As long as we're talking about the dangers of playing with fire . . .
I've been working on a rewrite of the 4th level druid spell, produce fire. I've renamed it, since I've upped the area of effect to a 30 foot circle, greater than the 12 foot circle described in the AD&D Player's Handbook. I'm calling it create wildfire.
It is a real horror of a spell - not because it does a lot of damage to individuals, but because of the terrifying speed with which the fire moves. I am basing that on this Toronto firelab guidebook, which is based on grasslands. I'm sure I must be reading some of the information wrong; of course, I'm grossly simplifying the numbers contained inside for game use. Nevertheless, if it is dry, there's no rain, there's plenty of fuel and the fire gets started in a high wind . . . well, holy hell, a lot of stuff is going to burn.
I'm going to let the wiki handle the details of the spell - I suggest following the link above if the reader wants details on fire movement. I'm also basing a lot of the spell's principles on my recent weather tables, which I am hoping helps keep many fires from skipping more than 20 hexes a round. After all, any sort of weather that produces more than a light breeze will probably also produce precipitation, particularly in cooler, wetter climes. Thankfully, hotter, dryer climes also tend to have fuel ratings that are very low.
Still, in perfect conditions, right at the cusp of there being plenty of fuel and in a month with low rainfall, I can see a disaster looming.
Will a player character pause and consider the implications of letting a wildfire break out just to kill a few enemies, if that fire also means a large burned out area covering hundreds of square miles.
At least we could be sure that a high level cleric or druid attached to the kingdom would probably get involved before vast territories were destroyed, potentially causing thousands to die from famine. Too, there's always the reality that a fire started in a big city will have a higher percentage of 1st level mages, druids and clerics with precipitation spells who, by virtue of population density, will be a block or two away when the fire starts. The player arsonist might find it hard to burn down a town when some do-gooder keeps wet blanketing the area.
For the time being, I'm going to leave these numbers and see. They fit with the research I've done, though they are scary. I doubt that an RPGer pulling numbers out of their ass would have arrived at numbers this high. I wouldn't have.