Insolation over the Globe

Topics: Sun, Equator, Earth Pages: 2 (516 words) Published: December 3, 2013
Insolation over the Globe

Most natural phenomena on the Earth’s surface—from the downhill flow of a river to the movement of a sand dune to the growth of a forest—are powered by the Sun, either directly or indirectly. Although the flow of solar radiation to the Earth as a whole remains constant, different places on the planet receive different amounts of energy at different times. What causes this variation? Incoming solar radiation is known as insolation. It is a rate of flow of energy and is measured in units of watts per square meter (W/m2).

Daily insolation is the average flow rate over a 24-hour day, while annual insolation is the average flow rate over the entire year. Insolation depends on the angle of the Sun above the horizon. It is greatest when the Sun is directly overhead, and it decreases when the Sun is low in the sky, since the same amount of solar energy is spread out over a greater area of ground surface. Daily insolation at a location depends on two factors: (1) the angle at which the Sun’s rays strike the Earth, and (2) how long the place is exposed to the rays.

At mid-latitude locations in summer, for example, days are long and the Sun rises to a position high in the sky, heating the surface more intensely. How does the angle of the Sun vary during the day? It depends on the Sun’s path. Near noon, the Sun is high above the horizon—the Sun’s angle is greater, and so insolation is higher.

The seasonal pattern of daily insolation provides a convenient way to divide the globe into broad latitude zones. The equatorial zone encompasses the Equator and covers the latitude belt roughly 10° north to 10° south. Here the Sun provides intense insolation throughout most of the year, and days and nights are of roughly equal length. Spanning the Tropics of Cancer and Capricorn are the tropical zones, ranging from latitudes 10° to 25° north and south.

A marked seasonal cycle exists in these zones, combined with high annual insolation....
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