Forecasters often refer to air as being unstable. The instability of the air is actually a reflection of the air’s tendency keep rising once rising motion is initiated by some force. The most unstable environments are a combination of warm and moist air near the ground and cooler, drier air aloft. When air rises, it cools. If air rising in an updraft remains warmer than the surrounding air, it will continue to rise rapidly — a condition critical to storm development.
Once air in an updraft reaches a point at which moisture is condensed, additional energy (called latent heat of condensation) is released, causing an explosion of additional rising air that is the catalyst to a strong thunderstorm. It is important to realize that instability is not something that can be readily sensed by people because it requires knowledge of the conditions high above the ground. This is why thunderstorms do not form every time it gets hot and muggy.