Ask Jack: Rising Air

By Jack Williams ©2015

   Latent heat supplies much of the energy for supercell thunderstorms like this one that pelted Chaparral, N.M., with two-inch hail stones in April 2004. NOAA photo by Greg Lundeen.

Latent heat supplies much of the energy for supercell thunderstorms like this one that pelted Chaparral, N.M., with two-inch hail stones in April 2004. NOAA photo by Greg Lundeen.

Q: Why does moist air cool less rapidly than drier air when rising? What would be a good analogy for this?

A: The fact that water vapor, which is condensing to form cloud drops, slows the cooling of rising air is a perfect example of the importance of latent heat in weather.

Chapter 4 of The AMS Weather Book explains latent heat in detail, including how eighteenth century scientists discovered it.  In brief, when water evaporates to become invisible water vapor (humidity) in the air, it takes up heat from its surroundings. For example, the evaporation of perspiration cools you on a hot day because the water vapor carries away heat from your body.

As air rises and cools, eventually its water vapor will begin condensing to form tiny water drops that make up a cloud. When water vapor condenses it gives up the latent heat it gained when it evaporated, which warms the surroundings.

For an analogy, you can think of rising air as being like the air in a room that an air conditioner is cooling. When condensation begins, it’s like turning on a separate heater in the room.  As the air conditioner is working to cool the air, the heater is adding heat, but not enough to completely overcome the cooling by the air conditioner. It does, however, slow the rate of cooling.

Such release of latent heat supplies much of the energy that powers thunderstorms and hurricanes. The AMS Weather Book’s chapters on thunderstorms and hurricanes explain in some detail how this works. In other words, latent heat is one of nature’s most powerful sources of energy.

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