We knew what was coming next.
While we were sitting in the van on Thanksgiving Day, waiting for more passengers to shuttle, the dark and tumultuous sky suddenly lit up. Several seconds later the thunder rattled.
We were treated to several flashes of lightning and echoes of thunder that day as the clouds pelted us with rain. Since Oregon gets better cloud development in winter than in summer, we also get more lightning and thunder in winter than we do in summer.
Clouds, such as the dense blanket roiling overhead this Thanksgiving, form as relatively warm, moist air cools off -- usually by rising to a cooler level in the atmosphere -- condensing invisible water vapor into visible droplets. While we might paint them as serene, clouds may be very dynamic, with currents forcefully tossing the various water droplets and ice crystals around the cloud. When such currents are violent enough, differently sized particles run into each other going up and down, sharing glancing blows.
In very turbulent, large clouds (such as the extremely tall thunderheads) the negatively charged particles tend to collect at the bottom and the positively charged particles at the top. The greater the distance between the positively and negatively charged particles, the stronger the electrical field and the more pronounced the resulting lightning.
Like scuffing your feet on the carpet, the friction builds up static electricity; like getting a shock off the doorknob, lightning is the sudden discharge that balances the electricity. That discharge may form a single line, branches, a sheet, or, rarely, a ball. (From our vantage point on Thanksgiving, the clouds obscured the lightning, so we couldn’t identify its shape.)
Occurring about 100 times each second, planet-wide, lightning usually takes place between clouds or within a cloud or between a cloud and open air. World-wide, only about a quarter of lightning occurs between a cloud and the ground (land or water). And lightning doesn’t form only with rain clouds. According to NOAA’s National Severe Storms Laboratory, lightning can develop in many severe atmospheric disturbances with particles that rub against each other, such as in heavy snowstorms, in the billows around intense forest fires, or around volcanic eruptions.
In most lightning types, the invisible first stroke is the flow of negative charge toward a more positively charged part of the cloud or a patch of land/water. That negative charge may “feel” its way to the positive source, taking many tentative, forking, zigzagging steps. As it travels, this “step leader” opens a one- or two-inch wide channel of ionized air for the return stroke.
As the negative charge approaches the positive source, the positive charge immediately rallies. When the flow of the two charges is close enough, a “return streamer” of positive energy passes through the channel towards the negative, connecting the current. This return stroke is the flash we see -- too fast for us to see the actual movement -- traveling back up the channel. Rapid repeating of the process makes the lightning flicker.
The huge amount of electricity (30 million volts or more) suddenly discharged by the lightning heats the air to very high temperatures -- up to 18,000 degrees F -- in a tiny fraction of a second. This explosive heating of the air expands it suddenly enough to create the massive sound waves we call thunder. Thunder rumbles, in part, because the step leader and the return streamer create different sounds, with the return being louder, as the lightning flickers.
In addition to causing thunder and setting trees aflame or instantly boiling water, lightning’s great heat may fuse rock, soil, or sand when it hits the ground, forming a "fulgurite." "Solidified lightning," fulgurites are usually long and slender, often tubular. Fulgurites typically have a glassy core, where the heat is apparently greatest, with a gritty skin of fused grains. The Reedsport office of the Oregon Dunes National Recreation Area has a locally-formed, foot-long fulgurite, offering concrete evidence of past Oregon coast thunderstorms. According to the Guinness Book of World Records, a Florida fulgurite, dug up in 1997, had a 17-foot-long branch and a 16-foot-long branch.
Thunder never occurs without lightning and lightning always produces thunder.
So, if lightning produces thunder, why do we usually say “thunder and lightning” and put the result before the cause?
For information on how to arrange a gift certificate of an exploration of our fascinating natural history, contact Marty at 541-267-4027, firstname.lastname@example.org, or www.facebook.com/wavecrestdiscoveries. Questions and comments about local natural history are welcome. www.wavecrestdiscoveries.com