Insolation (from Latin insolare, to expose to the sun) is the total amount of solar radiation energy received on a given surface area during a given time. It is also called solar irradiation and expressed as “hourly irradiation” if recorded during an hour or “daily irradiation” if recorded during a day.
Absorption & Reflection :
The object or surface that solar radiation strikes may be a planet, a terrestrial object inside the atmosphere of a planet, or an object exposed to solar rays outside of an atmosphere, such as spacecraft. Some of the radiation will be absorbed and the remainder reflected. Usually the absorbed solar radiation is converted to thermal energy, causing an increase in the object’s temperature. Manmade or natural systems, however, may convert a portion of the absorbed radiation into another form, as in the case of photovoltaic cells or plants. The proportion of radiation reflected or absorbed depends on the object’s reflectivity or albedo.
Direct insolation is the solar irradiance measured at a given location on Earth with a surface element perpendicular to the Sun’s rays, excluding diffuse insolation (the solar radiation that is scattered or reflected by atmospheric components in the sky). Direct insolation is equal to the solar constant minus the atmospheric losses due to absorption andscattering. While the solar constant varies with theEarth-Sun distance and solar cycles, the losses depend on the time of day (length of light’s path through the atmosphere depending on theSolar elevation angle), cloud cover, moisture content, and other impurities. Insolation is a fundamental abiotic factor affecting the metabolism of plants and the behavior of animals.
Heat Budget :
The Earth and the atmosphere are heated by energy from the sun. Theatmospheric heat budget of the Earth depends on the balance between insolation and out going terrestrial radiation. This budget has remained constant over the last few thousand years.
The amount of energy received from the sun is determined by;
• The solar constant – varies slightly and affects longer term climate rather than short term weather variations.
• The distance from the sun – the eccentric orbit of the Earth can cause a variation of up to 6% in the solar constant.
• The altitude of the sun in the sky – the equator receives more energy as solar radiation strikes the Earth head on, whereas at 60 N or 60 S the angle creates twice the area to cover and increases the amount of atmosphere to go through.
• The length of day and night –
The Earth receives energy from the sun as insolation. Some is lost as it passes through the atmosphere but overall the surface has a net gain of energy, the exception being the polar regions. Only about 24% of this insolation reaches the surface as it is either absorb, reflected or scattered
The atmosphere in contrast has a net deficit of energy. Because of this difference, heat is transferred from the surface to the atmosphere by radiation, conduction and by the release of latent heat
Heat budget by latitude
There are variations in energy and heat between latitudes. Low latitudes have a net surplus of energy, mainly because of their relative proximity to the sun. The high latitudes (pole wards of 40 N and 40 S) have a net deficit. As the tropics are not heating up and the poles are not cooling down, a transfer of heat must occur.
This occurs by:
• Horizontal heat transfers: air movement (winds, 80%, including the jet streams, hurricanes and depressions) and water movement (ocean currents).
• Vertical heat transfers: energy is transferred from the warm surface vertically by radiation, conduction and convection. Latent heat also helps to transfer energy, e,g, when water is evaporated. This energy is released when condensation occurs in the upper atmosphere