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Lifting by Convection
thermals bubbling upwards

When the earth is heated by the sun, bubbles of hot air, or thermals, rise up from the warm surface, cooling and expanding as they ascend. The thermal becomes diluted as it mixes with the surrounding air, loosing some of its 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.

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

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

If a deep unstable layer exists just above the cloud base, continued vertical growth is likely, possibly leading to the development of a cumulonimbus cloud. However, once the supply of thermals is cut off, the clouds begin to dissipate and eventually disappear. In contrast to convergence lifting, convective clouds are typically much more vertically developed and are fueled by stronger updrafts, sometimes in exceeding 50 miles/hour in the more powerful thunderstorms.

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.