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Perikles
2012-Jul-16, 03:43 PM
I've managed to get myself in an argument here, and realized I know nothing about the subject. We have a local wildfire which is spreading like, er, wildfire. This is on the south-western slopes of Tenerife, the area being below but very close to the nation park of Teide, a volcano surrounded by pine trees. The prevaling wind is west, which would work against the fire travelling up the slope to the national park, but everything is tinder dry, and temperatures almost 40 centigrade.

To the west of the fire zone, however, there is a massive ravine running north-south, I am just on the other side of it. If it were not for the ravine, I wouldn't be sitting here, that's for sure. But some official 'expert' has just commented that the presence of the ravine has prevented the fire spreading westwards, but that it has caused a tragedy for the national park, because the fire has changed directions and is now working its way north/north-eastwards.

The fire has been stopped from moving westwards, but does this actually have any effect on the progress of the fire northwards or north-eastwards? My common sense tells me that the progress of the fire in any one direction is not influenced by its progress in any other. In which case the presence or absence of the ravine is irrelevant to the spread of the fire in another direction.

Any thoughts?

Charlie in Dayton
2012-Jul-16, 05:14 PM
...snip...
The fire has been stopped from moving westwards, but does this actually have any effect on the progress of the fire northwards or north-eastwards? My common sense tells me that the progress of the fire in any one direction is not influenced by its progress in any other. In which case the presence or absence of the ravine is irrelevant to the spread of the fire in another direction.

Any thoughts?

Depends on what factors stopped the fire moving in direction X. If there's a firebreak (ravine, previously burned area), that's one thing. If the fire can only move against the prevailing winds, the progress would be slow...and once conditions calm, the fire will move more quickly in the direction it couldn't move before.
High winds, extremely dry conditions...those things will make a fire roar along faster than a man can outrun it.
My little brother lives in AZ, and has property west of Flagstaff up on the ridge...fire is always a concern out that way. The one thing that's always sure about fire is its unsureness...just about the time you think you know what's happening, it's sneaked around behind you and toasted your nether regions.

Swift
2012-Jul-16, 05:22 PM
The fire has been stopped from moving westwards, but does this actually have any effect on the progress of the fire northwards or north-eastwards? My common sense tells me that the progress of the fire in any one direction is not influenced by its progress in any other. In which case the presence or absence of the ravine is irrelevant to the spread of the fire in another direction.

Any thoughts?
I'm no expert. I wouldn't think the fire being stopped from moving westward would have any effect on it moving in another direction (otherwise, what would be the point of firebreaks, until you completely surrounded the fire.

I can however imagine that the ravine might have an influence on local winds (ravines around here do), there might be updrafts out of the ravine, for example, and those might push the fire in some direction. But that's a guess.

Gillianren
2012-Jul-16, 05:36 PM
Right--firebreaks Back Home (I grew up in Los Angeles County, an area with an ecology that requires fire on a regular basis) are mostly just intended to keep the fire from burning houses. They know that the fire will spread out, not just in any one direction. Even with prevailing winds going in one direction, the fire still spreads a little in others. Just a lot more slowly.

Ara Pacis
2012-Jul-16, 06:32 PM
Wildfires and firestorms don't just rely on wind but can create their own winds.

Noclevername
2012-Jul-16, 06:51 PM
Don't those winds usually blow in toward the center of the firestorm, where the lowest pressure is, and limit the fire's growth?

nosbig5
2012-Jul-16, 06:54 PM
I agree, the fire will travel predominantly with the wind, and to a lesser degree against it, but upon encountering a ravine it will not be pushed back and spread in another direction unless it would have spread in that other direction anyway. I don't see the ravine being at fault here.

The only mechanism I can think in which the ravine would cause the fire to turn is if there were strong updrafts sweeping up the ravine that carried burning bits high into the sky, and if the windstream flowed to the east at this higher elevation, and if upon being deposited further eastward the bits had retained enough heat to start another fire.

Ara Pacis
2012-Jul-16, 07:29 PM
Don't those winds usually blow in toward the center of the firestorm, where the lowest pressure is, and limit the fire's growth?

Up, more specifically, updrafts not indrafts, upwards can coincide with topography synergistically. Winds moving over an obstacle, such as a ridgeline, can also create vortices and turbulence and constrain the movement of hot air and embers away from the fire.

Here's a useful page (https://fp.auburn.edu/fire/topos_effect.htm).

starcanuck64
2012-Jul-16, 10:28 PM
I grew up in a family of foresters and they had some pretty hairy stories about fighting fires...most of which I've forgotten.

Fires can do some fairly unpredicatable things, like create their own winds and throw buring materials like pinecones hundreds of yards ahead of the fire. In crown fires the flames can rapidly spread from treetop to treetop and with candleling the entire tree essentially explodes. From what I recall fires also can rapidly move upslope.

The dryness of the ground is obviously an important factor as is the presense of material on the ground. Some fires will quickly burn through an area, removing the dry understory material and some burn everything and sterilize the soil.

