It has to do with the thermal structure of the atmosphere. In general, the atmosphere in the summer is warmer in the surface layer (the lower several hundred feet) than it is above that. Clouds, especially cumulus clouds, form when bubbles or columns of warm air rise upward from the surface into colder air aloft, chill and condense into visible clouds. Air surrounding the rising columns sinks to replace it. These motions produce scattered clouds with interspersed clear areas. But in the winter the temperature situation is reversed: It’s colder in the surface layer and warmer aloft. Clouds, mainly stratus types of clouds, form in the boundary between cold air underneath and warmer air above, and they tend to be flat and spread out and occupy most of the sky.
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