Forest Fires burn millions of acres in the United States each year and they all begin with a source of heat: a lightning strike, an arsonist torch, or something as simple as a discarded cigarette.
The heat can make bone-dry leaves or grass release volatile gasses that combine with oxygen in the air to burn.
That generates more heat, which releases more gasses, starting a chain reaction involving larger and larger vegetation. Soon, whole trees are burning.
Steep terrain helps the fire spread quickly as the hot air rises and heats nearby trees.
Wind spreads the heat as well and adds more oxygen to the fire, making it hotter. The forest canopy can get so hot that the fire crowns, or leaps from treetop to treetop.
To stop a fire spread, firefighters starve it of fuel by cutting or burning a zone of vegetation in the fire’s path.
Water, sometimes in the form of foams or gels, is used to cool the blaze because water absorbs heat as it turns to steam.
The steam molecules also crowd out some of the oxygen in the air, reducing the amount available for burning. Fire retardants, usually dropped by planes react chemically with the wood to produce fewer volatile gasses.
That gives firefighters a better chance to extinguish the blaze completely.