On a night when the sky is clear, the temperature may dip considerably; on a cloudy night the temperature will usually dip much less. The reason for this difference is that

The correct answer is A. the clear sky allows radiant energy to escape readily from the earth.

On a clear night, the earth’s surface radiates heat energy into space. This heat energy is in the form of infrared radiation. The atmosphere is transparent to infrared radiation, so the heat energy can escape into space without being absorbed by the atmosphere.

On a cloudy night, the clouds block the infrared radiation from escaping into space. The heat energy is instead absorbed by the clouds, which warms the clouds. The warm clouds then radiate heat energy back to the earth, which prevents the earth’s surface from cooling as quickly.

Here is a more detailed explanation of each option:

• Option A: The clear sky allows radiant energy to escape readily from the earth. This is the correct answer.
• Option B: On a cloudy night the clouds are warm and therefore retard the cooling. This is incorrect. The clouds are actually cooler than the earth’s surface, so they do not retard the cooling.
• Option C: The clear sky allows the cold of outer space to reach the earth. This is incorrect. The cold of outer space is not a significant factor in the cooling of the earth’s surface.
• Option D: Current of air carry the heat away from the earth on a clear night. This is incorrect. The currents of air do not carry the heat away from the earth on a clear night. The heat energy is instead radiated into space.