The ground is always cold at night because the

Step 1: Analyze the question It asks why the ground becomes cold at night. This is about heat transfer, specifically what the ground does with its heat once the sun has set.

Step 2: The concept All bodies above absolute zero emit thermal energy as infrared radiation.

• During the day, the ground absorbs solar radiation and gains more heat than it loses, so it warms up.
• At night, there is no incoming solar energy, but the ground keeps radiating heat out into the atmosphere and space. It loses more heat than it gains, so it cools.

Step 3: Check each option

• A. The atmosphere absorbs the sun's energy at night. Wrong. The sun's energy is not arriving at night, so there is nothing to absorb. This also does not explain why the ground cools.
• B. The earth radiates heat to the atmosphere at night. Correct. This is the actual cooling mechanism, called terrestrial radiation.
• C. The sun no longer shines at night. This is true, but it only explains why the ground stops heating. It does not explain why the ground actively loses heat. The ground is cold because of the heat it radiates away, which makes B the better answer.
• D. The atmosphere reflects the sun's energy at night. Wrong. There is no sunlight at night to reflect.

Step 4: Answer

Exam tip: Cold nights are clearer and colder when the sky is cloudless. Clouds trap and return some of the radiated heat, while a clear sky lets it escape. Remember: "Day: the ground absorbs. Night: the ground radiates."