Earth's Energy Balance

Albedo - values for different surfaces/global average

A portion of the incoming solar radiation is absorbed by the surface and a portion is also reflected away. The proportion of light reflected from a surface is the albedo. Albedo values range from 0 for no reflection to 1 for complete reflection of light striking the surface. Albedo can be expressed as a percentage (albedo multiplied by 100). For instance, grass has an albedo of about .23 (range.15-.25). This means that of the incoming solar radiation that strikes the grass, 23% of it is reflected away. On the other hand, highly reflective surfaces like snow have an albedo upwards of .90, or 90% of sunlight is reflected away. The average albedo of Earth is approximately 0.31 (31%).

Thermals (convective processes)

A thermal column (or thermal) is a column of rising air in the lower altitudes of the Earth's atmosphere; Thermals are created by the uneven heating of the Earth's surface from solar radiation, and an example of convection

Non-radiative heat transfer

Available net radiation is used to do work in the Earth system; The principal use of this energy is in the phase change of water (latent heat), changing the temperature of the air (sensible heat), and subsurface (ground heat); That is, conduction or convection/advection are responsible for the transfer of heat (thermal energy), not electromagnetic radiation

Influence of coastal environment on seasonal variations in temperature - importance of differences in specific heat of land and water

Continentality. Continentality is the tendency of land to experience more thermal variation than water, due to the land's lower specific heat capacity.

Geographic and seasonal changes in surface temperatures

Does not vary much at the equator, varies a lot at the poles.

Electromagnetic radiation

Electromagnetic radiation travels through space in the form of waves. Unlike heat transfer by convection or conduction, heat transfer by electromagnetic radiation can travel through empty space, requiring no intervening medium to transmit it. The quantity of energy carried in a wave is associated with the height or amplitude of the wave. Everything else being equal, the amount of energy carried in a wave is directly proportional to the amplitude of the wave. The type or "quality" of radiation depends on the wavelength, the distance between successive crests. The greater the distance between wave crests, the longer the wavelength.

Changes in atmospheric density, pressure and temperature with altitude

From the surface, temperature decreases with altitude, as the air becomes thinner. Air pressure reduces with altitude due to the reduction in the mass of overlying air. This reduction in pressure is associated with a reduction in air density.

Subsurface heat (ground)

Heat is transferred from the surface downwards via conduction; Heat is transferred downwards when the surface is warmer than the subsurface (positive ground heat flux); If the subsurface is warmer than the surface then heat is transferred upwards (negative ground heat flux)

Conduction/Convection - Transfer from surface (land and water) to atmosphere

Initial transfer of thermal energy from surface to air via conduction; Air at surface is warmer than surrounding air and becomes buoyant, transferring thermal energy via convection

Latent heat (Changes in energy state during evaporation/condensation)

Latent heat is the stored energy released or absorbed when a substance changes from one state to another (evaporation, condensation and freezing of water)

Energy Balance

Net Energy Exchange = 0 (Inputs-Outputs)

Net Radiation Balance

Net radiation can be positive, negative, or even zero. Net radiation is a positive value when there is more incoming radiation than outgoing radiation. This typically occurs during the day time when the sun is out and the air temperature is the warmest. At night, net radiation is usually a negative value as there is no incoming solar radiation and net longwave is dominated by the outgoing terrestrial longwave flux. Net radiation is zero when the incoming and outgoing components are in perfect balance, which doesn't occur too often.

Net shortwave radiation

Net shortwave radiation is the difference between incoming and outgoing shortwave radiation. During the day, K* is a positive value as incoming always exceeds outgoing shortwave radiation. At night, K* is equal to zero as the Sun is below the horizon.

Sensible heat

Sensible heat is heat energy transferred between the surface and air when there is a difference in temperature between them; Heat is initially transferred into the air by conduction as air molecules collide with those of the surface; As the air warms it circulates upwards via convection; Thus the transfer of sensible heat is accomplished in a two-step process

Shortwave

Shortwave radiation from the Sun penetrates through space to the outer edge of the atmosphere unimpeded by the vacuum of outer space. If one places a surface oriented perpendicular to an incoming beam of light, 1360 W m of solar radiation will be received. This value is known as the solar constant but actually varies by a small amount as the Earth-Sun distance changes through the year. When the solar constant is averaged over the entire Earth surface we get a value of 342 W m Once solar radiation begins to penetrate through the atmosphere this amount begins to decrease due to absorption and reflection.

Specific heat (relative values for water and air)

Specific heat is the amount of energy needed to raise the temperature of 1 gram of material by 1 C. Specific heat is different for every substance, a function of the molecular structure. Water has an unusually high specific heat.

Temperature/heat

Temperature is proportional to the average speed of the atoms or molecules in an object (matter).

Seasonal variations in incoming shortwave (solar radiation)

The Earth's axis (23.5 degrees tilted)--defines the tropics. Also, the revolution of the Earth around the sun over the course of the year.

Geographic (latitude) variations in incoming shortwave (solar radiation)

The curvature of the Earth. When the Sun shines directly over the equator, the equator receives the most intense solar radiation, and the poles receive little.

Net longwave radiation

The difference between incoming and outgoing longwave radiation is net longwave radiation

Longwave radiation

The energy absorbed at the surface is radiated by the Earth as terrestrial longwave radiation (L↑). The amount of energy emitted is primarily dependent on the temperature of the surface. The hotter the surface the more radiant energy it will emit.

Greenhouse effect

The gases of the atmosphere are relatively good absorbers of longwave radiation and thus absorb the energy emitted by the Earth's surface. The absorbed radiation is emitted downward toward the surface as longwave atmospheric counter-radiation (L↓) keeping near surface temperatures warmer than they would be without this blanket of gases. This is known as the "greenhouse effect".

Maximum wavelength - function of temperature

The maximum wavelength at which a body emits radiation depends on its temperature. Wein's (pronounced "weens") Law states that the peak wavelength of radiation emission is inversely related to the temperature of the emitting body. That is, the hotter the body, the shorter the wavelength of peak emission. The figure below shows the wavelengths over which the sun and earth emit most of their radiation. The Sun being a much hotter body emits most of its radiation in the shortwave end and the Earth in the longwave end of the spectrum.

Radiation balance

The radiation balance of the Earth system is an accounting of the incoming and outgoing components of radiation. These components are balanced (incoming = outgoing) over long time periods and over the Earth as whole. If they weren't the Earth would be continually cooling or warming. However, over a short period of time, radiant energy is unequally distributed over the Earth, resulting in periods of localized heating and cooling.

Direct/diffuse radiation

The remaining solar radiation makes its way to surface as direct and diffuse solar radiation. Direct solar radiation (S) is shortwave radiation able to penetrate through the atmosphere without having been affected by constituents of the atmosphere in any way. Diffuse radiation (D) is shortwave radiation that has been scattered by gases in the atmosphere.

Dominant components of the atmosphere

nitrogen and oxygen