TEMPERATURE ABOVE FORESTS. 



69 



g-raclients are somewhat alike in and out of woods. In other parts of 

 the year it generally grows warmer as one ascends. 



A higher temperature of the air above than l)elow is called a rever 

 sal of the vertical gradient, and it appears that in the woods this re- 

 versal occurs especially in summer. It also occurs in the open air 

 regularly at night and often becomes very marked ou a clear summer 

 night, especially toward morning. The gradient is at such times gen- 

 erally three or four degrees and may, under exceptionally favorable cir- 

 cumstances, be several degrees more. The same thing is true through- 

 out the day in the winter montlis, but the gradient is then not gener- 

 ally large. The action of the forest, therefore, tends to produce a ver- 

 tical distiibutiou of temperature like that over snow, or over level 

 fields on clear sunmier nights. It should l)e noted that this arrange- 

 ment is in favor of stability of tlie air. The warmer air is the lighter 

 and is on top. Still this tree-top air is, in the warm season, usually 

 cooler, and conserpiently heavier than the air at the same level out 

 side, as is shown by Fig. 38. This is true of the entire column of for- 

 est air — that is, air in the forest — and this heavier air will tend to flow 

 out. 



Fig. :i!).— Forest temiierature, (HHcrt-uce.s at Hulatte, iiiiilfr decitluous trees. 



TE3IPERATURE ABOVE FORESTS. 



Systematic observations above forests have been seldom taken. 

 Among them are those taken by M. Fautrat a few miles north of Paris, 

 in the forests of Ilalatte and Ermenonville. In the first the observa- 

 tions were under and over deciduous trees, oaks, and hornbeams, in 

 the second over pines. The soil at the latter place was a coarse quartz 

 sand, and the open field station was over a sandy plain. The surface 

 observations were taken at a height of 1.4 meters (4 feet 7 inches), 

 while those above trees were 14 meters (4() feet). The instruments 

 appear to have been close to the top of the pines, l)ut many feet above 

 the deciduous trees. The published observations are somewhat frag- 

 mentary, from two to ibur years being available. The: temi)erature dia- 

 grams near the ground are given in Figs. 39 and 40. The first is for 

 the deciduous forest and i.s of the familiar type. Fig. 40 exhibits some 

 striking peculiarities, c^hiefly in the exaggeration of the minima and 



