...and when it gets to the top of the hill, it stops (usually)
Boulder's Flagstaff Fire - Timelapse from Dustin Henderlong on Vimeo.
In the linked video, you can see time lapse photography from about an hour after the fire started until about 2 days later. The fire covers 1.5 miles over ground in the first few hours (you can't see this in the video, but you see the huge burst of smoke pouring over the ridge line) then, just as the fire crests the ridge that runs north and south from the peak of Bear Mountain, it grinds to a halt. In the next 36 hours, it basically only advances a few hundred yards downhill.
[...] at the peak of Bear Mountain, the vegetation is pretty thin, mostly rock. That, plus the difficulty of going downhill, makes a very nice fire break. Today, we could see plumes of smoke on the slopes just east of Bear peak, but they were only from backfires lit to clean up patches of vegetation that represented possible leaks in the firebreak that the fire created for itself when it stalled coming over the ridge.
Read the original Fire is a river that runs uphill post.