Moist convection

Moist convection

Warm *moist* air, however, behaves somewhat differently. This difference is crucial for the formation of thunderstorms. As the parcel of warm, moist air rises it initially cools by the dry adiabatic lapse rate, i.e. it cools by about 10 °C (9.8 °C) for every km it rises.

However, as it cools, the moisture the air contains begins to condense. As you may recall, the water going from vapour to liquid (to water droplets) releases heat, warming the air. Therefore moist warm air cools less quickly as dry warm air. As it rises and cools, more and more moisture condenses into water droplets, releasing latent heat and slightly warming the air. Additionally, as the moist air rises, water droplets begin to freeze, releasing more latent heat and slightly warming the air further.

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The rate at which rising moist air in which moisture has already began condensing, cools with height is the moist adiabatic lapse rate: it cools on average by less than 6 °C (typically 5.5-6 °C) for every kilometer it rises. So rising moist air cools only about 2/3 as fast as dry air.

Interesting fact: the moist adiabatic lapse rate is not constant – it depends on the temperature and pressure. At 1000 mbar and 20 °C it is 4.3 °C/km, at 0 °C and 600 mbar pressure it is 5.4 °C and -20 °C and 400 mbar pressure it is 7.3 °C/km.

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