An air parcel is unstable if its temperature is and remains higher than the temperature of its surrounding environment as it rises above the ground. When this occurs, the parcel continues to rise. This is known as moist convection.
Stable air parcels function in a manner opposite to unstable ones. When stable masses of air rise above ground, they are cooler than their surrounding environments. When this occurs, the air parcel falls back to the ground. Stable air masses are associated with stable weather conditions, meaning the weather does not change suddenly. Unstable air masses are associated with sudden changes in weather.