Positives and Negatives...
Posted March 3, 2009
Updated March 6, 2009
Everything has them of course, but this being severe weather awareness week, the topic here is lightning. Lightning is a phenomenon that is created by a separation of positive and negative electrical charges in cumulonimbus clouds, and that can occur in both a positive and negative fashion. Also, lightning has its pluses and minuses in terms of impacts, the up side being it keeps the earth's electrical field in balance and acts to "fix" some atmospheric nitrogen so that it is useful to plants. The down sides, of course, are that lightning can be dangerous and disruptive to humans and animals, can interrupt power supplies, and can start fires, both in buildings and in the wild.
It's worth noting that when it comes to severe weather, lightning isn't even technically a required or sufficient criteria. Storms are considered severe if winds exceed 58 mph (50 knots) and/or hail diameter exceeds three quarters of an inch (a value that may be revised upward in the near future). Even a routine thunderstorm that lacks those features can produce numerous lightning strikes, but may not rate a warning in the form of a crawl or a cut-in on television.
Here in the Tarheel state, lightning is an important weather hazard. Thunderstorms here generate an average of well over 500,000 cloud-to-ground flashes annually, and the National Weather Service in Raleigh estimates that central NC receives about 9-12 flashes per square mile each year. While definitive statistics on the number of people struck by lightning and injured in our state are hard to come by, the number of deaths is known to a better degree. During the 10 year span from 1998-2007, for example, North Carolina suffered 19 deaths due to lightning strikes, for an average of almost 2 per year. Of course, it's variable, and 2003 was an especially bad year with 5 fatalities. The statistics here lead to a ranking of 5th in the nation when it comes to raw numbers (behind Florida, Texas, Colorado and Georgia). On the other hand, when you factor in population, we come in 16th in the country in yearly lightning deaths per million people.
I mentioned lightning can come in both positive and negative "flavors," and I'm referring there to negative strikes and positive strikes. The first is the most common, routine lightning strike that transfers negative charge to the ground. It accounts for about 85-95% of strikes, has a very brief transfer of current to the surface, with an average current around 30,000 amps, and typically strikes beneath or very close to the storm cell itself. Positive strikes on the other hand, transfer a net positive charge to the ground, often originate from high in the cloud and can strike as much as 10-15 miles away from the storm (occasionally resulting in a "bolt from the blue" in a rain or cloud-free area), carry currents more like 350,000 amps, and the currents tend to "attach" and continue for 5-10 times the duration of negative strikes. This makes positive strikes more lethal, and also makes them more likely to create power disruptions and to start fires. None of this is to say that negative strikes aren't dangerous, just that positive strikes are an especially powerful jolt!
There are lots of good web sites out there with excellent background information on lightning generation and also on lightning safety, and I've added links to a couple of those here. Also, be sure and catch our severe weather awareness segments each evening this week.
Finally, writing about lightning always takes me back to my closest call with a strike, back in the late 1980s when I was a graduate student at Florida State University in Tallahassee. I had watched through sliding glass doors as a series of thunderstorms rolled across the area and finally tapered off, and after seeing and hearing nothing beyond a distant rumble for 15 or 20 minutes, decided to step out on the balcony. After a quiet 5 or 10 more minutes leaning against the balcony rail with mostly cloudy skies and no rain, a tearing sound, blinding flash and crashing boom left me about 5 feet across the balcony in an instant, while sparks flew and a branch about 10 inches in diameter split off a tree 20 feet in front of me. I was fine, but it was a visceral illustration of how instantly lightning can go from nothing to something, and I've been a little less cavalier about sticking around outdoors with lightning in the vicinity ever since!