Tuesday, November 02, 2004

Atmospheric Physics Part III

All this discussion of late about explosives has reminded me of another major flaw in the “nuclear winter” scenario. The premise was that using large nuclear weapons would raise firestorms in the targeted cities. This is quite unlikely. Freeman Dyson worked as a mathematical analyst for Sir Arthur “Bomber” Harris of Bomber Command in World War II. In his book, Weapons and Hope , he writes “Nobody understands to this day why or how fire storms begin. In every big raid we tried to raise a fire storm, but we succeeded only twice, once in Hamburg and once two years later in Dresden. Probably the thing only happens when the bombing releases pre-existing instability in the local meteorology.” (page 117). Of the dozens of massive fire raids on Japan, only one caused a fire storm, the raid on Tokyo. Neither atom bomb started a fire storm. So the suggestion that a bigger bomb would automatically start a fire storm should be met with skepticism.

From this historical distance, I cannot give a definitive answer to what causes a fire storm to start, but there are strong hints that Mr. Dyson’s observation was on the mark. Tokyo is the easiest to explain. There was a 50 knot gale blowing when the bombing took place! Mother Nature provided a forced draft system to feed oxygen into the fires. This was more of a massive forest fire than a fire tornado, but the results were similar.

The Hamburg raid took place in late July 1943. The bomb load consisted of 4000 pound block busters and incendiaries. The block busters to create a pile of flammable rubble and the incendiaries to light the pile. An important point is the use of thermite incendiaries. These do not require air to support combustion and burn at much higher flame temperatures than an ordinary flame. If the bomb runs are well aimed and concentrated in time, you can create a very hot column of gas to jump start a vertical circulation in a pre-existing, vertically unstable atmosphere. Once the circulation starts, it brings in oxygen to support combustion. The thermite combustion allows the fire to be sustained even during periods of oxygen depletion. This ability to sustain itself is not present in the aftermath of a nuclear explosion. (see links in previous post regarding explosives for discussion of the necessary structure to support detonation). The raid on Dresden was structurally similar. So there actually is good reason to believe a fire storm is unlikely to be created by a nuclear weapon. Without the fire storms, there would be much less smoke and therefore much less chance of a huge dark cloud blocking out the sun. Further confirmation of this skepticism was provided by the Kuwaiti oil fires. Once again, we find more hype than science in the TTAPS scenario. And since “global warming” is supposed to be caused by a combustion process, it means we should examine this a bit more in a future post.


Blogger Timothy Birdnow said...

Yeah, it`s tough to start a firestorm; especially with atomic weapons. The bomb gives off a tremendous burst of heat followed by the shock wave. This forms a vaccum inside the expanding ``shell`` of the wavefront, which snuffs out fires caused by the initial burst. Atomic bombs starve the fires for oxygen, plain and simple. The bombs put up a lot of dust (which settles as fallout)but firestorms just don`t stand a chance.

Sagan and company were nuclear freezenicks, and made science serve their purposes in this case. Turns out they were dead wrong.

Great piece!

5:13 PM  

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