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Don Paul: A primer on lightning and how it forms

Don Paul

There are about 1.4 billion lightning discharges globally in a year. Each second, about 40 to 50 strokes occur. In terms of numbers and types of discharges, 5 to 10 more intracloud, or cloud to cloud, discharges occur than cloud to ground. Talk about a lot of static.

The National Severe Storms Laboratory (NSSL) in Norman notes two other basic types of lightning categories. Natural lightning occurs from electrification of the environment, and artificial lightning is triggered by the presence of tall objects, airplanes and rockets. Lightning strikes that are triggered by tall objects are ground to cloud; that is, from the object upward. Natural lightning strikes to the surface are cloud to ground (CG) and are the most dangerous to us.

Cloud-to-ground strikes are initialized by invisible stepped leaders of a negative charge that zigzag to the ground in 50-yard segments. The leaders are attracted to positive charge streamers from objects on the ground. A return upward luminosity discharge occurs when the negative leader and positive streamers connect, and the discharge travels at 60,000 miles per second back toward the cloud. That’s what we see.

The flash can range from one to as many as 20 return strokes, which creates the flickering we see. Amazingly, the actual stroke is just 1 to 2 inches in diameter. It heats the adjacent air to around 18,000 degrees Fahrenheit, with the instantaneous expansion of the air molecules causing the thunderclap. Thunder can be heard as far as 25 miles away, at which it typically sounds like a low rumble. That rumbling sound results because that’s all that’s left after the higher frequency “cracking” sound from the original thunderclap has been absorbed by the environment en route to distant locations.

With NOAA’s newest weather satellite lightning sensors, GOES 16 can observe lightning discharges from space:

Despite voluminous study, much remains to be learned about the generation of lightning. One well-supported theory is tied to the electrification that occurs from the collision of different-sized raindrops, hail and graupel pellets. As these coalesced drops and hailstones grow heavier, it’s thought they carry more negative charge to the base of the thunderstorm cloud, known as a cumulonimbus cloud. The upward motion of convection, which is what develops the cumulonimbus cloud, it thought to transport positive charge higher in the cloud from the ground, while the negative charges move downward in the colder downdraft of the storm. Click here to see a modeled cross-section of a thunderstorm and its typical charge distribution.

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By the way, you can safely assume I’m oversimplifying the complexities of lightning, about which countless papers and books have been published.

Cloud-to-cloud or cloud-to-air discharges are more common, but we’re less concerned with them, except for aviation safety. As I mentioned in a previous article, airliners are commonly struck, with little or no damage.

Less well-understood types of lightning occur above the thunderstorm and cannot be seen from the ground. Red sprites are a large, weak flash that seem to occur at the same time as more powerful cloud-to-ground discharges beneath. Blue jets are even less well understood, last only a few thousandths of a second, and are sometimes observed by pilots. Elves are broad disc-shaped discharges lasting no more than one-thousandth of a second. They seem to be associated with high-lightning-content thunderstorms. Click here to see what these can look like.

Lightning detection is now widely used by meteorologists and other users to keep track of discharges in real time (within a few seconds). The data is supplied by the National Lightning Detection Network, developed by the New Mexico Institute of Mining and Technology. More than 100 ground-based sensors are located across the lower 48 states. The data is transmitted, to users such as WKBW and the other local stations, National Weather Service offices, utilities and other diverse customers, by Vaisala in Tuscon. The stored data reveals patterns that are very useful in lightning research.

Lightning forecasting is a burgeoning science, and complex algorithms are being developed to predict which storms pose a greater threat of cloud-to-ground strikes. We also know that increasing cloud-to-ground and cloud-to-cloud strikes are a sign a storm is intensifying. There is also a growing body of evidence that a rapid spiking in lightning discharges within a storm may be an indicator a tornado is forming. Such a spike in lightning count occurred in an EF-5 tornado that struck Moore, Okla., in 1999.

My next article will focus on lightning safety.

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