Instability - A condition in the atmosphere where a layer of warm, moist air near the earth's surface is situated below a layer of cooler, drier air above it. Warm air is lighter than cool air, and its bouyancy makes it tend to rise upward in columns in a process called convection. Convective activity often results in thunderstorms.
Atmospheric instability is created any time a warm, moist layer of air is given the potential energy to rise upward through the cooler air. Most instability is created when the sun heats the earth's surface, which in turn heats the atmosphere near the ground. The sun's heating is also called 'diurnal' heating. Southerly winds can also create instability at non-sun-heated locations by transporting warm, moist air from one location to another (called warm advection), such as from the Gulf of Mexico to the mountains of West Virginia.