Summary and Final Tasks

Summary and Final Tasks

Two applications of the theory of atmospheric radiation have been presented. The most important concepts used are:

• everything radiates
• solar visible irradiance strikes Earth on one side, but Earth radiates in the infrared in all directions
• the total energy for solar visible radiation absorbed in the Earth system closely balances the total energy for the infrared radiation going out to space
• the atmosphere is highly transparent in the visible and weakly transparent in the infrared.

For climate, these principles mean that water vapor, carbon dioxide, and other gases radiate energy to Earth’s surface, keeping it warmer than it would be if the atmosphere did not have these gases. This downward infrared radiation is the greenhouse effect, a natural phenomenon that has been enhanced by human activity, mainly fossil fuel burning. Clouds contribute to the greenhouse effect, but they actually do more cooling by reflecting visible radiation from the sun back to space.

For satellite infrared observations, some wavelength bands are in windows, so that the satellites see radiation from Earth’s surface. Other bands are completely absorbed by water vapor or carbon dioxide, so that the infrared getting to the satellite comes from the top of the water vapor column. Clouds are opaque in the infrared, so the satellite sees their tops, which are radiating at the temperature of that altitude.