Radiation

Radiation is the process by which the earth receives heat energy from the sun. The intensity of this solar radiation at the outer limits of the earths atmosphere is quite constant, but the amount that reaches the earths surface is highly variable, depending largely on the amount of clouds in the atmosphere.

The solar radiation that enters the atmosphere is reflected, scattered by molecules in the atmosphere, or absorbed by the atmosphere and the earths surface. On average, 25% of the incoming solar radiation is reflected by clouds with an additional 3% reflecting off the earths surface. Approximately 17% of the solar radiation will be scattered by molecules in the atmosphere (10 of the 17% ends up being absorbed by the surface while the remaining 7% is lost to space). The remaining incoming solar radiation is absorbed by the atmosphere and earths surface. Water vapor, ozone and carbon dioxide each absorb radiation within certain wavelengths, helping contribute to an average of 22% of incoming solar radiation being absorbed by the atmosphere. The remaining incoming solar radiation (33%) is absorbed by the earths surface, which when combined with the fraction absorbed after scattering leads to an average of 43% of the suns radiation is absorbed by the earths surface.

The radiation absorbed by the earths surface and atmosphere is converted to thermal energy, thus warming the substance. While the solar radiation that reaches the surface warms the surface, the earths average temperature does not change as the earth in turn radiates energy back to the atmosphere and space. The radiation emitted by the earth is very different from that emitted by the sun. Due to the much lower temperature the radiation is of a much longer wavelength which changes how the radiation passes through the atmosphere. Water vapor in the atmosphere is very effective at absorbing this longwave radiation and reradiating a fraction of it back toward the surface, slowing the net energy loss from the surface. This is why cloudy nights are typically warmer than nights with clear skies.

The net balance between incoming solar radiation and outgoing longwave radiation from the earth is responsible for controlling the timing and magnitude of daily high and low temperatures. The earth is continually trying to cool itself by emitting longwave radiation. Beginning at sunrise, solar radiation tries to warm the earth, with this heating peaking at noon and ending at sunset. The timing of maximum temperature does not coincide with the peak in incoming solar radiation as the earth will warm as long as there is a net gain in heat. Likewise the earth will continue to cool as long as there is a net loss of heat, leading to low temperatures occurring near sunrise.

The amount of solar radiation received by any point of the earth is greatly influenced by the tilt of the earths axis and its annual progression around the sun that create the different seasons. During the northern hemisphere summer, the northern hemisphere is tilted toward the sun, allowing the incoming solar radiation to strike the earths surface at a higher (more perpendicular) angle. At this time the southern hemisphere is tilted away from the sun causing the solar radiation to the earth at a lower angle. This lower angle diffuses the radiation over a larger area making it less efficient in heating the surface.