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Lifting by Convection
upward moving thermals

In meteorology, convection refers primarily to atmospheric motions in the vertical direction.

As the earth is heated by the sun, bubbles of hot air (called thermals) rise upward from the warm surface. A thermal cools as it rises and becomes diluted as it mixes with the surrounding air, losing some of its buoyancy (its ability to rise).

An air parcel will rise naturally if the air within the parcel is warmer than the surrounding air (like a hot air balloon). Therefore, if cool air is present aloft with warm air at lower levels, thermals can rise to great heights before losing their buoyancy.

Successive thermals following the same path usually rise higher than previous ones, and if a thermal is able to rise high enough to cool to its saturation point, the moisture within condenses and becomes visible as a cloud.

[Image: thermals fueling the development of a cumulus tower (76K)]
Photograph by: Holle

When a deep stable layer exists just above the cloud base, continued vertical growth is restricted and only fair weather cumulus are able to form. However, if a deep unstable layer (cold air aloft) is present, continued vertical growth is likely, leading to the development of a cumulonimbus cloud, which contains raindrops. Once the supply of thermals is cut off, the cloud begins to dissipate and eventually disappears. Convective clouds are typically much more vertically developed than those clouds generated by convergence lifting.

rising air
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Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.