rises, causing water vapor in the atmosphere to condense onto nuclei (small dust particles), forming water droplets and ice crystals. These conditions usually happen at the leading edge of a warm front, where cirrostratus clouds thicken and lower until they transition into altostratus clouds. Alternatively, nimbostratus clouds can thin into altostratus. Altostratus can even form from the spreading of the upper anvil cloud or the middle column of a thunderstorm.

Altostratus