The stability of the atmosphere depends on its ability to
resist vertical motion. A stable atmosphere makes
vertical movement difficult, and small vertical
disturbances dampen out and disappear. In an unstable
atmosphere, small vertical air movements tend to
become larger, resulting in turbulent airflow and
convective activity. Instability can lead to significant
turbulence, extensive vertical clouds, and severe
weather.
Rising air expands and cools due to the decrease in air
pressure as altitude increases. The opposite is true of
descending air; as atmospheric pressure increases, the
temperature of descending air increases as it is
compressed. Adiabatic heating, or adiabatic cooling,
are the terms used to describe this temperature change.
The adiabatic process takes place in all upward and
downward moving air. When air rises into an area of
lower pressure, it expands to a larger volume. As the
molecules of air expand, the temperature of the air
lowers. As a result, when a parcel of air rises, pressure
decreases, volume increases, and temperature
decreases. When air descends, the opposite is true. The
rate at which temperature decreases with an increase in
altitude is referred to as its lapse rate. As air ascends
through the atmosphere, the average rate of
temperature change is 2°C (3.5°F) per 1,000 feet.
Since water vapor is lighter than air, moisture decreases
air density, causing it to rise. Conversely, as moisture
decreases, air becomes denser and tends to sink. Since
moist air cools at a slower rate, it is generally less stable
than dry air since the moist air must rise higher
before its temperature cools to that of the surrounding
air. The dry adiabatic lapse rate (unsaturated air) is 3°C
(5.4°F) per 1,000 feet. The moist adiabatic lapse rate
varies from 1.1°C to 2.8°C (2°F to 5°F) per 1,000 feet.
The combination of moisture and temperature
determine the stability of the air and the resulting
weather. Cool, dry air is very stable and resists vertical
movement, which leads to good and generally clear
weather. The greatest instability occurs when the air is
moist and warm, as it is in the tropical regions in the
summer. Typically, thunderstorms appear on a daily
basis in these regions due to the instability of the
surrounding air.
