Atmospheric stability refers to how the atmosphere reacts to lifting forces. A
lifting force is something that causes air to be lifted. For example, if air
was moving over upsloping terrain, it would be forced upward. Air is also
forced upward near weather fronts. Even in flat terrain and outside the influence of weather fronts, air is lifted
by temperature differences on the surface.
Whatever the method by which the air was forced upward, it reacts to that
lifting based on its stability.
Adiabatic Cooling and Heating
Adiabatic cooling and heating in the atmosphere results from pressure changes on
a volume of air. Whenever air pressure is lowered, the air cools. If
pressure increases, so does temperature. In other words, changing the air
pressure also results in a change to the air temperature - but without the
particular volume of air being heated or cooled by the surrounding environment.
The higher you travel in the atmosphere, the lower the atmospheric pressure.
As a result, when air is forced upward, its air pressure drops. As a
result of this lowering air pressure, temperature drops due to adiabatic
So, we have a volume of air that is lifted by some lifting force. As it
has travelled upward a short distance in the atmosphere, its pressure and
temperature have been reduced.
In a stable atmosphere, the actual lapse rate is lower, so the air doesn't get
as cold as quickly with increasing altitude. This means after a volume of
air has been lifted a short distance and adiabatically cooled as a result, the
temperature in the surrounding air did not cool as much over that same vertical
As a result, the lifted volume of air is cooler than the surrounding air.
Because colder air is more dense than warmer air, the upward movement of the
volume of air is halted by its own adiabatic cooling.
A stable atmosphere stops the upward movement of lifted volumes of air.
In an unstable atmosphere, the actual lapse rate is higher. So, the air
temperature decreases more quickly with increasing altitude.
In these conditions, this small volume of air still cools adiabatically as it is
pushed upward by the initial lifting force. However, this time the
surrounding air cools more quickly than the volume of lifted air. This
results in a situation where the lifted air is relatively hotter than the
surrounding air. Since hotter air is less dense, the lifted volume of air
continues upward. As it travels upward, it continues to cool more slowly
than the surrounding air. This temperature difference carries the small
volume of air up to higher altitude.
An unstable atmosphere promotes the upward movement of air. All a volume
of air needs is an initial push by some lifting force, and the forces of the
unstable atmosphere cause it to take off vertically.
How stable or unstable the atmosphere is depends on the actual lapse rate.
The actual lapse rate can be used to determine the stability of the atmosphere.
Normally, the air gets colder and colder as you move upward in the atmosphere.
A temperature inversion exists when there is an increase in temperature with
The most frequent type of surface based temperature inversion is one that is
produced by terrestrial radiation on clear, relatively still nights. With
no cloud cover and still air, the ground radiates it heat out into space.
It becomes cold and cools the air at low altitudes, resulting in a temperature
Ground based temperature inversions lead to low visibilities. This is
because the inversion traps fog, smoke, and other restrictions to visibility in
the lower levels of the atmosphere.
The layer of air below a temperature inversion is stable, and convective
activity is suppressed.
Frontal systems can also produce temperature inversions. The presence of
ice pellets or freezing rain at the surface is a good indicator of a temperature
inversion, since they are produced when rain aloft falls into colder air below.
Where there is lightning, there is a thunderstorm. Thunderstorms indicate
a lot of air movement, up, down, and laterally. If you are taking off from
or landing at an airport anywhere near a thunderstorm, you can expect wind
Thunderstorms can exist inside an air mass, along frontal boundaries, or out
front of a cold front in a band of weather called a squall line. Squall
line thunderstorms generally produce the most intense hazard to aircraft.
Three things are required for thunderstorm formation:
Sufficient water vapor
Unstable lapse rate
Thunderstorms have a life cycle defined by three stages of development, which
are cumulus, mature, and dissipating. The cumulus stage is dominated by a
continuous updraft. In this stage, the thunderstorm develops from a small
cumulus cloud into a towering cumulus cloud. Precipitation indicates the
mature stage of the thunderstorm. Thunderstorms reach their greatest
intensity during the mature stage. Finally, the thunderstorm dissipates.
The dissipating stage is characterized predominately through downdrafts.