The Earth’s Heat Budget is a description of what happens to the
radiation Earth receives from the sun. The Earth needs this radiation to drive forces such as the circulation of the
atmosphere and the
ocean. This energy and the way the atmosphere and land respond to is, gives us our weather and climate.
In the summer months, June-August, the northern hemisphere receives more radiation than the southern hemisphere because the Earth is tilted on its axis (at 23.5 degrees). The equator receives more energy throughout the whole year than the poles because the sun’s rays do not strike at an angle.
The atmosphere interferes with solar radiation in two ways: scattering and absorption (or re-radiation). All molecules tend to scatter radiation, the character of the scattering depends on the wavelength of the radiation relative to the size the molecules or other particles in the atmosphere. Our atmosphere is made up of very small molecules relative to wavelengths of the sunlight, and because of this, shorter wavelength radiation is scattered more effectively than longer wavelengths through a process known as Rayleigh scattering. This is the reason for the sky being blue. The many tiny molecules in the Earth’s atmosphere favour the shorter wavelengths, such as blue light.
Gases and aerosols absorb some solar radiation. It is usually relatively small and less important than scattering. However, ultraviolet radiation is strongly absorbed by ozone, which results in shielding the surface from UV radiation.
Of all the radiation that is received by Earth’s atmosphere:
31% is reflected back into space by the atmosphere (mainly clouds)
4% is reflected back by the Earth’s surface
59.5% is re-radiated from the Earth's atmosphere back into space
5.5% is re-radiated from the Earth's surface back into space
Reflection means that the sun’s light ‘bounces’ off the surface, not changing the wave length.
Re-radiation means that the radiation is absorbed by the material and heats it. The mean global surface temperature of the earth is 15 degrees Celsius, however because much of the Earth’s atmosphere is cooler than the surface, Earth emits radiation to space equivalent of a body with a temperature of -18 degrees Celsius and it is long wave radiation. Much of this is trapped in the atmosphere by so-called
greenhouse gases, heating the atmosphere- producing the
Greenhouse Effect.
The proportion of this radiation that is reflected outward is known as the surface albedo. Some examples of the surface albedo values for Earth are:
Tropical forest- 13%
Woodland- 14%
Farmland/natural grassland- 20%
Semi-desert/stony desert- 24%
Dry sandy desert/salt plans- 37%
Water (0-60 degrees) - <8%
Water (60-90 degrees) - <10%
Sea ice – 25-60%
Snow-covered vegetation – 20-80%
Snow-covered ice – 80%
Most of the radiation that hits the Earth’s surface is absorbed by dark soil and rock. The oceans also absorb a lot of the energy it receives, (but mostly reflects if the rays are not perpendicular to the surface) Water has a high heat capacity, which means that even though it absorbs large amounts of heat, its temperature will only rise a little.
In this way, the Earth keeps itself at equilibrium, by not warming up or cooling down. (This is assuming no effects from the Greenhouse Effect.)
Sources:
http://www.sfos.uaf.edu/msl111/notes/heat.html
http://experts.about.com/q/1359/3033204.htm
For more indepth information:
http://www.oceansonline.com/heat.htm
Diagram, very helpful:
http://www.sfos.uaf.edu/msl111/notes/graphics/heatbudget.jpg