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article imageOminous-looking pyrocumulus cloud looked like nuclear bomb

By Karen Graham     Dec 12, 2017 in Science
Ventura - In a phenomenon only seen during the biggest wildfires, a pyrocumulus cloud, which resembled an erupting volcano or a nuclear bomb, was seen above the San Ynez Mountains of western Ventura County in Southern California on Monday.
The huge, billowing blackish-gray cloud, also known as a fire cloud or Flammagenitus cloud stretched up almost 30,000 feet high, although they usually only get to about 5.0 miles (26,000 feet).
And while shocked onlookers stood in awe of the giant formation that looked more like a pyroclastic display from an erupting volcano or a nuclear bomb, the cloud was actually the result of the massive Thomas Fire which has scorched over 230,500 acres (93,280 hectares) of Southern California and destroyed over 1,000 homes to date.
What is a pyrocumulus cloud?
Pyrocumulus clouds are actually cumulus clouds, those pretty white clouds that look like heads of cauliflower. Cumulus clouds form in the sky when hot, moist air is warmed by Earth's surface and begins to rise. The air cools as it rises, eventually condensing into water droplets that cling to airborne particles called condensation nuclei.
As more droplets condense, they release heat and form more droplets and quite often, this results in enormous white, fluffy clouds. Pyrocumulus clouds form in essentially the same way, however, there are a few differences. Number one - They only form in extreme circumstances of heating and condensation.
A pyrocumulonimbus cloud towers over thick smoke from fires burning near Canberra  Australia  in 200...
A pyrocumulonimbus cloud towers over thick smoke from fires burning near Canberra, Australia, in 2003. The cloud's strong winds caused the fires to explode into the Australian city.
NASA/New South Wales Rural Fire Service
Eric Boldt, a forecaster with the National Weather Service office in Oxnard, told San Francisco Gate that the cloud was created by forces similar to a thunderstorm. Using satellite imagery, he measured it at 30,000 feet tall.
"It's basically like a thunderstorm," Boldt explains. "When we see these clouds billowing so tall, it's the same mechanisms that are happening with a thunderstorm. You're causing updrafts and air that's pushing the smoke higher. It creates its own wind. If it starts to spin, that's where you can get more wind and fast-moving progression of the fire. It can become a dangerous situation for firefighters."
Creating its own weather system
Pyrocumulus clouds can be very dangerous and are quite often seen as a foreboding sign that a wildfire will spread. This is because the hot, fast jets of air create a turbulent atmosphere within the cloud formation, making it unstable. According to NASA, if a cloud column builds high enough, for example, it can become a pyrocumulonimbus cloud - basically a fire-fueled thunderstorm cloud.
A pyrocumulonimbus storm combines smoke and fire with the features of a violent thunderstorm. Pollut...
A pyrocumulonimbus storm combines smoke and fire with the features of a violent thunderstorm. Pollutants from these storms are funneled into the stratosphere.
NASA/Naval Research Lab/Mike Fromm
Pyrocumulonimbus is the fire-breathing dragon of clouds. Add smoke and fire to the mix and you have pyrocumulonimbus, an explosive storm cloud actually created by the smoke and heat from the fire, and which can scorch tens of thousands of acres. And in the process, "pyroCb" storms funnel their smoke like a chimney into Earth's stratosphere, with lingering ill effects.
"Pyrocumulus clouds can increase [wildfire] spotting, since you are lofting more embers higher into the atmosphere, which can increase the horizontal distance the embers travel," said Nick Nauslar, a research scientist for the Cooperative Institute for Mesoscale Meteorological Studies/Storm Prediction Center at the National Oceanic and Atmospheric Administration (NOAA).
And if a large pyrocumulus column becomes too unstable, it may even "collapse onto itself." Should this happen, embers flying about could spark new fires and be a risk for anyone close by. As a matter of fact, the plume did collapse, creating dangerous conditions, according to the Public Information Officer, Ventura County Fire Department (VCFD).
"As the plume collapses it creates a very dangerous wind shift on the ground from "downdraft" winds. These winds have potential to spread fire in an outward circle from the point of collapse. Firefighter's on the ground must remain aware of current and forecast weather forecasts," said the VCFD.
More about Thomas Fire, pyrocumulous cloud, International Cloud Atlas, own weather systems, Dangerous