A vivid description of this is described in "Young Men & Fire"
Early 50's Forest service is developing smoke jumpers. At the time teams didn't train together as a team. They drop into the Mann Gulch fire, above it. Their plan was to come down the gulch opposite the fire, cross over and fight it from below.
Fire crowned, and jumped the valley. Both sides of the Missouri River end of the gulch were burning.
Dodge, the crew chief, order a retreat. If they could get above timberline the fire would only have grass and small brush to burn.
The fire wasn't moving that fast. Maybe 3-4 mph. But timberline is at 8,000 some feet. Keeping ahead while climbing, dressed in boots, tin-pants, cotton plaid shirt isn't quite the same as doing a beach jog in Malibu, CA.
Dodge lit a book of paper matches, and torched a meadow of grass, stepped across the flame, and waited. Once the fire was a few feet out, he lay down, and waited.
Fire passed him by.
A very vivid song that, within the limited scope of a folk song, is pretty accurate, was done by James Keelaghan, "Cold Missouri Waters"
If you want the book:
Also available as an Audible book. https://www.audible.ca/pd/Young-Men-and-Fire-Audiobook/B072FDXGHR
In passing: The flame front in a grass fire is very narrow, often only being a few feet wide. (Dodges crew were caught above tree line in a dry meadow) While intensely hot, it's not unreasonable to run through it. I've read about this done by natives and settlers on the great plains. The key issue is to not inhale when passing through the flame. You will lose exposed hair, and have burns on any exposed skin.
McLean in the book describes the fire as moving at a fast walk on level ground. 3-4 mph. Flames ranging from 10 to 25 feet high. If you have been running in terror, however, you are at the ragged edge of what you can breath. Catching your wind enough to run back is daunting, probably requiring descending while going forward.
A grass fire isn't a forest fire. But even forest fires come in different grades and speeds. Ground fires move slowly, and often aren't very hot. There is more material to burn, so the flame front is deeper, and the ground hotter. With smoke it's hard to maintain a consistent direction. Crown fires are roast everything in their path.
If you wanted to test these ideas:
- Start small.
- Dress well.
- Have someone on the other side with a wet blanket if you emerge with your clothing on fire.
I used to sing ballads as part of the entertainment on long canoe expeditions with teens. Keelaghan's song was a hit.