Entry by Haifei Zhang, AP 225, Fall 2009
What is convective instability
Convective instability or stability of an airmass refers to 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, vertical air movements (such as in orographic lifting, where an airmass is displaced upwards as it is blown by wind up the rising slope of a mountain range) tend to become larger, resulting in turbulent airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and severe weather such as thunderstorms.
Why convective instability
Adiabatic cooling and heating are phenomena of rising or descending air. 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 and adiabatic cooling are terms used to describe this temperature change.
The adiabatic lapse rate is the rate at which a rising or falling airmass lowers or increases per distance of vertical displacement. The ambient lapse rate is the temperature change in the (non-displaced) air per vertical distance. Instability results from difference between the adiabatic lapse rate of an airmass and the ambient lapse rate in the atmosphere. If the adiabatic lapse rate is lower than the ambient lapse rate, an airmass displaced upward cools less rapidly than the air in which it is moving. Hence, such an airmass becomes warmer relative to the atmosphere. As warmer air is less dense, such an airmass would tend to continue to rise.
Conversely, if the adiabatic lapse rate is higher than the ambient lapse rate, an airmass displaced upward cools more rapidly than the air in which it is moving. Hence, such an airmass becomes cooler relative to the atmosphere. As cooler air is more dense, the rise of such an airmass would tend to be resisted.
Examples of convective instability
Moist air cools when rising at a lower rate (given the same vertical movement) than dry air, and hence has a relatively low adiabatic lapse rate. Thus, moist air is generally less stable than dry air. The dry adiabatic lapse rate (for unsaturated air) is 3 °C (5.4 °F) per 1,000 vertical feet. The moist adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to 5 °F) per 1,000 vertical 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. The ambient lapse rate differs in different meteorological conditions, but, on average, is 2 °C (3.5 °F) per 1,000 vertical feet.