By Ned Rozell
June 09, 2006
Because lightning is responsible for most of the acreage burned in Alaska every year, Bureau of Land Management technicians installed lightning sensors at Unalakleet, Bethel, Galena, McGrath, Tanana, Bettles, Ft. Yukon, Fairbanks and Tanacross. Most of the sensors are in the Interior because that's where the vast majority of lightning strikes happen.
Dorte Dissing and her academic advisor Dave Verbyla, both of the UAF department of forest sciences, use BLM sensors to track lightning strikes. While working on her Ph.D. degree, Dissing and Verbyla noticed that dots on a map representing lightning strikes neatly covered the range of boreal forest in Alaska. Boreal forest consists of spruce, birch, aspen, willow and other trees. The area in Alaska covered by boreal forest--the Interior, Yukon Flats, the Copper River Basin, and the Susitna River--received more lightning strikes than did higher ground covered with tundra and shrubs.
Lightning may favor the boreal forest because the forest absorbs less heat (and gives off more heat) than tundra or shrubs. This warmth can produce unstable air masses that help the growth of cumulonimbus clouds, the thunderheads that produce lightning and are known for their characteristic anvil shape.
A stroke of lightning is actually a series of steps too quick for the human eye to separate. Once charges build within a cloud, electrons jump from the cloud in an invisible line. As the electrical charge continues on a crooked path toward the ground, a visible current of positive charge starts upward, usually from a tree, building, or some other tall object. When the electrical path is complete, electrons flow to the ground. If our brains processed information fast enough to break down all the events that happen in one ten-thousandth of a second, we could see lightning strikes upward, rather than downward.
In studying lightning strikes from 1986 to 1997, the UAF researchers found that lightning touches down most frequently in the Yukon-Tanana uplands and flats, the Nowitna, Tetlin and Kantishna River flats, and the Ray Mountains. The lightning champ for the state during those years was the White Mountains north of Fairbanks, which received 23 percent of strikes recorded in the state per year. The Kuskokwim Mountains north of the Alaska Range came in second. They were hit by 10 percent of lightning strikes recorded statewide.
The BLM sensors have located an average of more than 26,000 cloud-to-ground lightning strikes per year. Dissing and Verbyla also found that lightning strikes most frequently in the Interior between 4 and 6 p.m. during late June and early July. Severe storm days in Alaska usually feature from 2,000 to about 5,000 lightning strikes, most of them in the boreal forest.
The boreal forest might be taking more lightning strikes because it's producing heat, or it might just be a coincidence. Either way, Dissing wants to figure out the relationship between boreal forest and lightning. "I think the connection is so striking that I want to pick at it, to see what the link is."
Ned Rozell [email@example.com]is a science writer at the institute.
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