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Radiation is a form of heat transfer that is completely independent of the medium through which it travels. All bodies, whatever their temperature, emit heat energy in the form of short electromagnetic waves that travel through space at the speed of light. The actual wavelength depends on the temperature of the radiating body. The hotter the body, the shorter the wavelength and the more intense the emission. At very high temperatures, the body emits both heat and light, eg fire. The surface temperature of the sun is something in the nature of 6,000 degrees celsius. Of the sun's radiant energy that strikes the Earth, much is absorbed and the surface temperature rises. The surface then emits long invisible heat waves back into space. Some of the incoming short-wave radiation from the sun is lost through absorption, reflection and scattering by cloud. A thick cloud will reflect nearly 80 percent of the radiation that it receives. Very little is absorbed, probably about 7 percent. Water vapour and cloud, when present, strongly absorb most of the outgoing long-wave radiation. Some of this is re-radiated into space and some re-radiated downwards to the Earth's surface. This partly compensates for the heat lost by outgoing radiation. This is called the greenhouse effect. It explains why, when there is a thick cloud layer at night, the fall in surface temperature during the hours of darkness is less than on nights when there is a clear sky allowing free terrestrial radiation.