Author: Matthew Cappucci / Source: Science News for Students
What’s scarier than a tornado? How about a tornado made of fire?
On July 26, 2018, the so-called Carr Fire outside Redding, Calif., spawned the strongest tornado in state history: a fire tornado, or firenado.This rare and terrifying phenomenon was only the second true fire tornado in recorded history — and the first witnessed in the United States.
Wildfires have become an all too common phenomenon in California. The region’s low humidity and scarce rainfall make it an environment ripe for blazes. In fact, much of the state should burn naturally every 50 to 100 years or so. An occasional fire can even help the ecosystem. It’s a way for nature to restore nutrients to the soil while cleansing the landscape of an overgrowth of moisture-robbing vegetation. But people have been building homes in these regions. So when a forest goes up in flames, so too can houses. (Witness the estimated 6,000-plus homes destroyed by the so-called Camp Fire, this month, in Paradise, Calif.)
The Carr Fire was first reported on July 23, west of Redding, Calif. An RV trailer suffered a flat tire, which caused the wheel’s metal rim to scrape against the roadway. Authorities believe that sent sparks flying, USA Today reported in August.
Dry debris nearby caught fire. Eventually, this blaze consumed an area three times the size of Washington, D.
C., according to CalFire. That’s the state’s wildfire-fighting agency. The flames spread from forest to neighborhoods. And by the time it finally died out, the fire had claimed 7 lives and 1,604 homes and other structures.But the truly remarkable part: This inferno grew so strong that it unleashed a massive tornado.
Wildfires can make for wild weather
Roughly half of all land burned in U.S. wildfires in 2017 was in California, Montana, Nevada, Texas and Alaska. That’s according to a November 2018 report by the Insurance Information Institute. And because of California’s large and dense population, wildfires in this state are among the most costly, both in terms of damage and lives lost.
Much of California is dry almost year-round. Large parts of it also get quite hot. Winter is usually the wettest season. That’s when big Pacific storms carry the Pineapple Express — a river of moisture that develops in the middle atmosphere. These storms target the California coast with a seeming firehose of moisture. Those rains fuel the growth of vegetation.
In the spring and summer, winds from the west pull in cool air from the Pacific Ocean. That gives San Francisco its famous fog. These winds also force moist air up the mountains. But when it sinks back down on the other side of the state’s mountains, that air dries out. This desert-like air can suck the moisture out of anything it touches. So any dead plant matter begins to dry up. By mid-summer, much of the ground throughout the state is littered with brittle sticks and leaves. This becomes a powder keg of fuel for a fire to gobble up. Lightning, unattended campfires, discarded cigarettes and sparks from vehicle tailpipes — all of those can ignite the dry forest debris.
Farther inland, winds swirl clockwise around a semi-permanent system of high pressure that parks itself near Reno, Nevada. This sends occasional spurts of wind and dry air westward through the Santa Ana Mountains and Sierra Nevada. These so-called Santa Ana winds can top 97 kilometers (60 miles) per hour. They dry the air out and can fan the flames of a wildfire.
If they get big enough, wildfires can create their own weather. The largest of them suck in so much air that the entering winds can flow at speeds of up to 130 kilometers (80 miles) per hour. These winds also supply fires with plenty of oxygen, which the blazes need to burn.
Once in a while, a wildfire will reach so high into the atmosphere that it causes rain. That happens when the warm, steamy updraft carries water vapor to a level where this gas condenses and falls out as liquid droplets.
Some wildfires even produce lightning. Soot, smoke, ash and tree-formed hydrocarbons can become electrically charged as they interact with ice crystals above 7,600 meters (some 25,000 feet). The ice takes on a positive charge. Raindrops become negatively charged. This charge-producing phenomenon has a really long name: triboelectrification (TRY-boh-ee-LEK-trih-fih-KAY-shun). When the electrical charges between the ice and rain grow large enough, a lightning bolt can pass between them.
The Carr Fire churned up some particularly wild weather — a true fire tornado. And one key factor behind that was the speed of the storm’s updraft.
The evolution of a fiery ‘tornado’
The National Weather Service, or NWS, releases weather balloons to collect a vertical profile of the temperatures, humidity, wind speeds and barometric pressure as they rise through the atmosphere. One of these daily soundings was taken with a balloon sent up before sunrise…
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