In meteorology, convective available potential energy is a measure of the capacity of the atmosphere to support the vertical movement of air that can lead to cloud formation and storms. As air rises in an atmosphere, it expands and cools. CAPE exists when a given mass of air can ascend and remain warmer than the surrounding air. The warm parcel is less dense than the surrounding air and accelerates upward. While the rate of cooling for an ascending parcel of dry air would quickly cool that parcel below the surrounding air temperature in most cases, water vapor within a parcel of moist air "releases heat" if it condenses. This slows the air parcel's rate of cooling and may keep the parcel warmer than the surrounding air across a particular depth of the atmosphere. The continued ascent of relatively warm and moist air can stimulate the formation of cumulus or cumulonimbus clouds and fuel thunderstorms.