Why is cloud cover very changeable?

Clouds form when relatively warm surface water (from oceans, lakes, snow, etc.) evaporates and rises into cooler air above. The cooler air cannot hold as much water as the warm air, so that the water vapor condenses back to it's liquid or solid form when the air temperature at that level matches its dew point, that is, when the air can no longer retain the suspended moisture, forming clouds in the form of water droplets, when the temperature of the air is above freezing, or ice crystals when the temperature is below freezing. These droplets or crystals gather around tiny dust particles that fall back to earth as rain or snow when they become heavy enough.
Clouds cover is changeable because not all water vapor condenses because the dew points are so low and the moisture content of the air is also low, as in the dry climates of Arizona where the sun almost always shines. It's not that there is no moisture in the air, but what little there is does form water vapor, and when it rises to higher levels, it cools, but the air is still so dry that it is able to retain that moisture without condensing back to its liquid form.

Thanks for your answer, gerry!