Page:Handbook of Meteorology.djvu/148

 To the pilot, the result is not merely impairment of visibility; it may be almost obscurity. The airmen who crossed the Atlantic emerged from the sea fog with plane wings thickly covered with ice.

The upward movement of turbulence, “the railway of the air” continues until resistance balances initial force—that is, to an altitude which practically is a lid. At this plane the fog spreads out, forming stratus clouds.

Convectional Movements.—A similar movement takes place when air is warmed. An ascending movement occurs at the focal area of warmth; descending air flows in to take its place. To a certain degree, these movements are planetary; in the tropics, ascending currents are the rule, and these are balanced by descending currents in higher latitudes. Planetary convectional movements are pretty well known and the limits of their procession and recession with the apparent motion of the sun are also known. The time, limits and location of the impairment of visibility resulting from these movements are also pretty well established. Indeed, the Coast Pilot Charts of the Hydrographic Office afford the information by which the loci of impaired visibility may be determined.

There are other examples of convectional movements which may be regional but are not planetary. The cyclonic movements are ascending convectional currents; the anticyclones are descending currents. The cyclone is very apt to be an area of impaired visibility, especially on the southeastern half, in which rain, snow and fog may be expected. The anticyclone may sweep snow or dust in blinding quantities up to a distance of a few hundred feet above ground; but it generally brings the conditions of best visibility.

Various causes bring about local updraughts of small area. Thus, during hot weather, a large area of bare rock surrounded by greensward becomes a local radiator of heat, and a sharp updraught results. Vision through such an updraught may become blurred, but it is not greatly impaired. The airman entering it gets a bump that rattles his plane, however. Descending currents in the shape of downdraughts of small area if rather strong are apt to be dust-raisers; they are “woollies” over water. There may be a slight blurring of outline due to refraction, but there is otherwise but little impairment of visibility.