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58 the shape of the shell of air composing it; but they may not cause a general circulation. The fact that the air of the stratosphere is warmer in high than in equatorial latitudes indicates that a circulation of some sort exists and that the general movement may be the reverse of that of the lower shell of air. The coldest air is over the warmest zone.

Winds Encountered by the Airman.—The marine pilot is concerned wholly with the horizontal movements of the surface air; he is not conscious of the updraughts or the downdraughts of convection. To the airman, on the other hand, the horizontal air movements are usually less of consequence than the vertical movements. Good air for flying must be free from holes and bumps.

An air hole is not a vacuous space, nor is it one in which the density of the air is abnormally low. Sometimes it is a downdraught; quite often it is convectionally still air. If the airman has been flying over hot, bare ground, where the updraught is strong, his plane takes a drop when he passes over a patch of greensward, where the updraught ceases; this is the airman's “hole.” In going from convectionally still air into an updraught, he gets a “bump.” The same result is apparent if, while traversing a downdraught, he strikes still air.

It has been noted that the air ranges itself in layers differing in density, temperature, and moisture content. In many cases an acquired sense born of experience enables the airman to discern these layers and to adjust the wings of his plane in encountering them. Frequently a sheet of smoke or dust separates two cloud layers and experience has taught how to avoid or how to penetrate it.

Air is either going up or coming down. Turbulent ascending currents are manifest in the rapid motion of cumulus clouds, both within the cloud and beneath it. Measurements have shown that ordinary updraughts may have a velocity of 10 feet per second; under a cumulus cloud of the thunderhead type the velocity may be as high as 40 feet per second. The updraughts that produce clouds give visible signs of their existence. Those caused during clear days by bosses of rock or by bare ground are not so easily detected. They are not apt to begin existence until the sun is high enough to heat the areas producing them; they rarely form during cloudy days.