Clouds typically form when moisture in the air condenses, and several factors can lead to this process occurring in a previously clear sky. Some of the most likely causes include:
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Cooling of the Air: When warm air rises, it can cool as it ascends. If the air temperature drops to the dew point, moisture in the air can condense into tiny droplets, forming clouds. This can happen when warm air is forced to rise over mountains (orographic lifting) or due to frontal systems.
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Increase in Humidity: An influx of humid air can introduce additional moisture into a clear sky, leading to cloud formation. This can happen with the approach of a warm, moist air mass or a change in wind direction that brings in moist air from over a body of water.
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Convergence: When winds from different directions converge, the air is forced to rise. This upward motion can cool the air and lead to cloud formation, even in previously clear conditions.
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Local Heating: Ground heating (from the sun, for example) can cause the surface air to warm up, rise, and cool at higher altitudes, resulting in cloud formation.
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Weather Fronts: The passage of a front (such as a warm front or cold front) can cause a transition from clear to cloudy conditions. Warmer air is forced over cooler air at a warm front, leading to cloud formation.
In summary, cooling of the air, increases in humidity, air convergence, local surface heating, and the impact of weather fronts are the key processes that can lead to cloud formation in a previously clear sky.