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Thunderstorms and Lightning

Extreme and violent vertical movements of air, know as updrafts, often lead to thunderstorms. This uplifting usually creates dramatic cumulus and cumulonimbus cloud formations that rise vertically, sometimes as high as 70,000 to 75,000 feet. The uplift of air can be caused by heating of the air from the ground (common in the Midwest), weather associated with a cold front, or temperature differences caused by the meeting of land and sea (common in the Gulf and southern Atlantic states).

Because temperature differences are the most common cause of thunderstorms, thunderstorms usually occur in the afternoon, when temperature differences between land and air or sea are the most extreme. Since temperature differences between land and sea are not as dramatic in the North Atlantic or West Coast (Pacific Ocean) regions, thunderstorms in these locations are not as frequent.

A thunderstorm forms as rising warm air cools and condenses. The higher the cloud stacks, the greater the level of cooling, and soon precipitation begins. As the falling rain and ice crystals cool the air within the cloud, the temperature difference between the cloud and the surrounding air begins to equalize, leading to the creation of downdrafts and heavy precipitation. The rush of cold air fans out ahead of the storm, sometimes as much as 2 to 3 miles, and is a good indicator of an impending thunderstorm.

As the downdrafts increase in intensity, the cloud becomes nothing more than downward moving air. Cooling of air ceases, and since no air is rising and condensing, the rain tapers off.

Lightning is caused by the attraction of unlike electrical charges within the storm cloud or the earth’s surface. The friction caused by rapidly moving air particles, churning from the violent updrafts and downdrafts, leads to a building up of strong electrical charges. As the electrical pressure builds, charges between parts of the cloud or from the cloud to the earth are released, taking the form of lightning.

Up to thirty million volts can be discharged by one single lightning bolt, and it is this power, or explosive heating of the air, that causes the compressions of thunder.

It is possible to roughly judge the distance of an approaching storm by observing the lightning’s flash followed by the resounding boom of thunder. For this method count slowly, “one-one thousand, twoone thousand, three-one thousand,” and so forth. This will approximate one second elapsed for each thousand counted. Once the lightning has flashed, begin to count. When you hear the thunder, stop counting. Every five seconds of elapsed time indicates 1 mile of distance. In other words, if the count reaches, “seven-one thousand,” it is safe to assume that the approaching thunderstorm is approximately 11⁄2 miles distant.

Thunderstorms are dangerous companions when you are traveling on a mountain peak. If a thunderstorm approaches when you and your hiking party are on an exposed peak or ridge, take the following precautions:

  • Get off the ridge or peak if at all possible. Even reaching a few feet below the highest point around you is better than not moving at all.
  • Get away from your pack, as the metal in the pack will conduct electricity.
  • Position yourself on a dry surface, preferably insulated, such as a sleeping pad. If the sleeping pad has become soaked in the rain, however, it will be useless, as water conducts electricity.
  • Do not lie down or sit. Rather crouch on the balls of your feet with your feet close together. The idea is to minimize the available surface area through which possible ground currents from nearby lightning strikes may move.

  • Do not huddle next to a single tree. That is rather like hugging a lightning rod and expecting to be safe. Choose a cluster of trees instead and place yourself in the middle (figure 5), preferably in an open area.

  • Spread the group out, at least 25 to 30 feet apart. If lightning does strike, the idea is to minimize the potential damage and injury. With the group spread out, chances are only one person will be injured, if, at all, and this leaves the rest of the party to provide lifesaving assistance once the storm moves on.

  • Avoid depressions or caves. Such areas usually have moisture present, which makes them more susceptible to conducting electricity.

  • If you are caught on a lake in a thunderstorm, assume a crouching position toward the middle of the boat. Try to minimize your contact with wet objects.

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    From the book Basic Illustrated Weather Forecasting, by Michael Hodgson. Copyright © 2008 by Michael Hodgson. Used by permission of FalconGuides, a division of Globe Pequot Press. Visit Falcon.com.

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