Wildfires
Source GM Core pg. 101
Wildfires travel mainly along a front moving in a single direction. In a forest, the front can advance up to 70 feet per round (7 miles per hour). They can move up to twice as fast across plains due to a lack of shade and the relatively low humidity. Embers from the fire, carried by winds and rising hot air, can scatter, forming spot fires as far as 10 miles away from the main wildfire. Wildfires present three main threats: flames, heat, and smoke.
Flames
Source GM Core pg. 101
Flames are hazardous terrain, usually dealing moderate damage and potentially setting a character on fire, dealing moderate persistent fire damage. The flames from a small fire are often less dangerous than the advancing heat from the front of a large fire.
Heat
Source GM Core pg. 101
Wildfires increase the temperature in advance of the front, reaching nearly 1,500°F at the fire's arrival, as hot as some lava. This begins as minor fire damage every round at a reasonable distance from the front and increases to massive fire damage for someone within the wildfire.
Smoke
Source GM Core pg. 101
Wind can carry smoke far in front of the wildfire itself. Smoke imposes a circumstance penalty to visual Perception checks, depending on the thickness. It causes creatures viewed through significant amounts of smoke to be concealed, and it cuts off all visibility at half a mile or less. Near or within the wildfire, the combination of smoke and heated air require characters to hold their breath or suffocate.