Disco inferno!

[The Shaman]

The SRD says it travels at wind speed, 120 feet per round in a moderate wind.

Wildland fire rate of spread is measured in chains per hour. According to standard fuel models used in North America, the rate of spread in a 5 mph wind for tall grass is 104 chains per hour, for chaparral (dense, flammable shrubs) 75 chains per hour, for pine forest about eight chains per hour.

The rate I suggested for the backing fire is about four chains per hour and for the head fire about eight chains per hour on average.

The rate suggested by the SRD is 1091 chains per hour, or about 14 mph.

Go with whatever makes for the best encounter for your game, of course.

 

[LordMelquiades]

Shaman, this is gold, I love it how people know such crazy stuff! You rock.

I think I'll vary the rate to keep them guessing. Start it slower to give them a chance to take action to douse it (as if--the only idea they've had so far is a fog cloud spell), and then pick the pace up after a few rounds, describing how the head of the fire expands and moves at a different rate...

So, a related question: what is it that causes forest fires to burn out? Given the forest I described, for how long is this thing likely to burn (weeks?), and is there a typical overall spread/area of effect for a forest fire?

 

[The Shaman]

So, a related question: what is it that causes forest fires to burn out? Given the forest I described, for how long is this thing likely to burn (weeks?), and is there a typical overall spread/area of effect for a forest fire?

It will likely burn much more slowly at night, and the first good rain shower will knock it down to a low smolder as the fuel will either be fine enough to soak up the moisture or thick enough that it's not burning very efficiently anyway. Topographic barriers will limit or halt the spread - streams, rocky outcrops, lakes. If the wind changes direction to force the head of the fire back toward the burned area (known as "the black"), then the fire will die out as it's starved of fuel.

On the other hand, if it stays dry and the wind picks up, it will burn until it runs out of fuel.

Glad I could help, by the way.

 

[LordMelquiades]

Whenever I need a gnomish, root-tossing forest expert, I'm coming to you!

 

[The Shaman]

Whenever I need a gnomish, root-tossing forest expert, I'm coming to you!

You may want to look for a different expert, I'm afraid: I made a math goof. :\

The SRD rate is 1091 chains per hour (not 6545), or about 14 miles per hour - still a bit fast, but not quite as off-the-scale as my earlier estimate.

A bad day for Bambi, either way.

 

[Piratecat]

A bad day for Bambi, either way.

Not if Bambi is a wild shaped druid, I suppose, but otherwise you're quite right.

Burning down a forest would reveal all the old ruins and dungeons hidden within it. Neat way to start some adventures!

 

[The Shaman]

Burning down a forest would reveal all the old ruins and dungeons hidden within it. Neat way to start some adventures!

Funny you should mention that...

I was reading over my 3.0 campaign notes years ago, and noticed that that something like 75% of my adventure sites appeared due to some natural phenomenon: a wildfire, an avalanche, a flood, even a tree falling over in a windstorm tearing open a long-lost tomb. Every game master has his or her cliches - that one is mine... :\

 

[Snapdragyn]

So very yoinked.

How many feet are in a chain, btw?

Also, I didn't see a mention of how recently it had rained before the wiz went pyro. I don't know that rain 3-4 days before would matter during summer, but surely rain the day (or 2?) before would slow things down.

While I understand the canopy interlocking effect in speeding up a crown fire, wouldn't that actually slow a ground fire as a) the forest floor would be more shaded to better retain moisture, & b) the undegrowth would be less dense?

Oh, & finally, is it deciduous forest everywhere? In a lot of temperate forest zones, decidous trees & coniferous trees both occur. Which dominates where w/in the forest depends on microclimate (coniferous more in drier spots, & deciduous in the moister spots; this pattern is reversed in boreal forests) & successional stage (conifers tending to grow faster after fire or logging & predominate until slower-growing deciduous trees mature beneath them). Since you say its been ages since the last fire, I wouldn't think you'd need to worry about early successional effects. Of course, I'm not sure how the varying ratio or dominance of coniferous vs. deciduous would effect rate of spread. Shaman?

 

[The Shaman]

How many feet are in a chain, btw?

66 feet = 4 rods = 1 chain.

Trivial note: Historically chains became the standard unit of measure for fire spread because the foresters who fought wildfires were the same people who surveyed timber sales - chains have been a surveyor's measure for centuries.

I don't know that rain 3-4 days before would matter during summer, but surely rain the day (or 2?) before would slow things down.

Yep.

While I understand the canopy interlocking effect in speeding up a crown fire, wouldn't that actually slow a ground fire as a) the forest floor would be more shaded to better retain moisture, & b) the undegrowth would be less dense?

Arguably once a fire crowns (moves into the canopy) you will end up with an intense ground fire as well - the heat from the crown fire pre-heats the ground fuels, and it's raining flaming debris under the trees at that point, so everything will burn, often down to mineral soil.

As far as a surface (brush and grass) or ground (brush, grass, and duff) fuel, yes, under a canopy you have less than in more open areas - that's reflected in the rates of spread for grass, shrubs, and woodland above. A clearing in a forest can become a fuel-sink (called a jackpot by wildland firefighters) - clearings are often the result of downed trees, which deposit lots of woody debris, and grass and shrubs can grow more abundant as they receive more light and less competetion for nutrients. The combination of woody debris and light, flashy fuels is pretty explosive.

Of course, I'm not sure how the varying ratio or dominance of coniferous vs. deciduous would effect rate of spread. Shaman?

Coniferous trees burn hotter and faster, increasing the rate of fire spread. Patchiness in a mixed forest strongly influences fire dynamics.

If you want to make charcoal, you need a hardwood, one that burns slowly without intense heat - softwoods burn too hot and fast. That's your fire spread in a mixed forest right there.

 

[Snapdragyn]

Cool. I can totally see doing a random table for a forest fire where the fleeing party runs into a stream (yay!) or downhill slope (yay!)... or a brushy clearing or patch of conifers (mwaha... I mean, eep!).