Embers, firenados, and modeling wildfires

bonfire new years netherlands
The Hague firefighters on an aerial ladder apply water to the roofs of buildings as embers from a bonfire shower the neighborhood during a New Year celebration, January 1, 2019.

Knowable Magazine has an interesting article by Alexandra Witze on a variety of physics principles that affect wildland fires. She covers the research by Michael Gollner of the University of Maryland on how embers start spot fires, how Janice Coen, an atmospheric scientist who studies wildland fires at the National Center for Atmospheric Research in Boulder, Colorado monitored the start of the Camp Fire as she sat in the back of a room at a conference, and the real time radar signature of the firenado (fire tornado) at the Carr Fire.

Below is an excerpt from a section about embers propagating spot fires.

It turns out that a single ember, or a handful of embers, can’t build up that much heat if it lands on a material such as a deck or a roof. But put one or two dozen embers into Gollner’s device and the heat flux goes up dramatically, he and his colleagues report in the March Fire Safety Journal. “You start to have re-radiation between them,” he says. “It glows, under the wind — it’s just beautiful.”

Just a small pile of embers can generate about 40 times the heat you’d feel from the sun on a hot day. That’s as much heating, and sometimes more, as comes from the fire itself. It’s also enough to ignite most materials, such as the wood of a deck.

So if there are a lot of embers flying ahead of a fire, but those embers land relatively far from one another, they may not build up the radiative heat needed to generate a spot fire. But if the embers pile up, perhaps blown by the wind into a crevice of a deck, they can smolder together and then trigger an ignition, Gollner says. Most homes that burn in the wildland-urban interface ignite from these embers, often hours after the fire front itself has passed.

Understanding the heat flux at these small scales can illuminate why some houses burn while others don’t. During the Tubbs fire, homes on one side of some streets were destroyed while those on the other side had hardly any damage. That may be because the first house that ignited radiated energy to its neighbor, which then burned neighboring homes like dominoes because of the radiative heat. When houses are closely packed together, there’s only so much homeowners can do to mitigate the danger by clearing brush and flammable material around the house.

Bushfire briefing, December 8, 2014

News about vegetation fires in Australia

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South Australia bushfire on the Fleurieu Peninsula

A fire in South Australia is being fought by more than 100 firefighters with the help of six air tankers.

From 9news:

An out of control bushfire on South Australia’s Fleurieu Peninsula is believed to have been started by an angle grinder.

The Country Fire Service on Monday made 71 aerial water drops on the 280-hectare fire near Mt Terrible Road and Louds Hill Road which was travelling in a westerly direction towards Sellicks Hill. By Monday evening the fire had shifted northwest towards Plains Road, Chaff Mill Road, Culley Road and Rogers Road.

Outlook for Victoria bushfire season upgraded

From the AAP:

Victoria’s bushfire season has been upgraded from above normal to major for 2014-15 following record warm October weather. Record-breaking heat and an ongoing warming trend in southern Australia have worsened the state’s fire warning, a new Climate Council report released on Tuesday said.

“These types of conditions drive up the likelihood of very high fire danger weather in Victoria this season,” report author Lesley Hughes said.

Residents of Victoria: “leave and live”

From ABC:

A new safety campaign has been launched for this year’s Victorian bushfire season. The campaign, titled ‘Leave and Live’, encourages Victorians to leave their homes early instead of taking a wait-and-see approach. Premier Daniel Andrews said the expert advice was it would be a long, hot, dry and dangerous summer.

He said nobody could expect Country Fire Authority crews to be door knocking homes when they were trying to fight fires.

“This campaign we’re about to launch is an attempt, and I think it will be successful, in driving home the message that the Government and its agencies have a job to do and will get on and do that but the community has a responsibility as well,” he said.

He said lives were lost by those who left too late.

City of Bunbury warns of ember attack

From ABC:

The City of Bunbury has changed its bushfire message to warn all residents they are at risk of an ember attack.

[…]

City’s spokesman Chris Widmer said Bunbury was not immune from the threat of fires.

“I guess the critical change to the message we have this year is that everyone is at risk from ember attack but if you are within 100 metres of any bushland, then you need to be concerned about the addition to radiated and convected heat from bushfires nearby,” he said.

“We’ve now done some assessments on bushfires risk using Australian standards and we can tell everyone in the City of Bunbury they are at risk from ember attack, which means they could lose their homes through embers from a nearby fire.