As of Saturday, August 2, 2014
We’ve experienced a couple of prolonged thunderstorms this summer, and will probably see more before we’re through. In addition to the risks of wildfire and injury, lightning offers the chance to see natural science on display.
Although most thunder and lightning occurs in tropical latitudes, the US experiences over 100,000 thunderstorms annually. The Pacific Northwest represents the region of lowest occurrence, but Oregon still averages over 50,000 ground strikes a year.
Thunderstorms usually form over land, particularly during the hot months of summer. While they can appear over large bodies of water, land surfaces are more likely to supply all three major factors that contribute to thunderstorm development: moisture, rising unstable air and a geologic feature that provides additional lift to the air.
When sunlight heats the Earth’s surface, which happens more quickly to land than water, the overlying air also warms. Warm air rises, and rises even faster when atmospheric conditions are unstable or when the moving air runs into hills, mountains or another cooler air mass.
As moisture in the air gets carried into the cold upper atmosphere, it releases heat and condenses to form clouds. The higher a cloud reaches the colder it gets, until water droplets freeze and enlarge, finally falling back down into the lower cloud. Powerful up and down drafts of wind and turbulence develop, carrying liquid and frozen water in both directions.
Collisions between the passing ice and raindrops strip negatively charged electrons from the water atoms, which tend to stick to the falling droplets. The result is a buildup of negative charges at the bottom of a thunderhead and positive charges near the top.
The air in the middle acts as an insulator, keeping the opposite charges separate until strong enough to jump the gap — the massive electrical discharge we know as lightning.
The negatively charged cloud bottom also attracts positive charges in the ground as it passes overhead. These ground charges follow along beneath the cloud, getting drawn up high points like trees and poles that extend closer to the cloud. Where the charge differences become strong enough to jump the air gap, lightning strikes. Because charges inside a cloud are usually stronger than those between the cloud and ground, most lightning occurs within clouds rather than striking the ground.
The average lightning bolt measures one or two inches in diameter, 5 miles in length and contains billions of watts of electricity. About 90 percent of this energy is converted to heat and 9 percent to light, leaving only 1 percent for the sound of thunder.
A lightning bolt almost instantaneously heats the air along its path to as high as 50,000 degrees Fahrenheit. This causes a sudden violent expansion of the air, producing a shock wave that travels up to 30 feet with destructive force. Beyond this distance the wave continues as sound vibrations in the air, or thunder.
While a lightning flash reaches us at the speed of light, thunder travels much more slowly at the speed of sound. Sound waves take nearly 5 seconds to travel each mile that light covers almost instantly. This explains the delay between seeing lightning and hearing its thunder. Count the seconds between a lightning bolt and the resulting thunder, divide by 5, and you have a rough measure of the distance in miles to the bolt. For instance, a 15 second count indicates a distance of about 3 miles.
Lightning can and does strike in the same spot twice (or more), and while you can see lightning up to 60 miles away thunder usually won’t carry more than 10 miles. Lightning can also strike from blue skies, the famous “bolt from the blue.” This is a bolt that travels from a distant thunderhead, striking the ground as far as 25 miles from the storm cloud itself.
The power and potential of even a small thunderstorm can be tremendous, something to always keep in mind (especially if you’re above timberline and the tallest thing around).
Basic safety rules for lightning are widely available online at sites such as www.ready.gov/thunderstorms-lightning.
Oregon native Fred Schubert, a The Dalles biologist, has a lifelong interest in general science and science writing. Feel free to submit comments on this article or suggestions for new topics to email@example.com.
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