temperatures. The evergreen 

 forest shows a symmetrical increase and decrease throughout the year. The deciduous forest 

 shows a variable influence which diminishes from the midwinter to springtime, but increases 

 rapidly as the leaves appear and grow, becoming a maximum in June and July and then 

 diminishing rapidly until November. The annual average effect is about the same both for 

 evergreens and deciduous forests. (Pp. 58-60, Bui. 7.) 



Forests situated at a considerable elevation above the sea have sensibly the same influence 

 on the reduction of the mean temperature as do forests that are at a low level. (P. 60, Bui. 7.) 



Young forests affect the air temperature very differently from mature forests; in the former 

 the minimum temperatures are always reduced, but the maxima are exaggerated. The observa- 

 tions oh a\ hich this conclusion is based ought, perhaps, to be considered as pertaining rather to 

 the case of temperatures in the tree tops. (P. 60, BuL 7.) 



(3) Air temperatures icithin 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. (Pp. 61-66, Bui. 7.) 



Evergreen forests show less difference between the temperature in the crown and below, and 

 altogether more uniformity in temperature changes throughout the year, than deciduous growth. 

 (P. 67, Bui. 7.) 



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. 

 (P. 68, BuL 7.) 



A reversal of the vertical gradient, namely, a higher temperature 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 produce a vertical distribution of temperature like that over snow or level fields on clear 

 nights. (P. 69, Bui. 7.) 



(4) Air temperature above the crowns. — The temperature, 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 tfow below? at least during the clear sfcy and calm air? Tbe wind affects the temperature 



