This chapter discusses the temperature and thermal processes in the earth. The average temperature at the earth's surface varies between roughly 0°C and –40°C in the region of the North Pole, roughly –10°C and –50°C in the region of the South Pole, and is about 26°C throughout the year in the equatorial belt. The mean temperature at the earth's surface is about 15°C with average variations of about 2°C during the year. The temperature at a given point at the earth's surface depends mainly on the radiation from the sun that reaches it and the angle with the surface at which the radiation arrives. Other quantities involved include the radiation back towards the sky and the heat exchange by air currents. The average solar heat flow reaching the ground in the continents is of magnitude of 10–2 cal/cm2sec. Consequently, the heat flow from the earth's interior of magnitude 10–6 cal/cm2 sec is negligible in comparison. The temperature of ocean bottoms is controlled by the local temperature of the water, which in deep oceans is near 0°C. In continents, thermal gradients have been determined by measuring the temperature at various depths in many boreholes
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