BI Answers: How does lightning form when it is not raining?
Last week, a rare lightning strike hit Venice Beach in southern California, killing one and injuring 13. But the strangest thing was that the lightning came out of nowhere, without any rain or any warning.
“It was not raining. It was slightly overcast [with] light clouds,” Inspector Rick Flores of the L.A. County Fire Department told Business Insider in an interview. “We weren’t expecting it.”
So where did this freak lighting storm come from, and why wasn’t there rain?
What is a thunderstorm?
Usually, rain, thunder, clouds, and lightning come as a package deal which we confusingly call a “thunderstorm.” Here is how you brew one.
When the sun heats the air close to the surface of the Earth, it rises. As it rises, the air cools — and so does the moisture it holds, according to NOAA. As the moisture cools, it condenses and forms heavy clouds. These clouds are well-known as the source of rain, but they are also the source of lightning.
Scientists still debate the details of how exactly lightning forms, but it likely has to do with what happens when a cloud is floating in the freezing air high above us. Tiny ice particles form inside the cloud, according to NOAA. And as those particles collide with water droplets and with each other, it generates the electrical phenomenon that’s crucial to the development of lightning.
National Geographic explains: “Ice particles collide as they swirl around in a storm, causing a separation of electrical charges. Positively charged ice crystals rise to the top of the thunderstorm, and negatively charged ice particles and hailstones drop to the lower parts of the storm. Enormous charge differences develop.”
As you can see below, the lightning forms because the positively charged bits of precipitation rise to the top of the storm, while the negatively charged bits hover towards the bottom. The negative charges, attracted to the positively charged ground, create a “channel” in the air. When enough charge builds up, “the subsequent electrical transfer in the channel is lightning,” according to National Geographic.
In turn, lightning creates thunder. The energy released by lightning heats the surrounding air to 50,000 degrees F according to NOAA. The hot air rapidly expands and contracts creating a sound wave in the process that we hear as thunder.
So how did California’s rogue lightning and thunder form without the rain? The short answer is that despite appearances down at the beach, there actually was rain — just too high in the sky for us to notice.
“This what we we often call dry lightning,” William Patzert from NASA’s Jet Propulsion Laboratory told Business Insider. The rain was falling, but the dry atmosphere made it evaporate before it hit the ground.
Dry lightning storms are actually quite common in much of the Southwest U.S. They are often the cause of wildfires — since it is so dry, the forest lights up like kindling. But these storms don’t usually find their way to the coast.
“If you looked at, say, a square mile of land in southern California, you would have to wait four years before you got a lightning strike on that square mile on average,” said Joseph Dwyer of the Florida Institute of Technology. You can compare that with Florida, the lightning capital, which has 30 flashes per square mile per year, he said.
But that day, “we had a funny condition,” said Patzert. “We had a very high pressure system over the Southwest and what it was doing was pulling in very warm, moist tropical air from Mexico and the Gulf of California,” he said. That created the thunderstorm, which generated no rain on the ground — at least in Venice where the lightning struck.
Usually that tropical air would head over to New Mexico, Arizona, or California’s eastern desert. Not the coast, he said, “but this time it did,” and with devastating consequence.
When lightning’s charge hits the ocean, as it did in California that day, it can be especially dangerous. This is because salt water is a great at spreading electricity. “It’s really highly conductive, so it strikes and people can actually be affected within a 100 foot radius,” Patzert said.
“In Florida, when lightning is approaching, then they clear the beach, but it’s such a rare occurrence here in California,” he said. “It totally took everybody by surprise.”
This post is part of a continuing series that answers all of your “why” questions related to science. Have your own question? Email [email protected] with the subject line “Q&A”; tweet your question to @BI_Science; or post to our Facebook page.
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