Rising Air
a key process in the production of clouds and precipitation

Imagine a block of air, or air parcel, rising upward through the atmosphere. The air parcel expands as it rises and this expansion, or work, causes the temperature of the air parcel to decrease.

As the parcel rises, its humidity increases until it reaches 100%. When this occurs, cloud droplets begin forming as the excess water vapor condenses on the largest aerosol particles. Above this point the cloud droplets grow by condensation in the rising air.

If the cloud is sufficiently deep or long lived, precipitation will develop.

The upward motions that generate clouds and lead to precipitation can be produced by convection in unstable air, convergence of air near cloud base, lifting of air by fronts and lifting over elevated topography such as mountains.

Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.