In this example, the fire is travelling at 5kmph on flat ground.

The slope of the hill has increased by 10° doubling the speed of the fire to 10kmph.

The slope of the hill has increased by 20° doubling the speed of the fire to 20kmph.

The slope of the hill has increased by 30° doubling the speed of the fire to 40kmph.

The slope of the hill has increased by 40° doubling the speed of the fire to 80kmph.

As the fire travels downhill, its speed will decrease roughly by half.