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24 13.5 hours to 24 hours. The possible hours of daily sunshine vary according to month and according to latitude.

Polar and tropical circles are the boundaries, not of climatic, but of light zones. The duration of daylight is of great importance; it governs, in no small degree, the maturing of crops, and therefore concerns practically all agricultural industries. In general, the regions of greatest productivity of staple food-stuffs are those in which the summer days are from 14 hours to 16 hours long. Both the navigator and the aviator must know whether he heads in the direction of increasing or of decreasing hours of daylight at any particular time of the year.

Climatic Zones.—Climatic zones correspond pretty closely to light zones, in position; but their boundaries are very irregular lines, called isothermal lines—that is, lines along which the annual mean temperature is the same. For all practical purposes, the climatic torrid zone is the zone where frost does not occur except at very high altitudes. Similarly, the southern-most line at which frost may occur is the southern boundary of the north temperate zone; and the line of mean temperature of 32° (0° C) may be considered its northern boundary. A more practical boundary is sometimes fixed at the northern limit at which barley will mature.

Climatology is chiefly concerned with the regions which will produce foodstuffs, and therefore sustain life. To a lesser degree it is concerned with the problems which affect transportation. In any case the problems are mainly those of temperature, pressure, moisture, wind and sunshine.

The Diffusion of Warmth.—The warmth of the various parts of the earth is modified chiefly by the movements of the air. Because of the vertical rays of the sun in equatorial regions, the air is not only warmed to a much higher temperature, but it is also warmed more quickly than in higher latitudes. Being expanded by the greater warmth, it becomes specifically lighter and is pushed upward by the denser cold air which flows in to take its place. The updraught of air flows poleward in upper currents, until it is chilled and descends to the surface again. A part of the descending current continues poleward but a considerable part flows back to tropical regions as a surface wind.