What is an unstable air mass?
To be unstable, the lowest layers of an air mass must be so warm and/or humid that, if some of the air rises, then that air parcel is warmer than its environment, and so it continues to rise. Normally in the atmosphere, if an air parcel rises, it will be colder than its environment (due to expansion), and it will then sink back down again. If the rising air parcel contains enough water vapor, however, then condensation of the water vapor in it occurs, and a cloud is formed. This condensation releases heat, which warms the air parcel, which can cause the parcel to rise. The most unstable air mass is one in which the lower layers are unstable to the first type of instability (dry),and which also contain enough water vapor to form clouds, providing the second type of instability (moist) as well. This is the basis for thunderstorm formation. The more unstable the air mass, the more energy these is for thunderstorms to feed off of.
