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INSTABILITY
You often hear weather forecasters talk about an unstable atmosphere and how
it helps spawn showers and thunderstorms. Stability is one of the main factors
that determine where showers and thunderstorms will develop and how strong they
will become. Due to the principle of density, a parcel of air will rise if it
is heated and becomes warmer than the surrounding environment. Unsaturated air
cools at a rate of 5.5 F per 1000 feet of altitude. In an unstable atmosphere,
the environment cools quicker than the parcel, allowing the parcel to continue
to rise. On the other hand, a stable atmosphere cools slower with height than
the parcel, causing the parcel to become colder than the surrounding air and
sink.
Moisture can make what
normally would be a stable atmosphere, unstable because of condensation. A saturated
air parcel cools at a rate of only 3.3 F per 1000 feet of altitude. This is
much slower than the unsaturated air parcel, and as a result, the rising, saturated
parcel remains warmer than if condensation were not taking place. The difference
in cooling rates is due to latent heat released by the condensing water.
Showers and thunderstorms
develop in areas of instability where air parcels are allowed to rise uninhibited
through the atmosphere. Any weather process that causes colder conditions aloft
relative to warmer conditions at the surface will lead to an unstable atmosphere.
Some of these weather processes include:
- Heating of the surface.
Sunshine or low level warm air advection heats the surface and the air near
it, making it relatively warmer than aloft.
- Cold advection aloft.
Cold air moves into the higher altitudes and causes temperatures to cool faster
than at the surface.
- Mixing of the air. Rising
air lowers the temperatures toward the top of the layer while sinking air
warms temperatures toward the bottom.
- Large scale rising of
the air. A layer of air expands and becomes thicker as it rises. The upper
part of the layer rises more than the bottom and cools more as a result. This
creates colder temperatures aloft relative to the surface.
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