Because the earth is a sphere, only one point on earth - that upon which the sun's noon rays are perpendicular - presents a surface at right angles to the sun's rays. In all directions away from this point, the earth's curved surface becomes turned at a decreasing angle with respect to the rays until the circle of illumination is reached. Along that great circle the rays are parallel with the surface.
At any particular place on the earth, the quantity of insolation received in one day will then depend upon two factors:
(1) the angle at which the sun's rays strike the earth, and
(2) the length of time of exposure to the rays.
These factors are varied by latitude and by the seasonal changes in path of the sun in the sky.
Intensity of insolation is greatest where the sun's rays strike vertically, as they do at noon at the latitude equal to the sun's declination ranging between the tropics of Cancer and Capricorn. With diminishing angle, the same amount of solar energy spreads over a greater area of ground surface. Hence, on the average, the Polar Regions receive the least heat per unit area. This fact helps to explain the general distribution of average air temperatures over the globe, from a maximum at low latitudes to a minimum near either pole.
The earth's inclination produces seasonal differences in insolation at any given latitude, and these differences increase towards the poles, where the ultimate in opposites (six months of day: six of night) is reached. Along with the variation in angle of sun's rays operates another factor, the duration of daylight. At the season when the sun's path is highest in the sky, the length of time it is above the horizon is correspondingly greater. The two factors thus work hand in hand to intensify the contrast between amounts of insolation at opposite solstices.
Notice that the equator receives two maximum periods (corresponding with the equinoxes, when the sun is overhead at the equator) and two minimum periods (corresponding to the solstices, when the sun's declination is farthest north and south of the equator). At the Arctic Circle, 66 1/2°N, insolation is reduced to nothing on the day of the winter solstice, and with increasing latitude pole ward this period of no insolation becomes longer. All latitudes between the Tropics of Cancer and Capricorn have two maxima and two minima, but one maximum becomes dominant as the tropic is approached. From 23 1/2° to 66 1/2° there is single continuous insolation cycle with maximum at one solstice, minimum at the other.
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