Wish I could remember more specific stuff about fires, but after a few experiences fighting fires as a teenager, it's a subject I've tended to avoid. One of the hottest, dirtiest and hardest jobs I've ever done.

Jerry
2012-Jul-17, 03:16 AM
Many canyons in the Rocky's are aligned E/W with the ranges running north south. My daughter lives at the bottom of a canyon where strong winds blown West in the morning (higher pressure from the sun heating Wyoming) and East every afternoon with the sun is over Nevada.

It is a similar state in southern California when a large High settles in over Utah and Nevada and pushed the dreadful Santa Annas to the coast.

Another important thing to remember in the propagation of a fire is that the radiant heat from the fire will first dry, and then slowly cook trees up to a kilometer or more in in front of the flame front. This is critical: In a heavy forest, once dried the baking includes releasing clouds of hydrogen gas within the forest and a single ember can cause an exposive propagation well downwind of the advancing wind. This is way rivers and roads will not stop a strongly wind-driven fire; and also why they are so deadly and dangerous to fight: they advance in leaps and bounds.

Pine beetles have also contributed to this years wildfires: dry dead wood is good kindling.

Ara Pacis
2012-Jul-17, 04:34 AM
Many canyons in the Rocky's are aligned E/W with the ranges running north south. My daughter lives at the bottom of a canyon where strong winds blown West in the morning (higher pressure from the sun heating Wyoming) and East every afternoon with the sun is over Nevada.

According to the link, it's smaller scale reversals caused by heating of the slope in the sun and then cooling of the slope in the shade.

DoggerDan
2012-Jul-21, 06:31 AM
Fires can do some fairly unpredicatable things, like create their own winds and throw buring materials like pinecones hundreds of yards ahead of the fire.

During our recent Waldo Canyon fire, the on-scene manager was saying some of the cinders were jumping upwards of half a mile due to the 65mph on that fateful Tuesday.

Perikles
2012-Jul-21, 07:27 AM
Fortunately, the fire which caused me to start the thread has now been extinguished by a fleet of helicopters and airplanes bombing it with water. They were helped no doubt by the complete lack of wind, so the fire fronts were moving very slowly. Villages all around us were evacuated, but not ours. We saw the fire start on Sunday afternoon, and it could have been extinguished there and then, had they reacted more quickly.

It is incredible and depressing how such an emergency is transformed into a political issue - which firefront to tackle, which area to let burn. Some locals are convinced that the fire was not extinguished immediately because the helicopter had no immediate access to local water reservoirs, because these were empty having been used to water the golf courses on the coast which only tourists use. It was also locals who started the fire because they thought it was a brilliant idea to burn off some stubble in a field, next to a forest, in the middle of summer with a temperature around 40 degrees Centigrade, almost zero humidity where it hasn't rained for over a year. Incredibly, this is not illegal. Why isn't stupidity illegal?

swampyankee
2012-Jul-21, 12:20 PM
Fortunately, the fire which caused me to start the thread has now been extinguished by a fleet of helicopters and airplanes bombing it with water. They were helped no doubt by the complete lack of wind, so the fire fronts were moving very slowly. Villages all around us were evacuated, but not ours. We saw the fire start on Sunday afternoon, and it could have been extinguished there and then, had they reacted more quickly.

It is incredible and depressing how such an emergency is transformed into a political issue - which firefront to tackle, which area to let burn. Some locals are convinced that the fire was not extinguished immediately because the helicopter had no immediate access to local water reservoirs, because these were empty having been used to water the golf courses on the coast which only tourists use. It was also locals who started the fire because they thought it was a brilliant idea to burn off some stubble in a field, next to a forest, in the middle of summer with a temperature around 40 degrees Centigrade, almost zero humidity where it hasn't rained for over a year. Incredibly, this is not illegal. Why isn't stupidity illegal?

Because we'd have to lock up everybody.

publiusr
2012-Jul-21, 04:44 PM
Two of the worst incidents:

http://en.wikipedia.org/wiki/South_Canyon_Fire

http://en.wikipedia.org/wiki/Peshtigo_Fire
Surviving witnesses reported that the firestorm generated a tornado that threw rail cars and houses into the air.

There are actually several kinds of fire whirls. On Tornado video classics, we learn of a 'tornado twin.'

More
en.wikipedia.org/wiki/Fire_whirl
http://wildfiretoday.com/2012/03/09/fire-vortices-on-video/

Ara Pacis
2012-Jul-21, 10:09 PM
It is incredible and depressing how such an emergency is transformed into a political issue - which firefront to tackle, which area to let burn. Some locals are convinced that the fire was not extinguished immediately because the helicopter had no immediate access to local water reservoirs, because these were empty having been used to water the golf courses on the coast which only tourists use. It was also locals who started the fire because they thought it was a brilliant idea to burn off some stubble in a field, next to a forest, in the middle of summer with a temperature around 40 degrees Centigrade, almost zero humidity where it hasn't rained for over a year. Incredibly, this is not illegal. Why isn't stupidity illegal?

I can haz freedumb. :)