How does higher humidity help fire fighters?

The answer seems obvious unless I think about it. Rain, yes, I understand how that could dampen everything. But raising humidity from 20% to 40% shouldn’t do much, immediately, in the face of flames.
So what’s happening?

In wildland fires, increased humidity aids in breaking apart the fire triangle. As we progress into the summer fire season, you will note the season intensifies later in the summer, and fires themselves are most dangerous in late afternoons. Both may be attributed to moisture, but on different scales. In terms of the season, fuels take time to dry out over the course of the summer, with heavier fuels reaching their lowest moisture content in late summer. The same applies on a daily basis with lighter fuels reaching their lowest moisture content in late afternoon. Of course, local factors can speed up the process (extended dry weather, wind, global warming) so that the timelines may be accelerated.

Rain has very little affect on wildland fires, unless the intensity levels are very high and/or the time length is substantial. I’ve been on hot wildland fires where we are hit with summer thunderstorm deluges of rain. Sure, the actual flames may die down for a bit, but deep in the logs, the fires are red hot. Fire rekindles rather quickly. In very large fires (fire storms), the heat intensity is such that the rain never makes it to the heat sources, let alone the flames. The moisture vaporizes long before it reaches the heat sources.

As for structural fires (and car fires, for example), fire fighters often turn to using large sprays of water, rather than a heavy stream. The fog sprays effectively creates high humidities around a fire, reducing the heat levels (not to mention actual water on fire) and breaks apart the fire triangle. The fog sprays need to be large enough to stop break the fire triangle permanently of the fire will restart. A decent fog spray effective blows the flames away from the heat source, will reducing the heat as well. That’s why a controlled and managed fog spray can put out a gasoline fire quite easily.

So an 80% humidity can have an effect on an established fire, as the one now burning in the Santa Cruz mountians? I sure hope so. Those people on the ground have it hard enough, imo. What a job, eh?

A fog, or wide pattern, spray also “trains” a lot of cool air along with it. When switching from a stream to a fog, there’s a sudden wind at your back.

[Mild Hijack] The thing I always wondered about in the weather report is the heat. They always mention that the fire will be easier to contain when the weather is cooler. I understand why it’s easier on the firefighters when it’s cool, but they make it sound like the fire won’t burn as well when it’s, say, 75 degrees as opposed to 90. I can’t imagine the extra 15 degrees would make that much difference in the rate of spread or the intensity. [/Mild Hijack]

Not a hijack at all, BlackNGold. That’s actually part of what I’m talking about.
They talk about both temperature and humidity together. I just heard them doing it tonight, and saying that if the present cooler, drier conditions continue they’ll get the upper-hand on the fire sooner. These were CalFire officials, not reporters.
Wind, I understand.

A 15-degree temperature variant from 75F to 90F greatly increases the atmosphere’s ability to absorb moisture. In essence, if absolute humidity remains constant, the relative humidity takes a sharp drop when the temperature climbs just 15 degrees. As the day warms up, (relative) humidities drop. In late summer with relative humdities already low, the fire danger level greatly increases.

Fire weather is a fascinating science. It’s nothing like the weather reports you hear on the radio or watch on TV. The attention to detail will blow your mind. I’ve been on fires with some of the best fire behavior analysts in the business. Combine a FBA with an onsite top notch fire forecaster from the National Weather Service and you have a skillset where the two can predict weather at a level that would make a TV weather person crap in their pants, over and over again. It’s not uncommon to hear the weather forecast during the 6am morning briefing for the major weather indicator nine hours later in a specfic square mile of the fire with an accuracy rate approaching 99.99%.

Part of the problem is the amount of heat needed to start something (ie, trees) burning.

Solids and liquids don’t burn. Seriously. Any time a solid or liquid is “burning” it is because they were heated enough to start driving off combustible vapors, a process called pyrolosis. The more heat you need to start that process, the more fire resistive that material will be.

Now, lets take a look at water’s role in this. It takes one Btu to heat one pound of water one degree F. Once I’m up to 212F, it takes 965 Btu to change that pound of water at 212F to one pound of steam at 212F. So, from 90F water, thats 1087 Btu per pound. That’s quite a bit of heat. That still doesn’t include the heat needed to start the pyrolosis of the trees in our wild fire.

