Chimney Sweep

Formation over Creek Hearth stated to be largest in US historical past

A huge thunderstorm hovered over the Creek Fire on Saturday, shooting clouds of smoke into the stratosphere as flames ripped through the Sierra National Forest – and an obscure meteorological term briefly broke into the popular lexicon: pyrocumulonimbus.

This is the name for a rare formation that NASA called “the fire-breathing dragon of the clouds”. It occurs when the scorched air from a wildfire or volcano meets moist, floating air a few miles above the earth. The resulting mass is essentially a rainless thunderstorm sitting on top of a huge fire, said David Peterson, a meteorologist at the Naval Research Laboratory in Monterey.

Scientists believe the pyrocumulonimbus that took shape over the Creek Fire could be the largest ever produced on U.S. soil.

The record-breaking cloud hurled huge amounts of pollutants into the Earth’s already warming atmosphere. Stunned passengers of a passing plane took photos that showed the peculiar shape and immense size of the cloud.

Today I flew from San Jose to Las Vegas with SWA and looked out my window and saw this cloud. I found it to be a cumulonimbus flammagenitus cloud, also known as a pyrocumulonimbus cloud, a type of cloud that forms over a heat source such as wildfire #CreekFire

– Thalia Dockery (@SweetBrown_Shug) September 6, 2020

It was just one of the many anomalous events that kept all eyes on the sky in the past few weeks as the early arrival of the forest fire season spawned one bizarre sky event after another.

Like the thunderstorms of mid-August and the mix of fog and smoke that created an eerie orange sky over the Bay Area on Wednesday, weather forecasters say the Creek Fire pyro cloud heralds a new era of unpredictable – and increasingly volatile – climatic events.

The largest pyrocumulonimbus events ever recorded worldwide have all occurred in the past three years, a trend that suggests that as the forest fire season worsens, they may become much more common.

This could create more dangerous conditions for firefighters on the ground as the vacuum effect created by pyrocumulonimbus creates irregular winds that can disperse embers. Some are known to produce dry lightning or, in rare cases, tornado-like eddies over forest fires, although this does not appear to have happened with the Creek Fire.

The Creek Fire Pyrocumulonimbus formed a day after the fast moving fire was ignited for unknown reasons. The flames had covered 175,893 acres by Thursday and threatened more than 14,000 buildings in Fresno and Madera counties. The fire remains 0% contained and full containment is not expected before October.

The massive cloud formation broke up before the end of Labor Day weekend, but not before it generated an online following and caught the attention of a NASA satellite that captured the evolution of the cloud in a series of images released Tuesday .

NASA scientists analyzed the cloud of smoke at high altitude using the satellite data and compared it with the historical records. The results indicated that it was “one of the largest, if not the largest, pyrocumulonimbus events in the United States,” said Colin Seftor, atmospheric researcher at NASA’s Goddard Space Flight Center.

Pyrocumulonimbus clouds are easy to see from space as they suck in smoke and “become one of the dirtiest clouds on earth, much like volcanic eruptions,” said Peterson, who has studied the phenomenon extensively. “It looks like a giant chimney that picks up smoke from the ground and injects it at the height of a jet plane.”

The conditions for pyrocumulonimbus have been right over the past few weeks, Peterson said. He estimates that between five and ten wildfires have formed in California this year alone, but the size of the Creek Fire cloud stands out.

The size and distinctive shape of the cloud caught the attention of passengers on planes that flew near the fire on Saturday. Thalia Dockery was on a flight from San Jose to Las Vegas when she spotted the pyrocumulonimbus in an otherwise clear sky. After a shocked moment, she started taking pictures and went online to identify the cause of the oddly shaped ledge.

“I’d never seen anything like it,” said Dockery. “The shape reminded me of the giant tree of life at Disney World because it looked like it had roots.”

Kris Mattarochia, a meteorologist with the National Weather Service who covers the central Sierra Nevada, noticed the cloud’s cauliflower-like head and protruding triangular shapes that resembled a ship’s hull.

These characteristic shapes occur when strong winds and emitted smoke squeeze the top of the cloud and fan it horizontally.

“It looked like a Star Wars Star Destroyer,” said Mattarochia.

Scientists around the world have observed pyrocumulonimbus more frequently in recent years. Five of these occurred almost simultaneously in British Columbia, Canada in 2017. And just last year, the record breaking Australian bushfires produced a series of pyrocumulonimbus, the clouds of which were trapped in the stratosphere and circling the globe for months.

Scientists are only beginning to understand the phenomenon, which was first documented by the Naval Research Laboratory and the Canadian Meteorological Center in the late 1990s. The clouds’ plumes of smoke were believed to have originated from volcanic eruptions until scientists attributed them to a large wildfire.

Today researchers are working to track the long-term effects of the volcanic clouds, the smoke of which can circulate over the earth for weeks or months. Peterson was part of a groundbreaking NASA mission that last year flown a DC-8 aircraft into the center of the pyrocumulonimbus Williams Flats Fire in Washington state.

The team will present the results of the flight next year – just in time for another forest fire season.

Nora Mishanec is a contributor to the San Francisco Chronicle. Email: Twitter: @nmishanec

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