In this sketch, the environmental temperature again shows "lapse" conditions,
but the lapse is much stronger. That is, the decrease of temperature with
altitude on this day was very strong, exceeding the rate of 10 degrees per
kilometer.
Suppose an air parcel in this atmosphere is forced to rise a bit by some
external influence. Again the parcel cools at 10 degrees per kilometer of
rising, under control of the basic laws of thermodynamics. But since the
surrounding environment is different, the results are different as well.
The rising parcel immediately becomes warmer than the environmental air
around it. The parcel is therefore warmer than its surroundings, less dense, and
lighter than its immediate surroundings. It therefore will continue to rise
under the influence of this buoyancy.
An atmosphere whose environmental temperature lapse is greater than the adiabatic
expansional cooling rate of 10 degrees per kilometer is said to be Statically Unstable
since the environmental air tends to enhance the upward motions of rising air. We might
also describe this as a positive feedback situation since pushing a parcel
upward leads to the environment pushing it further upward. In nature, statically unstable
conditions are usually accompanied by lots of vertical air motions, cloud formation,
and possibly storminess.
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