Page:Handbook of Meteorology.djvu/15

Rh on the other hand, increases with altitude, and they probably form the outer layer of the atmosphere. Because of their lightness it is not unlikely that hydrogen and helium are gradually escaping from the earth.

Depth of the Atmosphere.—Observations on the twilight arch indicate that at a height of 40 miles above sea level the air has a density sufficient to refract, reflect, and diffract light. A measurement of the parallax of a meteor seen by two observers

at different stations indicates the existence of air at a height of 200 miles. Actual measurements, however, have not extended much higher than 20 miles, the height to which sounding balloons have reached.

At an altitude varying approximately from 6 to 7 miles, according to latitude and also according to the season, a plane of contact occurs which apparently separates an upper from a