Lightning Seminar Part 2: Oddities and Questions 1/2
This is Part 2 of my notes from the Vaisala Lightning Seminar. Please see the other entries in this series for definitions and more information.
WHATS SO IMPORTANT ABOUT POSITIVE STROKES?
In Lightning Lingo yesterday we talked about the Polarity of strikes. Positive Strokes are not necessarily stronger, but are associated with "long-continuing current" which is especially damaging and more likely to be fatal, but there hasn't been much research on this. Safe to say: You probably won't notice whether you have been struck by a positive or negative lightning strike.
Positive Strokes are generally associated with ice storms in the winter in the continental U.S. in the winter; scientists are not sure why.
Positive Strokes were discovered in Japan when an electric utility there used a lightning network that detected negative strokes, and was getting equipment severely damaged by phantom strikes that didn't appear on the maps.
WHY DO SOME STORMS HAVE MORE LIGHTNING THAN OTHERS?
While scientists aren't sure for certain, storms have to be tall enough to gain ice crystals in their tops and graupel (hail) in an updraft. Lightning appears to be at a maximum with storms of 50 to 55,000 feet, though it can occur in much "shorter" storms. A more severe storm doesn't necessarily mean more lightning, as radar reflectivities of around 45 dBZ appear to be most conducive to lightning.
Scientists aren't sure why this is, but some possible reasons included: 1.) Strong storms have stronger updrafts which reach higher into the atmosphere, where lower pressure is more conducive to lightning, 2.) A dense, lower charge region (which has been observed in strong storms) may stop lightning from reaching the ground. Regions are pushed higher by stronger thunderstorm updrafts in stronger storms).
CAN LIGHTNING BE USED TO SIMULATE RADAR OR RAINFALL RATE MAPS?
Based on these numbers, it is possible to roughly simulate weather radar over ocean areas, where radar data does not exist, but more research is needed. You can also assign a rainfall rate to a lightning flash, but it varies greatly depending on where you are, but once a relationship is established, it can be helpful on a local basis.
HOW FAR AWAY CAN LIGHTNING STRIKE FROM A STORM?
Vaisala didn't have much for numbers in this category; the average air-mass storm will bring lightning strikes less than 5 miles from its center, which may not even be outside the storm. Before I have blogged about "bolt from the blue" lightning that stretched out to 30 miles; you can read below about one "spider" bolt that travelled for 190 kilometers. Suffice to say: If you can hear the thunder, the storm is close enough to strike you.
HOW MUCH DOES LIGHTNING VARY NATIONWIDE EACH YEAR?
There are millions of CG lightning strikes each year in the U.S. Looking over a period of several years, low variability rates were found for the Continental U.S. (+/- 6%). Of course, this varies regionally, as New England has on occasion seen one day of lightning that might drop half the year's average strikes.
HOW WILL WORLDWIDE LIGHTNING BE AFFECTED BY CLIMATE CHANGE?
Al Gore said in his latest Global Warming diatribe that "Scientists in the Department of Geophysics and Planetary Science at Tel Aviv University tell us that for every one degree increase in temperature, lightning strikes will go up another 10 percent.". When asked for comment, Vaisala said "There are theoretical models that have been around in the literature for about 30-40 years indicating that, in general, lightning amount is very sensitive to temperature and cloud-top altitude."
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