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26 convectional movements with which are classed the cyclones and the anticyclones.

Temperature and Altitude.—The effects of altitude on temperature may be considered in two aspects—altitude along a sloping surface, such as that of a mountain range, or a high plateau, and altitude above the surface, directly into the air. Altitudes are measured usually from mean sea level.

The variations in temperature of the various plains, plateaus, and mountain ranges are very great. In general, the temperature decreases with altitude until, in tropical regions, the limit of perpetual snow is reached at a height of about 16,000 feet; it decreases with increase of latitude until, in circumpolar regions, the snow line is not much above sea level. The variations of temperature with height are governed by so many conditions that specific rules apply to specific localities only.

The study of the relations between temperature and vertical altitudes is a matter of great importance in meteorology, and it has been prosecuted diligently during the last quarter of a century in various parts of the United States, Canada, Europe, South America and Africa.

Many thousand flights have been made by kites, manned balloons, captive balloons, pilot balloons, sounding balloons, airplanes and dirigible airships. At Uccle, Belgium, a pilot balloon reached an altitude of 20.1 miles, or 32,430 meters. Up to an altitude of about 9 miles, temperature and pressure statistics of the air have been obtained for about every thousand feet of altitude; beyond that plane the measurements are incomplete.

The fall in temperature with the increase in altitude has been in the traditional ratio of 1° F for every 300 feet —the conventional temperature gradient. This has been a convenient ratio for general purposes, but it cannot be used in specific cases. Within the first 2 miles the temperature gradient is very irregular; at times there is even a rise in temperature with increased altitude; that is, the temperature gradient becomes negative. The rise in temperature with increasing altitude is technically known as inversion.

Inversion may occur in winter, when comparatively still air