On a winter walk with my grandson yesterday, I pointed out the long narrow scar of a lightning strike on a black locust tree in our wood lot. The tree must have been hit several years ago, mid way up its trunk about fifteen feet from the ground. A prominent six inch wide scar, descending all the way to the ground exposed the now weathered wood. A well healed thickened rim of bark had grown on either side of the long scar. The tree survived. The scar elicited a nice conversation regarding lightning and the particular dangers of cloud to ground strikes.
Coincidentally today, I came across an article (“Why was there less lightning during Covid lockdowns? by Will Sullivan, “Inside Science”, 1/9/22 ) on the interesting fact that during the pandemic and it’s lockdowns there were fewer lightning strokes , than in “normal” non-lockdown years.
The difference was significant….during the pandemic lockdown there was almost 20% fewer lighting strokes! That is quite a large difference.
The story of pandemics and lightning revolves around a common culprit, pollution! During the pandemic, the whole of humanity simply did less—- and polluted less. There were fewer cars on the road, fewer or no airplanes overhead, less burning of fossil fuels and thus less pollution. Our nasty acts against the purity of our atmosphere are the result of our penchant for using the pure air as a cheap way to dispose of our combustion wastes. These nasty acts happen less when we are lockdown.
Combustion (in our auto engines, home furnaces , factories and airplanes) produces tiny solid particles (or often droplets) called aerosols. These tiny particles enter the air and remain suspended, often causing the lack of transparency and clarity we can observe over cities and other centers of human activity.
Recent studies (by Prof E Williams at MIT) reported at a conference of the American Geophysical Union, that fewer aerosols in the air are correlated with fewer lightning discharges!
What is the connection? Aerosols act as sites which attract water and water droplets in clouds. The more aerosols there are in the air the smaller the cloud water droplets, (many aerosols must spread the available water around) and conversely with fewer aerosols (there are fewer sites for water to condense) the water droplets are larger. Larger droplets tend to grow faster and to fall to earth more readily as precipitation.
However, when there are many aerosols in the air the resulting water or cloud droplets are smaller and more numerous. These tend to stay suspended. And in larger clouds (thunderheads) they can be lofted up to great heights where they freeze and form ice crystals.
These very large (billowing storm) clouds develop strong internal currents (updrafts and downdrafts) which carry these now solid icy particles ( snow, ice pellets, graupel) into violent internal patterns of movement which can cause them to strike each other as they move within the clouds. These collisions (billions of them ) are effective generators of static electricity. As they strike each other they may pull off electrons (negatively charged) particles and carry them away, and leave the “collision particle” behind with a resulting positive charge.
Just as electric charges (static electricity) can be separated when a plastic rod is rubbed with a silk cloth., or you, wearing a woolen overcoat, slide across your auto’s plastic seat cover and charge your body with excess electrons, which jump across the gap when you touch the metal door handle. In the same way, static electricity in a cloud is produced when ice crystals strike each other in their violent movements within the cloud.
But an interesting fact is that in a cloud, the particles bearing a positive charge, tend to accumulate near the top of the cloud (perhaps because they are lighter) , while particles with a negative charge accumulate in the lower or middle, parts of the cloud. When the differences in charge are great enough, or the insulating air between charged zones is somehow reduced, a discharge inside the cloud may occur. These are called an “inter cloud” lightning strike. Viewing these, we may see the whole cloud light up from within.
Of course, discharges occur between clouds, or between the ground and cloud (the typical lightning stroke) as well. All for the same reason.
And according to Professor Williams of MIT these discharges are about 19% less frequent during pandemics, when humans are forced into lockdowns and produce fewer aerosols!!! So one upside of a pandemic is there is a near 20% lower chance of being struck by lightening during a pandemic! Well that is something positive about pandemics …. but not much.
Now getting back to that tree in our wood lot which was hit by a cloud to ground discharge. How did that happen?
As in the above explanation, recall that the base of a thunderhead cloud has developed a strong negative charge as a result of the separation of charged particles in the cloud. As the cloud moves over the earth’s surface it induces a positive charged “shadow” in the ground just below it.
This induced charge happens because loose electrons within matter —like trees, metal fence lines and even the earth—can move in response to the positive cloud in the cloud base. As these electrons “sense” the strong negative charge just above them they are repelled away.
This repulsion of elections on the ground directly below the cloud base cause these substances to become strongly positively charged.
The negatively charged cloud base can then discharge its electrons to the “induced” positively charged object —a church steeple, ships mast or a tall tree—on earth in a cloud to ground discharge
That tree top in our wood lot became strongly positive, as the negatively charged cloud base moved overhead. The loose electrons in the tree’s tissues moved through it toward the ground. The top of the tree became more highly positive charged as so many electrons were repelled. The cloud base then may have discharged its static electric electrons to that tree….and caused that awful scar.
So such places are to be strictly avoided in thunderstorms.
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