The sun's path in the sky, which determines the flow of solar energy reaching the earth's surface-and hence governs the thermal environment of man-provides a basis for dividing the globe into latitude zones. It is not intended that the specified zone limits be taken as absolute and binding, but rather that the system be considered as a convenient terminology.
The equatorial zone lies astride the equator and extends to 10° latitude north and south. Within this zone, the sun throughout the year provides intense insolation, while day and night are of roughly equal duration. Astride the Tropics of Cancer and Capricorn are the north tropical zone and south tropical zone respectively, spanning the latitude belts 10° to 25° north and south. In this zone, the sun takes a path close to the zenith at one solstice and is appreciably lower at the opposite solstice. Thus a marked seasonal cycle exists, but is combined with a potentially large total annual insolation. The word tropics have been widely used to denote the entire belt of 47 degrees of latitude between the tropics of Cancer and Capricorn.
Immediately poleward of the tropical are transitional regions, which have become widely accepted among geographers as the subtropical zones. For convenience, these zones are here assigned the latitude belts 25° to 35° north and south, but it is understood that the adjective "subtropical" as applied to environment regions may extend a few degrees farther poleward or equator ward of these parallels.
The middle-latitude zones, lying between 35° and 55° north and south latitude represent regions in which the sun's path shifts through a relatively large range of neon altitudes, so that seasonal contrasts in incoming solar energy are strong. Strong seasonal differences in lengths of day and night exist as compared with the tropical zones.
Bordering the middle-latitude zones on the poleward side are the sub arctic zones. 55° to 60° north and south latitudes, transitional between middle-latitude and arctic zones.
Astride the Arctic and Antarctic circles, 66 1/2° north and south latitudes, lie the arctic zones, which may be further differentiated, if desired, into an arctic zone and an Antarctic zone. The latitudinal extent of the arctic zones is here specified as 60° to 75° north and south, but these limits should not be imposed severely. The arctic zones have an extremely large yearly variation in lengths of day and night, yielding enormous contrasts in incoming solar energy from solstice to solstice. Notice that classical usage, as found in standard dictionaries, considers the "arctic" or "arctic region" as the entire area from arctic circle to pole.
The polar zones, north and south, are circular areas between 75° latitude and the poles. Here the polar regime of six months day and six months night yields the ultimate in seasonal range of incoming solar radiation.
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