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In the mid-latitudes clear skies are typically associated with warm conditions during the summer, but are also often associated with very cold conditions in the winter. Why is this so?

Energy comes into the surface of the earth through clear skies and it leaves the earth's surface through clear skies. In the summer with long days the net energy gain is greater than the loss. In the winter with shorter days more is lost. Clouds act like a blanket that slows the temperature exchange and moderates the temperature of what is under it (you and me). We do not get as hot or as cold when there is a cloud cover.

You might want to research the term albedo.

Larry Krengel


In both cases, summer and winter, the physical cause of generally clear skies is the same, subsiding air in a high pressure area.

When air subsides (moves lower in altitude), its temperature increases, thereby reducing the relative humidity of the air, thus preventing condensation and therefore clouds from forming.

However, cumulus clouds can form even in high pressure areas if convective plumes of warmer and moister air (produced by the heating of small areas on the Earth's surface by the Sun) rise from the surface, and in the process, cool to the dew point, resulting in cumulus clouds in a generally clear sky.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory

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