air at altitude X, the air parcel willsink spontaneously, and this air is termed abso-
lutely stable. On average, the troposphere is absolutely stable because the average
environmental lapse rate is less than the saturated and unsaturated adiabatic lapse
rates. How, then, does the troposphere sometimes destabilize? The answer is
straightforward: the environmental lapse rate is quite variable over short amounts
of time. The atmosphere destabilizes as the environmental lapse rate increases to
be greater than the adiabatic lapse rates. The environmental lapse rate might have
been increased by warming near Earth’s surface or by cooling of the air aloft.
Warming of the lower air is accomplished by air moving over a warm surface,
and daytime heating by the influx of solar energy, or advection of warm air. Air
aloft can be cooled by air and clouds radiating energy to space or by cold air
advection resulting from disturbances in the jet stream. Moreover, both the warm-
ing of the lower air and cooling of air aloft can be simultaneous and result in rapid
destabilization. Over the southern Great Plains of the United States maritime
tropical air flows at low level and the polar front jet stream flows at the top of
the troposphere. The combination makes for thousands of meters of instability
and some of the largest thunderstorms on Earth. The presence of plentiful water
vapor helps makes the air more unstable. Although air can be unstable with modest
amounts of vapor, greater amounts of vapor make the air more unstable to greater
altitude. The key is in the saturated adiabatic lapse rate. Moister air parcels
become saturated with less lift than dryer parcels. The retardation of the cooling
of a saturated parcel via the saturated environmental lapse rate keeps the rising
air parcel warmer, less dense, and unstable to heights greater than possible in
unsaturated air.
Cloudsprovide visible depictions of stability conditions. Clouds can be present
when the air is stable, but are generally taller when the air is unstable. Clouds of
vertical development such as cumulus, altocumulus, and cirrocumulus are indica-
tors of instability at various elevations. Cumulonimbus clouds sometimes reach all
the way through the troposphere and bring lightning, thunder, rain, hail, and high
winds because of massive instability in a moist environment. The atmosphere’s
lifting mechanisms (seePrecipitation) are made much more potent by instability.
If an air parcel is nudged upward by one of the lifting mechanisms, it will sink or
remain at the lifted altitude if the air is stable or neutrally stable. However, if
unstable air is lifted it rises spontaneously above the initial lift and is much more
likely to cause significant cloudiness and precipitation.
There is a strong geography to the stability conditions of the atmosphere. They
are largely dependent on the global circulation. The intertropical convergence and
the polar front are zones where instability and, hence, storminess are common.
Conversely, regions dominated by the polar highs and subtropical highs are quite
stable and are associated with lack of strong ascent and sparse precipitation.22 Atmospheric Stability