Page:Handbook of Meteorology.djvu/39

Rh settles on a level surface or in a basin. It is pretty apt to be noticeable when a cloud layer separates two layers of air; the upper layer may be the warmer; indeed, the airman is quite apt to find a higher temperature above than below. Above a height of 2 miles, when the air is moderately still, the fall in temperature is apt to be fairly uniform. At a height varying from nearly 7 to 10 miles the fall in temperature ceases. Above this plane it remains stationary, or perhaps it rises. In one instance, a steady rise of temperature was observed between the altitudes of 8 miles and 20 miles.

The plane which separates the stratum of falling temperature from that of stationary temperature is sometimes, but rather loosely, called the isothermal layer. It varies in height, being highest at the equator; it is likewise higher in summer than in winter. It separates the shell of the atmosphere into two distinct layers—the stratosphere, and the troposphere.

The air of the stratosphere is remarkable chiefly for its apparent inertness. At its lower part the temperature does not vary much from &minus;67° F (&minus;55° C). If, as seems probable, there is a rise of temperature with increase of altitude, the rise is normal rather than abnormal. It seems to be due to the fact that the base of the stratosphere is chilled