68 



FOREST INFLUENCES. 



TEMPERATURE GRADIENTS. 



So far the discussion has been as to change of temperature horizon- 

 tally, or the horizontal temperature gradient. The tree-top observa- 

 tions permit also the study of the vertical distribution of temperatures, 

 and this is a matter of considerable interest, because from it can be 

 di-awn some conclusions as to the stability of forest air; that is, as to 

 its tendency to originate meteorological disturbances or to take part 

 in those which approach it. This can be studied within the forest, 

 because there are two stations — one above the other; but it can not, 

 unfortunately, be studied outside the forest, as the station there, in the 

 usual meteorological services, is^ single. In order to get around the 

 difficulty that the stations are not at the same height, the gradients 

 are expanded to the uniform height of 100 feet; that is, the vertical 

 temperature gradients in the figure are those which would occur in for- 

 ests if the temperature continued to change at the same rate through 

 100 -feet (30.48 m.). The height of this assumed station is its height 

 above the ground, decreased by 5 feet (1.5 m.) (the height of the lower 

 station). As the irregularities in the vertical differences are exagger- 



Decichious trees. Kveigrecn tnes. Average of all. 



Fig. 38.— Vertical temperature, gradient in wood.*, degrees Falirenheit for 100 feet. 



ated in increasing them for 100 feet, only the data from the eight high- 

 est stations were used. These give a distance between lower and upper 

 instrument of from 31 to 48 feet, averaging 38 feet (11.0 m.). Three of 

 these stations were for evergreen trees (average di.stance43 feet) (13 m.) 

 and five deciduous (averaging 35.5 feet) (10.8 m.). These were arranged 

 separately. 



The results appear in Fig. 38. The unbroken line represents the gra- 

 dients for the average of all eight stations, the broken line for the ever- 

 greens, and the dotted line for the deciduous trees. The vertical gra- 

 dients are surprisingly large, when compared with the average. In no 

 case would the latter be more than Oo.L'5, and it would always be nega- 

 tive. Here the gradients vary from OOOI F. per 100 feet in April (for 

 deciduous trees 0^.77, for evergreens 0^.35) to + 2^.50 in July (decidu- 

 ous + 20.62, evergreens + 2o,31). In summer the average gradient 

 under trees is about -f 2'=^; that is, it grows warmer as we ascend at the 

 rate of two degrees per 100 feet (31 m.). Outside in the^general aver- 

 age it grows colder by abont a quarter of a degree. In early spring the 



