insolation

Amount of solar radiation (heat energy from the Sun) that reaches the Earth's surface. Insolation varies with season and latitude, being greatest at the Equator and least at the poles. At the Equator the Sun is consistently high in the sky: its rays strike the equatorial region directly and are therefore more intense. At the poles the tilt of the Earth means that the Sun is low in the sky, and so its rays are slanted and spread out. Winds and ocean currents help to balance out the uneven spread of radiation.