Convective instability

In meteorology, convective instability or stability of an air mass refers to its ability to resist vertical motion.

In an unstable atmosphere, vertical air movements (such as in orographic lifting, where an air mass 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.

The ambient or environmental lapse rate is the temperature change in the (non-displaced) air per vertical distance.

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.

It was first introduced as a simple but useful measure of the strength of the inversion that caps the planetary boundary layer on earth, and also indicates the level of convective stability of an air column at a given location.

[3] Regions with negative LTS have a larger potential temperature on the surface than in the mid-troposphere, which makes the air column unstable and encourages convection.

There is a major limitation of this measure of stability however, which is that it does not take the thermodynamic properties (saturation mixing ratio and therefore the shape of adiabats in the lower troposphere) of the air into account.

A more refined measure of stability has since been developed, named the estimated inversion strength, which pays closer attention to the thermodynamic properties of the air in the lower troposphere.