Solar Radiation, Heat Balance and Temperature — Class 11 Geography

The earth receives almost all its energy from the sun as short-wave incoming solar radiation, called insolation; on average the top of the atmosphere receives 1.94 calories per sq cm per minute. Because the earth is a geoid, the sun's rays fall obliquely and only a small portion is intercepted. The earth is farthest from the sun (aphelion) on 4 July and nearest (perihelion) on 3 January. Insolation varies with the rotation of the earth, the angle of the sun's rays, the length of day, atmospheric transparency and configuration of land. The atmosphere is heated by conduction, convection, advection and terrestrial radiation. The heat budget keeps the earth's temperature constant: of 100 units of insolation, 35 are reflected (the albedo), 65 are absorbed and ultimately returned, balancing the total received. There is a surplus of net radiation between 40°N and 40°S and a deficit near the poles, redistributed pole-ward. Temperature is controlled by latitude, altitude (normal lapse rate of 6.5°C per 1,000 m), distance from the sea, air masses and ocean currents. Temperature distribution is shown by isotherms, and at times the normal lapse rate is reversed in an inversion of temperature.