Atmospheric Stability
The great ocean of air that is theatmosphereis far from quiescent. Place-to place
heating differences make air flow horizontally (seewinds and pressure systems)
and also vertically. The vertical rise of air has particular significance because it is
the rise of air that is associated withcloudsandprecipitation. How strong is the
rise of air? The answer is that it depends on the stability of the atmosphere and that
stability differs by location, season, and time of day. If lift is gentle or air is
descending there is not likely to be significant precipitation. It is the strong ascent
of unstable air that is associated with all of the world’s great storminess. The basis
of understanding of atmospheric stability is rooted in the ready ability of air to
compress and decompress. From physics, the Gas Law (Equation of State) holds
that temperature, pressure, and density are interrelated. As air is heated and
cooled, its density can change dramatically. In this regard, air is usually considered
in small, discrete pieces known as air parcels. Air parcels are useful constructs
with which to consider the thermodynamic properties of air. A few meters across,
an air parcel has consistency oftemperature, pressure, and density within and is
compared to the air around it (known as the environmental air).
As air parcels rise they decompress; that is, the same mass takes up more vol-
ume and the density lowers. This decompression decreases the number of molecu-
lar collisions in the air parcel and causes a corresponding drop in temperature.
Importantly, the temperature drop is the result of the spatial rearrangement of the
molecules andnota loss of energy to the surrounding environmental air. This is
known as the adiabatic process and is a profound influence on the temperature
the rising and sinking air. Rising air parcels that are not saturated cool at the
unsaturated (dry) adiabatic lapse rate of 10°C for every kilometer of ascent. In
saturated air, the decrease of temperature averages 6°C for every kilometer (the
saturated or wet adiabatic lapse rate). This retardation of the saturated adiabatic
lapse rate is the result of the conversion of latent heat to sensible heat during the
cooling of the air parcel. Descending air parcels warm at the unsaturated adiabatic
lapse rate, whether or not they begin the descent saturated: descending air warms
so that it soon becomes unsaturated. Air parcel temperatures can change dramati-
cally in a few minutes because of adiabatic processes; similar temperature change
because of gain or loss of radiant energy would take hours to occur. Consider an
air parcel rising from the surface. The air parcel is lifted to altitude X and its tem-
perature decreases at the appropriate adiabatic lapse rate. The air parcel at altitude
X is less dense and warmer than the environmental air, so the air parcel willrise
spontaneously, and this air is absolutely unstable. An air parcel already at altitude
X has the same density as the environmental air, will remain at altitude X, and it
has neutral stability. If an air parcel is denser and cooler than the environmental
Atmospheric Stability 21