|STABLE VS. UNSTABLE CLOUDS
METEOROLOGIST JEFF HABY
It may seem that all clouds represent unstable air but it is not so. The words to distinguish between clouds
produced by instability and other clouds are stratiform for clouds forming in a stable layer and convective
clouds for clouds developing in an unstable environment. Convective clouds have vertical development and
often extend through a significant depth of the troposphere. Stratiform clouds are basically flat and
Instability is a situation in which rising air will continue to rise since it is less dense than the
surrounding air. Clouds developing in a deep layer of instability will often develop into thunderstorms.
Think of bubbles rising in a pot of water- that is instability being released in action. The bubbles
rise on their own without being forced to.
Stratiform clouds develop when the air becomes saturated and at least a minimal amount of forced lifting takes
place. Forced lifting is different from instability in that the air is not rising on its own but
rather being forced up (through low level convergence or upper level divergence). Think of a weight
lifter doing a bench press. The weights do not rise
on their own but rather are being force lifted by the muscles.
Convective clouds have texture on visible imagery. The texture is produced by shadows being cast from the
cloud. Convective clouds are bright white on visible imagery since they are thick (high moisture
content and vertically developed). Convective clouds on infrared imagery have very cold cloud tops
when thunderstorms occur. The overshooting top will have the coldest temperature. Convective clouds
can be seen ahead of cold fronts and within warm and humid air masses.
Stratiform clouds can cover great areas. They are often seen around warm front boundaries and where
one air mass is moving over another air mass. They are also seen spiraling around
low pressure systems. Most winter precipitation falls from stratiform clouds. Fog development is
a stratus cloud on the ground.
When you see clouds in the sky or on a satellite image
think about if it is a convective or stratiform cloud. Some clouds
have characteristics of both. Some times convective clouds will be within a shallow layer of
instability and thus will not extend a significant vertical distance (fair weather cumulus).