HKDSE Geography/M7/Radiation Budget

The earth's solar radiation budget is made up of two components: Incoming radiation and outgoing radiation.

Incoming solar radiation

What is insolation? Simple.

Insolation - Incoming solar radiation

Energy transmitted from the sun to the earth is solar radiation. They are in the form of electromagnetic waves. The type of radiation depends on the electromagnetic spectrum. Around 99% of the insolation is belongs to the middle range of the electromagnetic spectrum: ultraviolet, visible light and infrared.

Solar radiation is also known as short-wave radiation. There is also long-wave radiation, which we will look at later.

Solar radiation is measured in Watts per square metre (W/m²).

Surplus and Deficit

When incoming radiation exceeds outgoing radiation, the net radiation is positive and a surplus occurs. When outgoing radiation exceeds incoming radiation, the net radiation is negative and a deficit occurs.

Low latitudes tend to have surplus budgets and high latitudes tend to have deficits. This will be explored later.

Transfer processes

Short-wave radiation processes:

• After insolation enters the atmosphere, it may be:
  • Reflected and scattered by the clouds
  • Reflected by the atmosphere
  • Absorbed by the atmosphere
• After it reaches the ground,
  • It can be reflected by the earth's surface.
  • It can be absorbed by the earth's surface.

Long-wave radiation

• After heat energy is absorbed by the earth's surface,
  • It can be re-radiated back to the atmosphere. This is long-wave radiation or terrestrial radiation.
• The terrestrial radiation will then go these processes:
  • It may be radiated directly into space.
  • It may be radiated into the atmosphere.
    • Some of it will be radiated into space.
    • Some of it will be trapped in the atmosphere by greenhouse gases. This is the greenhouse effect.
      • The heat trapped by greenhouse gases will be re-radiated back to the earth's surface as counter-radiation.
  • It may return to the atmosphere through conduction, convection and evaporation.