Visible energy that comes in from the Sun is a short wavelength. This is absorbed and reradiated as a longer wavelength, such as infrared. This is because energy is lost during the transition. These longer waves are heat waves. If the reradiated energy is trapped, a greenhouse situation is created. The trapping mechanism can be clouds or greenhouse gases, as is the case on Venus. Carbon dioxide gas in the atmosphere acts like the glass in a hothouse. Basically, the Sun's rays check in, but they don't check out. A greenhouse situation is shown in Figure 1 . Earth's main greenhouse gases are carbon dioxide, water vapor, and methane.

Figure 1 Greenhouse effect.

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