When the atmosphere is unstable, thunderstorms form in sudden, billowing masses of white vapor. Here, you see an image of a developing storm. Over the continental United States, storms can reach heights of 60,000 feet -- twice the cruising altitude of a passenger jet! Or, another way to think of it: at 60,000 feet, you have shot above the troposphere (the layer of atmosphere in which we live) and have entered the stratosphere!