Lightning Myths: The "world record lightning strike" means that lightning can strike 200 miles away from a thunderstorm
TRUTH: Relax! Lightning can travel no more than 20-25 miles away from a storm, in even exceptional cases. You don't need to worry about distant storms 100 miles away zapping you!
A recently-released science journal paper describes a lightning strike in Oklahoma on June 20, 2007 that was nearly 200 miles long, a world record. News articles picked up the story, raising alarm that "we'll have to rethink our lightning safety rules" because the event showed "lightning can strike farther from a storm than previously thought". These fears are unwarranted, originating from a misunderstanding of the data.
The world-record-length lightning event was not a classic "bolt from the blue" (what we call a strike leaping out into clear air away from a storm). Rather, the record event happened entirely within an expansive thunderstorm formation called a "trailing stratiform" region. The stratiform region is an area of electrified clouds and light precipitation that extends some distance behind the heavy leading cores of a squall line. Here is the radar image of the storm complex that produced the event:
Fig. 1: Radar image showing the convective squall line and attendant trailing stratiform precipitation region that produced the record-length lightning discharge over Oklahoma on June 20, 2007.
Thunderstorm squall lines like this are far from rare - they are commonplace in the central USA, with a couple dozen or more happening each year. They can extend for hundreds of miles, many times spanning across several states. The attendant stratiform regions of these systems are usually one contiguous mass, extending a hundred miles or more behind the leading line storms.
These stratiform shields are highly electrified, fed by the charge-generating updrafts at the leading edge of the complex. The lightning discharges that result will spread horizontally across the stratiform region, growing in a chain-reaction fashion as they tap into the vast regions of electrical charge. Storm chasers call these long, horizontal lightning discharges "anvil crawlers", a colloquialism to describe the way their channels visually propagate across the sky. These "crawlers" routinely cover vast distances - lightning bolts dozens of miles long are routine events during these types of storms.
Fig. 2: An "anvil crawler" lightning discharge within a squall line's trailing stratiform shield.
As these discharges progress, they often send out periodic interconnected vertical channels that strike the ground:
Fig. 3: An "anvil crawler" lightning discharge with an interconnected cloud-to-ground component.
You've likely experienced many of these storm complexes yourself: the storm arrives with torrential downpours and frequent, loud cloud-to-ground lightning and thunder. Those are the leading cores of the squall line. As the stratiform region arrives and passes overhead, the rain tapers off to a steady light shower, while the lightning becomes less frequent with softer, longer rolls of thunder.
So, to summarize, the record-length lightning flash was an "overachieving" anvil crawler-type discharge in just such an environment, contained within a long squall line's stratiform precip area. Contrary to what the news articles say, there are no new lightning safety implications from the record event. Since the record-length channel was entirely within a large thunderstorm complex, anyone at risk from getting struck from any part of it would already be inside the storm, experiencing lightning and thunder before and after the event.
The takeaway? The adage "when thunder roars, go indoors" still holds true. If you can hear thunder, you're within range of the next strike. There is, however, no need to worry about a storm more than 30 miles away sending a lightning bolt out to zap you. However, you should be mindful of new thunderstorms that might develop nearby or overhead, as any storm in your vicinity means the conditions might be ripe for additional storms areawide. Those storms 100 miles away also could be moving in your direction!
So, what about actual "bolts from the blue", the ones that reach out from storms into clear air? Those indeed are a threat if you are less than 25 miles from a storm. This photograph by Louise Denton from Australia is the longest "bolt from the blue" lightning strike I've seen documented, probably on the order of 15-20 miles away from the storm:
Flickr photo link
That example is near the upper end of horizontal distance seen for these types of events - most "bolts from the blue" travel less than a third of that range.
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