Several readers of my wildfire blog suggested that I had given lightning strikes short shrift as causes of forest fires.  In fact, lightning as a cause is not even mentioned because it is not in the top four causes, at least until 2015, as reported in the the paper I used, which is cited at the end of this piece. They plot the proportion of number of fires and area burned by cause of ignition in (a) the Santa Monica Mountains and (b) San Diego County. I am not reproducing the plot here for copyright reasons but interested folks can go to the link in the citation (Syphard and Keeley 2015) below.

Most such studies concentrate more on the area burned, than numbers of ignition, presumably because that is the net effect on the public and on the environment.  Lightning as a cause is present in both areas studied by Syphard and Kelley, but the incidences are few and importantly, the areas burned are low when compared with the four largest contributors, arson, campfires, power lines and equipment (this last is a grab bag including chain saws and mowers).  These data beg the question whether something has changed in the last half dozen years to make lightning a bigger factor.  I examined a later (2018) paper by the same authors and found no change in this area.

Then I found a recent story which indicated that between August 16 and August 19, 12,000 bursts of lightning were recorded. Many of these ignited fires, burning more than 1.3 million acres across California.  That is nearly a third of the total area burned to date, certainly not in line with the conclusion that lightning is a minor player.  The Science News story linked above gets into an explanation, with the gist of it being “a bit of bad luck and a landscape primed for fire devastation due to climate change”.  The bad luck part is a combination of two events, each rare for that time of year, and leading to the production of “dry lightning”.  Before getting into what that means, and why it happens, first a quick primer on lightning.

Storm clouds usually form by the collision of colder and warmer air.  Moisture can be in the form of water, ice, or a soft hail.  Air movements cause collision of the species and these collisions produce static charges. The negatively charged particles sink in the cloud and the positively charged ones go to the top.  When the charge balance is sufficient to break down the insulating medium between, a lightning flash occurs, within a cloud or between clouds. The heat generated can be intense, which produces a shock wave, which we hear as thunder.

Image of a dry thunderstorm. Courtesy National Oceanic and Atmospheric Administration

Dry lightning is lightning associated with thunderstorms producing very little rain on the ground.  Ordinarily rain can be expected to render the vegetation less susceptible to ignition.  In a forest with a canopy, 0.1 inches are needed, although as little as a tenth of that is sufficient for open grassland.  Dry thunderstorms do produce precipitation, but evaporation in the hot dry air on the way down results in very little on the ground.

The dry lightning which caused many of the August fires in California this year were described by experts as “freak” events.  Tropical storms far south were wind driven to California to supply cloud forming moisture.  At the same time, the remnants of a Sonoran Desert storm conspired with this tropical moisture to form high storm clouds at elevations of 9000 feet.  Copious lightning was not accompanied by rain on the ground because of the hot and dry conditions below the clouds.  Wildfires were the result.

What are we to make of this and what is the connection to climate change? Prior to the dry lightning events, California was experiencing record-breaking hot and dry conditions.  August 16, the first day of the dry lightning onslaught, somewhat coincidentally, was the date that Death Valley, CA recorded the highest ever temperature measured on the planet: 130 o F.  These conditions certainly carry the imprint of climate change.  But connecting dry thunderstorm conditions to climate change is more of a stretch. No modeling to date has been disclosed. Other phenomena, such as increased frequency and intensity of El Nino, are grounded in mechanisms.

I am forced to conclude that lightning as a source of large wildfires is a chance occurrence.  But lightning could strike again, symbolically* and really.

Vikram Rao

*” Lightning’s striking again” in Lightnin’ Strikes, by Lou Christie (1965), written by Lou Christie and Twyla Herbert

Syphard, A. D., & Keeley, J. E. (2015). Location, timing and extent of wildfire vary by cause of ignition. International Journal of Wildland Fire, 24(1), 37.


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