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What is an unstable air mass?
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| To be unstable, the lowest layers of an air mass
must be so warm and/or humid that, if some of the air rises, then that air parcel is warmer than
its environment, and so it continues to rise. Normally in the atmosphere, if an air
parcel rises, it will be colder than its environment (due to expansion), and it will then
sink back down again. If the rising air parcel contains enough water vapor, however, then
condensation
of the water vapor in it occurs, and a cloud is formed. This condensation releases heat,
which warms the air parcel, which can cause the parcel to rise.
The most unstable air mass is one in which the lower layers are unstable to the first type of
instability (dry),and which also contain enough water vapor to form clouds, providing the second
type of instability (moist) as well. This is the basis for thunderstorm
formation. The more unstable the air mass, the more energy these is for thunderstorms to feed off of.
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| Interesting facts:
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| On a sunny day over land, the lowest 1,000-5,000 feet or so of the atmosphere is continuously unstable,
with warm air parcels rising from the surface, mixing the whole "boundary layer". This causes turbulence, which you
will feel when you take off or land in an airplane on a sunny day.
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