This figure is a symbolic sketch of temperature versus altitude for the
atmosphere. The black curve represents a "sounding" made, for example,
by a weather balloon or meteorological satellite showing typically decreasing
temperature with increasing
altitude. A vertical distribution of air temperature which shows decreasing
temperatures with altitude (as in the sketch) is said to exhibit "temperature
lapse" conditions. If the environmental temperature increases with altitude,
it is said to exhibit "temperature inversion" conditions.
If an "air parcel" in this atmosphere is forced to move upward
for some reason, such as flow over a mountain range, the law of adiabatic
expansional cooling causes the
parcel temperature to decrease at the particular rate of 10 degrees centigrade per
kilometer of
lifting, as represented by the red line with an arrowhead. This rate of adiabatic
cooling upon lifting is based upon a fundamental law of thermodynamics, and is
independent of the general thermal stratification of
the rest of the air surrounding it.
In this particular illustration, the "Environmental Temperature Lapse" is less than
the 10 degree per kilometer rate of expansional cooling so that the air parcel
rising along the red curve arrives at an upper level colder than the environment
at that level. Since it is colder, it is also more dense than its surroundings,
heavier than the surrounding air, and will tend to sink back to its original level. An
atmosphere whose environmental temperature lapse is less than this adiabatic
expansional cooling rate of 10 degrees per kilometer is said to be Statically Stable since the
environmental air tends to suppress the upward motions of rising air. We might
also describe this as a negative feedback situation since pushing a parcel
upward leads to the environment pushing it back downward.
Notice that while the air parcel at its initial location is warmer than air above it,
the effect of the adiabatic expansional cooling is such as to suppress its tendency
to rise. Thus, the statement "hot air rises" must refer to the temperature
or air relative to its immediate surrounding, and not to the fact that this
air parcel was initially warmer than air several kilometers above it. Think about that!
In nature, conditions of static stability are usually accompanied by fair weather, and a general
lack of storminess.
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