To understand macro patterns of forest fires, it turns out, you need just two key parameters. I labeled the horizontal axis in the forest fire phase diagram a few pages before “wind speed.” But a better term, which captures what really matters for the spread of fires, might be “virality.” High wind speeds, dry ground, and low humidity increase virality: they make fires more likely to spread. Low wind speeds, moist ground, and high humidity decrease virality: they make fires less likely to spread.
