Clouds are most likely to form in a previously clear sky when there is an increase in humidity and a cooling of the air. This can occur due to several factors:
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Temperature Changes: As the temperature drops, the air can’t hold as much moisture, leading to condensation and cloud formation.
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Weather Fronts: The approach of a cold front can lift warm, moist air, causing it to cool and condense, forming clouds.
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Surface Heating: On sunny days, the ground heats the air above it. As this warm air rises, it can cool and condense if there's enough moisture, leading to cloud formation.
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Orographic Lift: If air is forced to rise over mountains or hills, it can cool and create clouds even in clear conditions.
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Humidity Increase: Local sources of moisture, such as bodies of water or changes in vegetation, can raise humidity levels, contributing to cloud formation.
In summary, a combination of rising air, cooling temperatures, and increased humidity is necessary for clouds to form from a previously clear sky.