Thunderstorms are common throughout the United States, but they occur most frequently during the summer months in the Southeast, Midwest and Great Plains. They form when the air rapidly heats and lifts clouds to high elevations. Mountain thunderstorms usually form in the early to mid-afternoon. So as a general rule of thumb, hike to peaks in the early morning so that you're able to head down from high elevations when the thunderstorm threat is the greatest.
You should always plan trips in advance and be aware of the weather forecast for the day. Despite your preparations, weather can be anything but predictable, so it's a good idea to be on your guard at all times. Keep your eyes on the sky and be prepared to take a rain check for your hiking excursion if a storm develops. Clouds indicating fair weather are usually puffy, short and show little vertical development. When clouds begin to build up darker bases, they're on their way to becoming thunderstorm clouds. A cloud that is tall and flat at the top is definitely an indicator of thunderstorm activity. A dark sky, flashes of light, heavy wind and claps of thunder indicate that a storm may soon be approaching.
If you can hear thunder, you're close enough to the storm to be in danger of being struck by lightning [source: American Red Cross]. You can judge the distance of an oncoming storm by the time between flashes of lightning and claps of thunder. Count the number of seconds between flashes and claps and divide this number by five to estimate how many miles away an approaching storm is [source: Lobe].
When setting up your camp, pitch tents in areas protected by a lot of tree growth. This will make it less likely you'll be struck by lightning. Before the storm hits, remove dead trees and branches that might cause damage if felled by high winds. Put all equipment in your tent so that nothing blows away. Check your camp for structures that might become unstable in a storm.