Every year, lightning kills 100 to 300 Americans. Only a handful are backpackers; the rest are mountaineers, golfers, boaters, swimmers and others.

Intense thunderstorms generate lightning by a mechanism that's still poorly understood, although it's known that the difference in electrical potential between the ends of a lightning stroke may reach hundreds of millions of volts. When lightning flashes, the air along its path is heated momentarily to perhaps 27,000 degrees F. The heated air expands explosively, causing thunder. Sound travels about one mile every five seconds, so the interval between a lightning flash and the sound of thunder tells you the distance to the storm. If lightning is frequent, keep track of the time between a lightning bolt and its thunderclap. If the time is decreasing, the storm is approaching you. Find shelter immediately.

Thunderstorms are characterized by violent updrafts within the storm itself that can reach 60 mph. Rapid condensation created as the updraft cools is the cause of a thunderstorm's pounding rain. When the rising air tops out at the top of the cloud, it cascades down the outside of the storm and spreads out along the ground, creating the sudden, cool breeze that heralds a thunderstorm's arrival. Severe thunderstorms can be distinguished from less violent ones by the size, shape and position of the cloud. Cumulus clouds with low bases should be watched carefully. The low base indicates air with abundant moisture that needs to cool only a little to begin condensing. Anvil-shaped thunderstorms have updrafts powerful enough to reach the upper limit of the troposphere, which is several miles high. Overshooting thunderstorms, in which the normally flat top of the anvil bulges, are more dangerous still.

Prevention is obviously better than treatment when confronted with an immensely powerful force of nature like lightning. Lightning most often strikes tall, isolated objects – a peak, a rocky spire, a single tree in a meadow – or a person standing in an open area. If a thunderstorm threatens, get off the summits and ridges. If a dense forest is nearby, plunge in and relax – you're safe. Stay away from isolated trees and clumps of trees, which can act as lightning rods. If you're caught in the open with nowhere to hide, set metal objects such as tent poles, ice axes and tripods aside and move several hundred yards away. Kneel down and put your hands on your knees. The idea is to reduce your height to the extent possible to avoid acting as a lightning rod, yet to minimize your contact with the ground so that the ground current set up by a lightning strike has the smallest possible avenue to enter your body. If possible, crouch on an insulator such as a pack or climbing rope. Ground currents caused by lightning tend to flow along the paths taken by the rain as it runs off cliffs or hillsides. Shallow caves and overhanging rocks don't necessarily offer good protection from ground currents, which can flow along the cave walls or arc across the cave's mouth. A person hit by lightning frequently stops breathing. In addition, the victim's heart may stop beating. Be prepared to start cardiopulmonary resuscitation, a technique which requires specialized training in advance. Prompt CPR has saved the life of many lightning victims.

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