A sky-watcher's guide to summer's electric show.
July and August are most active lightning months in the West. When dry, heated ground air rises and meets moist tropical air, the moisture condenses into cloud droplets, which eventually form a cumulus congestus cloud with billowy cauliflower shape. As it continues to pile up, it may form a cumulonimbus cloud or thunderhead, with a telltale anvil shape.
The upward-moving water droplets carry a positive charge; the cloud base carries a negative charge that induces a positive charge in the ground beneath. When the difference in charges builds up enough to overcome resistance, lightning strikes.
The crack and boom of the thunder are created by the explosive heating and expansion of the air channel through which the bolt passes. The compression waves radiate like waves from a pebble dropped in a pond, causing a rolling rumble. (You can estimate the distance in miles to a lightning strike: count the seconds between the flash and sound of thunder, and divide by 5.)
If you happen to be close to a lightning strike, you can often smell the pungent bleach-like odor of ozone gas generated by the electrical discharge.
Actually, two-thirds of all lightning discharges never touch the ground: they just stay within the thunderhead. This is called cloud-to-cloud lightning. (In hot weather, so-called heat lightning is the flashing of distant lightning beyond the horizon.)
Cloud-to-ground strikes occur in steps: first, a stepped leader begins a branched downward thrust to the ground, then a return stroke flashes from the ground back up the channel created by the leader. A dart leader heads from the cloud down the original channel but sometimes forges a different path to the ground, adding a fork to the bolt. Strikes are so rapid that most flashes contain two to four return strokes so fast the eye perceives them as a single flickering stroke. how much danger? What to do?
Each year in America, lightning kills some 100 people--more than tornadoes do. Most are struck when (in order of frequency) they]re under a tree, on open water, on a tractor, on a golf course, or talking on the telephone.
Roughly 70 percent of lightning injuries occur in the afternoon. And in the Rockies and deserts, most thunderstorms come in the late afternoon and early evening. Keep this in mind when scheduling outdoor activities such as hikes.
Colorado]s Rocky, Mountain National Park sees lightning almost every summer afternoon; a third of the park is above timber line. Glen Kaye, the park's chief naturalist and a lightning strike survivor, advises: "If you're walking above timber line, you're the highest point around and a natural target. Don't hesitate to head back the minute skies begin to darken." If you plan to hike, camp, or boat in lightning country, get a forecast before you go. Bring a radio for forecast updates, keep an eye on the sky, and be ready to seek shelter.
When a thunderstorm threatens, here's what the National Weather Service recommends: Try to get inside a house, large building, or all-metal vehicle such as your car (except a convertible, which offers no protection). Get off metal equipment such as bikes or golf carts; stay away from wire fences, pipes, rails.
If you're caught outdoors in a thunderstorm, get down off ridges and high places. Do not go into your tent or any small isolated structure in an open area. Instead seek a shelter in a low-lying area or depression, or dense woods. Never stand under a single large tree in the open--it's a natural lightning target. Hikers in groups should spread out at least 10 yards apart to lessen the danger to all.
Get off open fields, golf courses, tennis courts, parking lots.
Get away from open water--swimming pools or lakes. Enclosed metal boats or small boats that have been protected against lightning can offer safe shelter. If you're caught in a small, unprotected boat, crouch in the lowest part.
In a thunderstorm, if you should ever feel your hair stand on end, drop to your knees and bend forward, putting your hands on your knees. According to TRIP physicist E. Philip Krider of the University of Arizona, it's a common misconception that you should lie flat on the ground. "that's dangerous because you're maximizing your contact with the current flow area. It's better to assume a 'Moslem-style' prayer position," he advises.
If someone near you is struck and severely shocked by lightning, they may need cardio-pulmonary resuscitation (CPR) until medical help arrives.
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|Date:||Jul 1, 1984|
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