. Bulletin. Forests and forestry -- United States. 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 exceptional
. Bulletin. Forests and forestry -- United States. 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 Fig. :i!).—Forest temiierature, ( 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 meters (4 feet 7 inches), while those above trees were 14 meters (4() feet). The instruments
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