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David B. Fankhauser, Ph.D.
Professor of Biology and Chemistry
University of Cincinnati Clermont College
Batavia OH 45103

This tree was struck up the valley from my house and produced a very interesting spiral pattern of damage. Thanks to my student Kasey Stopp for bringing it to my attention.

My hypothesis: Tree trunks grow in spirals over the years, providing conductive channels down which lightening electricity can easily flow. The bark has been blown off above these channels as the charge is transmitted.

source: David B. Fankhauser, PhD, Clermont College


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Alaska Science Forum
More on Why Tree Trunks Spiral
Article #783

by Larry Gedney

This article is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Larry Gedney is a seismologist at the Institute.

In an earlier column, I asked if any readers could explain why the grain in trees seemed to spiral up the trunk-in a clockwise direction. That is, spiral marks in old trees crack open from the upper right to lower left around the trunk. Professor (now Emeritus) Neil Davis, the originator of this column, posed the same question in this column over ten years ago, and it's time for an update.

I received numerous replies to the query, and thank all those who answered. I'd like to repeat two of the better responses here. First, David J. Friis of Anchorage included not only some solid reasoning, but a bit of whimsy. He said:

"When I worked for NOM 's Environmental Research Labs in Boulder, we were once asked why lightning sometimes spirals down the trunk of a tree. While the answer was not proven, we observed that the path of least resistance might follow the spiral grain of the wood. I eventually found a tree with a spiral lightning mark and it followed the spiral grain exactly. One tree, of course, proves nothing.

"But why should the tree spiral? More speculation here: Foliage tends to be thicker on the south side of the tree because of better sunlight. Prevailing winds, in most of the tree-growing northern hemisphere, are from the west. Combine these factors, and the westerly wind pushing on the thicker south side of the tree, year after year, causes an asymmetrical wind loading which slowly twists the tree around in the observed direction.

"This reasoning is so obvious that there must be something wrong with it or else it would be well known to plant biologists. I can think of several ways to test this hypothesis. My favorite is to examine trees in the southern hemisphere for an opposite rotation. I have several countries in mind. Do you know where I can get a grant?"

(I should add here that I checked with retired University of Alaska Professor Tunis Wentink, an expert in wind power, and John Lingaas of the National Weather Service in Fairbanks to find the prevailing direction of wind in the Alaskan Interior. Both agreed that it was extremely variable, but that it was most often from the north-northeast during the winter, with stronger winds from the southwest during the growing season of June and July.) .

I applaud Mr. Friis's ingenuity, but it was matched by that of Hans Nielsen of the Geophysical Institute. Professor Nielsen was the first to come up with the clockwise-spiraling question in this column back in 1976. He apparently has had time to think the matter over further.

Hans told me recently that he thinks the matter can be related to the Coriolis effect of the earth's rotation. In the northern hemisphere, all moving objects are diverted ever so slightly to the right. (This is why hurricanes rotate counterclockwise--air moving toward the storm is diverted to the right and thus imparts a counterclockwise spiral to the storm at the center.) Nielsen thinks that possibly when a tree is rocked by winds, the tip might tend to rotate in a counterclockwise circle when viewed from above. This would lead to a clockwise spiral twist. (That sounds like a contradiction until you think about it for a while.)

Granted, not all trees exhibit the same twist, but the majority of them do. The phenomenon can be likened to the claim that water will always spiral out of a drain in a counter-clockwise direction in the northern hemisphere. It is well known that you can make it spiral out in either direction, if you give it a little shove first. Local effects such as topography of the landscape (or irregularities in the bowl) play a much greater role than does the very minuscule Coriolis effect.

I found it intriguing that even though they had entirely different suggestions about the cause, both Friis and Nielsen suggested that a good test would be a trip to the southern hemisphere to check the direction of twists in trees there. In the southern hemisphere, the prevailing winds at similar latitudes would be in the same direction, but the Coriolis force would be to the left and the sun would lie to the north, rather than to the south. The observed sense of twisting in trees should therefore be clockwise around the trunk, resulting in a counterclockwise spiral of cracks (upper left to lower right). If all this makes you slightly dizzy, as it does me, try twisting a roll of putty or bread dough, and you'll see what I mean.

A final note: Barry Donnellan, a Fairbanks attorney, observes that the term "spiral" is not correct in the sense that we've been using it here. The preferred use of spiral, he points out, is the description of a plane curve like a neatly coiled garden hose lying flat on the driveway. If you raise one end of the coiled hose, you would have the shape that we're talking about, which is a helix. But, as he says, who ever speaks of a "helical" staircase?

source: More on Why Tree Trunks Spiral, Alaska Science Forum


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The Strike

When the charge exchange pathway is connected, the visible light of a lightning stroke moves at one-third the speed of light, spewing light and instantaneously heating the air. Usually, several charge exchanges (three or four strokes on average) occur within one lightning strike. Each stroke can last many milliseconds. The duration of the complete flash of lightning, including periods between individual strokes, is usually about one-half second. The human eye can just catch individual strokes within each strike, which makes the lightning appear to flicker.

Because of rapid changes in air resistance and ground streamer locations, each stroke within one strike may not follow the identical pathway. Ground points exchanging charges within one lightning strike can be separated by more than a mile but usually are closely grouped. Several trees in a row can show damage from several different strokes within one lightning strike. Anywhere along a strike path, lightning can jump ("side-flash") from one object to another, such as from a tree to a building or person.

The charge exchange carried by a single lightning strike is highly variable. Average electrical values for a strike are 100 million volts and 35,000 amps. The inner core of a lightning strike ranges from 2 to 12 inches in diameter, with a bright, surrounding halo ranging from 3 to 15 feet across. Internal core temperatures of a strike path can exceed 30,000¼F. This heat source causes rapid air expansion, generating a shock wave heard as thunder.

Tree Damage

Most trees along a lightning path are not killed. More than 20 percent of trees along a lightning strike path carry no visible injuries. Nevertheless, trees presenting no immediate sign of lightning damage are still prone to increased stress, inefficient defenses, and pest attacks. Tree damage mirrors the strength and duration of the charge exchange in a lightning strike. The most serious tree injuries caused by lightning are from the acoustic wave (shock wave) radiating from the lightning path core (momentarily reaching approximately 500 to 1,500 psi of pressure). Additional damage is caused by water heating and steam explosions within tree tissues and by electrical disruption of living cells.

Identifying Scars

Lightning-strike damage to trees is as variable as individual lightning strikes and individual trees. Most people are familiar with the long bark and wood strips, or bark sheets, ripped away from a tree by a lightning strike (Figure 4). All parts of the tree and the soil can show impacts from lightning's passage. Lightning strikes can often leave a tree with some form of bark damage following the longitudinal axis of the tree. Most lightning scars in trees are shallow and continuous between a point at least 80 percent of the tree height above the ground to within several feet of the tree base. In any woodland or park, a number of living trees may show scars of a past lightning strike. Of all trees with scars, about 10 percent have more than one scar. These scars do not always lead to immediate death of the trees.

Trees damaged in the past by lightning present a number of key symptoms and signs, usually represented by scars (woundwood along damaged tissue perimeters). Most lightning scars in trees follow the longitudinal axis of the xylem cells (wood grain). Xylem grain orientation develops based on mechanical loading in trees applied through bending and torque (twist). Some trees have straight grain, and some have unequal wind forces applied to their crowns (lopsidedness), causing xylem grain to spiral down the stem. Lightning scars can spiral down the stem following the longitudinal spiral pattern of the xylem elements. The initial electrical flow along the grain offers the least initial electrical resistance within a tree.

source: International Society of Arboriculture