Take a cool, humid day. The higher the relative humidity, the more water will be absorbed into the fuels. More water = more heat to keep the fire going = slower moving fire. On the other end of the spectrum, at hot, dry day = less heat needed = faster moving fire. Even the 20 or 30 degrees difference between a hot and a cool day does have an effect - you still have to heat the materials up to get them burning. Relate it to your car warming up on a cold morning vs a hot morning. It might only be a 30 degree difference, but the car will warm up a lot faster on the 60 degree morning than the 30 degree morning.

I much prefer buildings and cars to western-style brush fires. At least we have water to put our fires out.

I’ll take a wildland fire over a structural fire any day.

Most wild fires include structures. True wild fires, ones that don’t threaten people, should maybe be left to their own devices.


As for letting wildland fire burns, how do you address accidental and deliberate (arson) wildland fires? In 2007, there were 12,261 lightning-caused wildland fires that burned 5,878,691 acres in the US, and 73,446 human-caused wildland fires that burned 3,449,360 acres. As of this past Friday , 2008 has already seen 25,100 wildland fires burn 1,437,637 acres across America.

A considerable number of wildland fires are “allowed” to burn. That’s one reason why the acreage is so high. But what do you mean by allowing fires that don’t threaten people? Are you talking about only those fires sensationalized on the news where homes are at stake? What about fires that threaten transmission lines? Those fires don’t directly threaten people but their destruction in a fire might deny millions of people electricity for days and weeks? Or fires that threaten transportation corridors? A fire that destroys hillsides in summer would be ripe for landslides and avalanches in the wetter seasons and cut off commerce for weeks. Don’t those fires threaten people?

That is what I said, isn’t it? Threaten people? You added the limitations.
Most of those things you mention can be abated by prevention. Steel transmission towers, for example. Even home fires, many of which are burned because people want to be close to the woods. If more owners followed the forrest service’s advice, far fewer structures would burn. But they don’t want their homes to look like those in the suburbs, with tile roofs and manicured yards all around.
One takes ones chances.
Anyway, my question has been answered, partly by the mass media.

Even in California, there are very few areas where a wildfire could burn and not threaten any people. And for an object lesson on what happens when that policy is implemented, we have, of course, the disastrous Yellowstone wildfires of 1988.

They were not limitations. They were examples. The media loves to exploit the big flames threatening houses because it makes for great, sensationalist images. We’ve had big news stories about big fires in Florida and California. Great pictures. Yet those are only two large fires out 375 large fires reported so far this year.

If this were only partially true, wildland fire statistics would take a nosedive, but it’s not. Prevention is a loaded word these days when it comes to wildland fires. Since this is GQ I’ll leave the politics out but will offer big business stands to make lots of money if wildland fires are “allowed” to burn and destroy trees, homes and lives. As for steel transmission towers, they are no match for wildland fires that cause the towers to buckle and lines to collapse and burn. The San Diego fires last year took out several transmission routes, but not all. At one point there was a fear that if the season continued just a wee bit longer with hot dry weather, it was conceiveable that several million people would lose power for weeks with the last lines going down. The media barely touched on that.

And you are quite accurate here. Those who choose to live along the urban forest boundaries have responsibilities to not just themselves, but their neighbors, the greater communities and the ecosystems they want to live adjacent. Again, big money is playing bigger and bigger roles here, at the expense of forest, homes and lives.

Have a look at post #9, in which I come a long way from advocating a willy-nilly “let it burn” policy. In fact, my suggestion is quite limiting in it’s language.
You’re addressing duckster’s interpretation of what I said in that post.

The media covered that very well, including the fact that material was allowed to grow right up to and under these towers. That very fact was much bemoaned by the forrest authorities and well reported by the media. Of course they’ll show the big flames. I want them to. It’s fascinating, and I’d like to see it in person. From a safe distance. But most of the airtime is devoted to interviews with the firefighters and authorities.

Me too! I have little to add to what Duckster and KCB said, but Here’s an interesting site with information on fuel moisture. Basically, the humdity affects the fuel moisture of things that aren’t already burning which then affects the rate of spread of the fire.

Also, IIRC from S-290, with the same absolute amount of moisture in the air, a 20-degree increase in temperature will halve the relative humidity.

St. Urho
Former USFS wildland firefighter