Oklahoma Climatological Survey

 

Earth's Energy Budget

Part 1

 

 

Principle

Absorption and re-emission of radiation at the earth's surface is only one part of an intricate web of heat transfer in the earth's planetary domain. Equally important are selective absorption and emission of radiation from molecules in the atmosphere. If the earth did not have an atmosphere, surface temperatures would be too cold to sustain life. If too many gases which absorb and emit infrared radiation were present in the atmosphere, surface temperatures would be too hot to sustain life.

 

 

Figure 1 - Radiation "Budget" for Incoming Solar Radiation

Incoming Solar Radiation
  • The sun radiates mostly in the visible band, but also in the ultraviolet (shorter wavelength).

 

  • When averaged globally and annually, only 51% of the solar radiation striking the earth and its atmosphere is absorbed at the surface.

    The atmosphere absorbs 19% of incoming solar radiation and the remaining 30% is reflected back into space.

  • The shortest wavelengths of solar radiation (those most dangerous to life) are absorbed by molecules in the upper and middle atmosphere.

    In particular, ozone selectively absorbs ultraviolet radiation while allowing visible radiation to pass through relatively unhindered.

  • Reflection significantly affects the solar radiation that reaches the ground, as the sun's rays could be reflected off of air molecules (termed scattering), clouds or the ground itself.

    Light-colored or shiny objects reflect more radiation than dark objects. Energy that is reflected cannot be absorbed or transmitted through an object.

  • Different surfaces have different albedos (see Table 1).

    Meteorologists refer to the percent of radiation returning from a surface compared to the incident radiation as the "albedo". For example, the earth reflects an average of 30% of the incoming radiation, so the average albedo is 30%, or 0.3. Most of the earth's average reflection results from clouds.

  • Visible satellite imagery can be used only during the day, when visible solar radiation is striking the earth's atmosphere and surface.

 

 

Table 1- Average Reflectivity of Surfaces to Solar Radiation

 

Surface Type

% Reflected

Albedo

fresh snow

~90%

~0.9

thick clouds

~80%

~0.8

thin clouds

~40%

~0.4

ice

~35%

~0.35

soil

~20%

~0.2

forest

~5%

~0.05

ocean (high sun angle)

~5%

~0.05

ocean (low sun angle)

~95%

~0.95

 

 

Continue to Part 2


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