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Firenado!


BBC had a Dan Snow documentary about PTSD on Nov 12th, and very harrowing it was too. So harrowing that I feel a little bad to have been distracted by the physics of one story that a survivor told....

He had been a prisoner of war in WW2, and was held near Dresden, so he was there for the massive bombing in Feb 1945. He described how fierce the fire was and how it created a wind in the surrounding area that was so powerful he had to hold tightly to a wall as he saw women and children being sucked into the flames.

What was happening presumably was that the rising hot air created an area of low pressure directly around the flames, and that the surrounding air rushed in to fill that near vacuum. I knew that that would happen, but had no idea it would be so powerful, nor that it would happen on such a large scale.

The New York Times here, seem to describe a similar effect. The in-rushing air from any one direction, of course, meets similar high velocity air from every other direction and can create what they call a firenado. This not only provides the fire with a renewed source of oxygen so that it grows in intensity but also - if there is enough fuel nearby - means that it will grow in size.

The NYT article is from August and talks about how California experienced a fire last July that created a wind measured at 143 mph. They have records of winds strong enough to pick up a fire truck, and one incident where four fire fighters were pulled into a blaze (but survived).

Ominously, the article also mentions how there has a problem in recent years with bark beetles in the US west which has killed up to 129 million(!) trees in California. They speculate how this could provide fuel for more damaging fires in the future - a prediction that would be worth revisiting as California experiences unprecedented November fires this year.

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