Because of its temperature, the atmosphere emits infrared radiation. For example, on clear nights Earth's surface cools down faster than on cloudy nights. This is because clouds (H2O) are strong absorbers and emitters of infrared radiation. This is also why it becomes colder at night at higher elevations.
Consider the following text:
David was visiting Europe last summer. First he visited London. To his dismay, it was overcast all night. Then he flew to Rome. There the sky was as clear as you could get at night. David was very happy about it.

In which place less infrared radiation would be absorbed in the atmosphere, London or Rome?