Radiational cooling

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Radiational cooling is the cooling of the Earth's surface through thermal radiation in the infrared frequency. Although our eyes are not sensitive to IR, we can feel changes in temperature. The invisible effect of radiational cooling is ongoing day and night, however; we only notice the drop in temperature at night when there is no solar heating to mask the effect. Radiational cooling is especially noticeable in clear sky conditions when the effect is most pronounced.

In general, the Earth re-radiates heat into outer space at about the same rate as the sun delivers heat, thus maintaining a relatively constant temperature on earth. For this reason, radiational cooling has a critical influence on climate and weather. Essentially, the Earth's temperature can only be balanced by the effects of radiation, since the Earth is perfectly insulated by the vacuum of space.

Radiational cooling may be inhibited by the insulating effects of greenhouse gases. However, the largest man-made inhibitor of radiational cooling is urban heat islands. Urban heat islands may disrupt the radiational cooling layer enough to keep cities up to 10 degrees warmer than the countryside. This has, in fact, caused some climate records to be overly warm, as some weather stations are surrounded by cities.