Little side project on fantasy story with dragons (or wyverns to be exact, but they're equivalent for the purpose).
I have dragons roaming the country side, laying waste and ruin in their wake. As most known draconids, those creatures are able to breath fire to devastating effect. They breathe in massive amounts of air in their dragon lungs (this will be important later) and then expel it while their throats ooze a substance that catches fire when exposed to air. Now, this amazing hunting and defense mechanism has a big drawback, as it creates a lot of heat the dragon has to get rid of. So much heat, in fact, the dragon radiates it, and nearby vegetation tends to catch fire, making it really dangerous to fight one in a forest.
Wandering on the stack, I found some great unrelated answers, mentioning interesting ways to do what is stated above. What I've come to is an alternative breathing system.
Most of the time, a dragon breathes in and out through mouth and nose alone. When it has to breathe fire, it fills its lungs through the biggest air opening it has, namely the mouth, block, then exhale. During exhalation, cooling "vents" open, that are direct pipes to the lungs. They help chase the air and reduce the heat build-up. Next inspiration, the dragon now takes an even deeper inhalation, and simultaneously chase air through the vents, creating hyper-heated jets of air, biologically jettisoning air to cool down.
(For a visual representation, take a look at the fire drakes from Warcraft. Shorter and stubbier than full dragons, with rows of larges scales from head to end of the tail, with spikes protruding from said rows. For this question, the spikes are hollow, and are tubes used as cooling vent.)
Now my question is how hot those air jets have to be for surrounding vegetation to reach autoignition temperature?
- We're in common "Western-European" medieval forest, not Australia, so the average tree is not eucalyptus. More the vegetation you expect in north of France, Germany and South of Great Britain.
- I don't need the whole vegetation to catch fire on the spot, but the leaves with lower than average autoignition temperature in the area have to. Consequence being fires starting spontaneously around the dragon.
- I suppose the answer differs a lot depending of the humidity around. The forest has to catch fire only during dry periods of summer, with at least a week without rain.
- Handwave everything happening "inside" the dragon, even if it should cook itself, it is able to withstand the temperature the time it needs to cool down.
- The cooling process takes the time the dragon empties its massive lungs, so the air venting lasts in average 7-8 seconds depending on the size of the dragon.
- The dragon will expel around 150 L of air during the cooling process.
I'm asking to know what else would catch fire and picture what it would looks like from an external point of view to describe it. Wikipedia has an article about auto-ignition, with the equation and some examples, but I have been unable to locate any data regarding trees and leaves (from the concerned regions) autoignition temperature and lack the theoretical knowledge to deduce the answer.