Thunderstorms--which also induce flash floods--are one of the top weather-related killers in the United States, especially in the arid west. Out here normally dry canyons, gullies, and streambeds can suddenly fill with raging torrents of muddy water spawned by distant storms. Watch the sky for clues as to what is coming.

Here are cumulus clouds in an early stage of vertical development, south of Lake Tahoe. To avoid becoming a grim statistic, the National Weather Service (NWS) suggests that at the first sign of lightning, get into a fully enclosed building or a hard-topped car or truck.

If you can chose a building, pick one with wiring and plumbing running into the ground. Partially enclosed carports, covered patios, and tents are NOT safe shelters.

This is a picture of a summer thunderstorm Lake Tahoe. This is a warning for boaters to head for land that is away from the storm.

Hikers should get off exposed mountain ridges, and people outdoors on land should not stand under a tree.

If you are exposed on land and there is no safe refuge, squat down with your feet together, cover your ears, and close your eyes. Then lean forward on your toes with your heels off the ground to minimize contact with the ground.

These are rare, mammatus clouds that indicate very moist and unstable atmosphere and vigorous thunderstorms in the region. Their appearance usually follows the most active growth stage of storms.

A few years ago, dry lightning storms sparked huge wildfires in tinder-dry northern Nevada. Brush fires east of Elko, Nevada, temporarily shut down portions of Interstate 80, and hot, windy weather hampered fire crews.

The July 2004 "Waterfall fire" near Carson City, Nevada, burned nearly 9,000 acres and destroyed 31 homes. Suppression cost close to $6 million; no lives were lost.

Wildfires around the United States blackened about 3.3 million acres in 2006, compared to 1.1 million acres in 2005.

Editor’s Note: The story and photos are by Mark McLaughlin. He is a weather historian living on the North Shore. Mark has four award-winning books.