Why does rising air usually mean cloudy weather?

Answer 1

Air that is rising experiences cooling because as it moves up in the atmosphere, it expands (density decreases) and its temperature cools. As the temperature decreases, the air's ability to hold water vapor decreases, meaning that at some point on its ascent, the air parcel will become saturated (it is holding the most water vapor that it possibly can). As the air rises even more, it will continue to cool (at a different rate than before, however), and as it does so, its ability to hold water vapor will continue to decrease, because saturation specific humidity-an air parcel's ability to hold water vapor-is directly related to temperature. At this point, the air parcel has more water vapor than its saturation specific humidity allows for, meaning that some of this water vapor must CONDENSE out, forming clouds. If the parcel cools even more, more condensation will happen, and eventually enough water droplets will form and coalesce, causing precipitation.