How they form When the sun heats up a moist air mass, or when a cold front wedges underneath it, that mass can rise violently upward—think afternoon thermals rising over the Rockies or a low-pressure front rolling into the Smokies. As the air rises, its water vapor condenses, lifting the cloud even higher. The result: massive thunderclouds piled miles high.
What to expect Cumulonimbus clouds can develop in as little as 20 minutes. The faster, taller, and darker they build up, the faster and more violent the energy release. Translation: fierce downpours, wind, lightning, and hail. In North America, thunderstorms are most common over the Florida peninsula and Colorado’s southeastern plains, but they also form commonly on the windward side of mountain ranges; above unevenly heated terrain; and wherever two air masses collide.
Warning signs Watch for altocumulus (mid-level, puffy clouds), especially in warm weather. These typically signal a cold front; once it collides with warm air, afternoon thunderstorms might result.
What to do Cut short your outing, descending from high points or jagged terrain and avoiding isolated, tall objects, like lone trees. If there’s lightning, spread 50 feet away from others, squat on top of insulating material, with your head down, feet together, and arms wrapped around your legs (for a safer lightning path in case of a strike).