ATPL Intensive Program (IP) Practice Exam

The process that increases the stability of an air mass is related to the cooling by the underlying surface. When the surface cools, it can lower the temperature of the air directly above it. This cooling can create a situation where the air experiences a temperature inversion, where the air at the surface is cooler than the air above it. In stable air masses, any upward movement of the air is inhibited because the cooler surface air is denser than the warmer air above it. This density difference prevents vertical mixing and can maintain clear skies and calm weather conditions.

In contrast, when the air mass is warmed from below, it can lead to instability as the warmer air rises and the cooler air sinks, promoting convection and possibly leading to clouds and precipitation. Therefore, cooling by the underlying surface is a clear factor in promoting the stability of an air mass.