A thunderstorm is an atmospheric process by which the atmosphere in a mesoscale region is stabilized through violent heat exchange between the lower and upper atmospheres. Through the atmospheric processes of diurnal heating, UV radiation, moisture advection and other factors, the lower atmosphere (typically referred to as the boundary or mixed layer becomes excessively warm and moist. If the temperature gradient is great enough between this boundary layer air mass and the cold, dry air in the upper atmosphere, the lower atmosphere becomes bouyant and its warm, moist air attempts to rise (called convection)- at this point it is considered to be unstable. If allowed to rise without a strong lift mechanism or some inhibition, that air mass would cool as it rises slowly upward at what is called the adiabatic rate (typically a few degrees C for each 1000 feet). When the temperature approaches the dew point, the air cannot hold the moisture that it has and so it condensed into water droplets, which we see on a large scale as a cloud. If allowed to continue its ascent, more moisture would become water droplets and these droplets would grow until their weight causes them to fall in the form of rain. This produces a common rain or thunder shower. If, however, an inhibition to convection exists and/or a strong lifting mechanism is present to accelerate the rate of convection, the atmosphere can be thought of as a pressure cooker - the inhibition, usually caused by an inverted layer of air (warm air over cold air) in the middle atmosphere called the "mid level cap" or just "cap". This cap inversion is very common and if strong enough, prevents convection from breaking through from below. This is sort of like the lid to a pressure cooker - if the air below the cap continues to warm and become more moist, the lower atmosphere becomes extremely unstable because it can't rise and thus stabilize itself. If the cap then begins to weaken and/or the lifting mechanism (usually provided by a cold or warm front) is strong enough, the lower atmosphere will finally break through the inversion layer. This is something like the eruption of a volcano - all that force and pressure trying to get out, and tons of rock and earth keeping it from doing so, until the pressure becomes so great that it finally breaks through. When the breakthrough occurs, it can be very violent and rapid convection occurs as the air mass in the boundary layer attemps to skyrocket upward to stabilize the atmosphere. As the air cools, it consenses rapidly and becomes a cumulus cloud. With the release of latent energy caused by water condensation, convection is further accelerated and within minutes a towering cumulonimbus cloud is forming. Sometimes they can get up to 60 or 70 thousand feet high! As heavy rain, lightning and strong winds begin to be produced as the process continues, it becomes a thunderstorm.