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Thunderstorms pose threat to safety

By Ann Bond
Special to The SUN

Summer thunderstorms pose a threat to your safety in the backcountry.

Plan trips to descend from high elevations to avoid afternoon thunderstorms above timberline. A good rule of thumb is to descend to tree line by noon.

When skies begin to look threatening, they already are. Anytime clouds develop vertically and cloud bases grow dark, a lightning hazard exists, even before you hear thunder or see lightning.

Monitor approaching storms on the horizon, as well as thunderstorm development overhead or nearby. Lightning bolts can strike 10-15 miles out from a thunderstorm.

If you recognize a hazard is developing, seek cover in a building or vehicle, if possible. That means a substantial building (not picnic shelters, bus stops, dugouts, etc.) or inside a metal, covered vehicle (not open Jeeps, golf carts). If inside a vehicle or building, don’t touch anything that could conduct electricity.

If caught out in the backcountry during a thunderstorm, retreat from high exposed areas to minimize the chance of a direct hit, and don’t be the tallest object in the vicinity. Lightning often seeks higher objects, because it’s a shorter gap for the electrical charge to bridge, so being near a single tall tree or group of tall trees in an opening is also dangerous. Move into a stand of smaller trees in a lower position on the landscape to increase the odds that lightning will strike higher points or taller objects instead.

The majority of lightning strikes are not direct hits, but rather arc from another object or through the ground after a nearby strike. Dangerous ground currents can reach out up to 100 yards from the object that is struck. Minimizing your contact with the ground will minimize the likelihood of being hit by a ground arc. In a worst-case scenario, squat down with your head on your knees, feet and legs together, and balance on the balls of your feet.

In dangerous situations, it’s advisable for groups to separate, not huddle together. If a ground arc hits one person, it will probably affect everyone nearby. By spreading out, you reduce the odds of everyone being hit.

Avoid things that conduct electricity, like water channels or metal fencing. Lightning can follow streams or barbed wire fences for long distances, flow into the ground and spread out.

Even when a thunderstorm is far away or has already passed over, lightning danger may remain. While most lightning strikes originate from the negatively charged lower levels of a thunder cloud and bridge the gap to corresponding positive charges on the ground, far more powerful strokes can originate from the positively charged upper portions of a cumulonimbus cloud, extending miles into clear air (often behind the thunderstorm), and angling to the ground many miles from the storm itself. Because of the much greater gaps these positive strokes bridge to corresponding negatively charged ground objects (including people), they are much more powerful.

Likewise, such strokes can precede a storm by miles for the same reason.

For more information on the subject of lightning behavior and safety, go to www.crh.noaa.gov/pub/ltg.php.

This story was posted on July 17, 2014.