Lifting Forces
Atmospheric stability refers to how the atmosphere reacts to lifting forces. A lifting force is something that causes air to be lifted. For example, if air was moving over upsloping terrain, it would be forced upward. Air is also forced upward near weather fronts. Even in flat terrain and outside the influence of weather fronts, air is lifted by temperature differences on the surface. Whatever the method by which the air was forced upward, it reacts to that lifting based on its stability.
Adiabatic Cooling and Heating
Adiabatic cooling and heating in the atmosphere results from pressure changes on a volume of air. Whenever air pressure is lowered, the air cools. If pressure increases, so does temperature. In other words, changing the air pressure also results in a change to the air temperature - but without the particular volume of air being heated or cooled by the surrounding environment.
The higher you travel in the atmosphere, the lower the atmospheric pressure. As a result, when air is forced upward, its air pressure drops. As a result of this lowering air pressure, temperature drops due to adiabatic cooling.
Stable Atmosphere
So, we have a volume of air that is lifted by some lifting force. As it has travelled upward a short distance in the atmosphere, its pressure and temperature have been reduced.
In a stable atmosphere, the actual lapse rate is lower, so the air doesn't get as cold as quickly with increasing altitude. This means after a volume of air has been lifted a short distance and adiabatically cooled as a result, the temperature in the surrounding air did not cool as much over that same vertical distance.
As a result, the lifted volume of air is cooler than the surrounding air. Because colder air is more dense than warmer air, the upward movement of the volume of air is halted by its own adiabatic cooling.
A stable atmosphere stops the upward movement of lifted volumes of air.
Unstable Atmosphere
In an unstable atmosphere, the actual lapse rate is higher. So, the air temperature decreases more quickly with increasing altitude.
In these conditions, this small volume of air still cools adiabatically as it is pushed upward by the initial lifting force. However, this time the surrounding air cools more quickly than the volume of lifted air. This results in a situation where the lifted air is relatively hotter than the surrounding air. Since hotter air is less dense, the lifted volume of air continues upward. As it travels upward, it continues to cool more slowly than the surrounding air. This temperature difference carries the small volume of air up to higher altitude.
An unstable atmosphere promotes the upward movement of air. All a volume of air needs is an initial push by some lifting force, and the forces of the unstable atmosphere cause it to take off vertically.
How stable or unstable the atmosphere is depends on the actual lapse rate. The actual lapse rate can be used to determine the stability of the atmosphere.
Temperature Inversions
Normally, the air gets colder and colder as you move upward in the atmosphere. A temperature inversion exists when there is an increase in temperature with higher altitude.
The most frequent type of surface based temperature inversion is one that is produced by terrestrial radiation on clear, relatively still nights. With no cloud cover and still air, the ground radiates it heat out into space. It becomes cold and cools the air at low altitudes, resulting in a temperature inversion.
Ground based temperature inversions lead to low visibilities. This is because the inversion traps fog, smoke, and other restrictions to visibility in the lower levels of the atmosphere.
The layer of air below a temperature inversion is stable, and convective activity is suppressed.
Frontal systems can also produce temperature inversions. The presence of ice pellets or freezing rain at the surface is a good indicator of a temperature inversion, since they are produced when rain aloft falls into colder air below.
Thunderstorms
Where there is lightning, there is a thunderstorm. Thunderstorms indicate a lot of air movement, up, down, and laterally. If you are taking off from or landing at an airport anywhere near a thunderstorm, you can expect wind shear.
Thunderstorms can exist inside an air mass, along frontal boundaries, or out front of a cold front in a band of weather called a squall line. Squall line thunderstorms generally produce the most intense hazard to aircraft.
Development
Three things are required for thunderstorm formation:
Sufficient water vapor
Unstable lapse rate
Lifting Force
Thunderstorms have a life cycle defined by three stages of development, which are cumulus, mature, and dissipating. The cumulus stage is dominated by a continuous updraft. In this stage, the thunderstorm develops from a small cumulus cloud into a towering cumulus cloud. Precipitation indicates the mature stage of the thunderstorm. Thunderstorms reach their greatest intensity during the mature stage. Finally, the thunderstorm dissipates. The dissipating stage is characterized predominately through downdrafts.