Every year, enough forest-land burns on Earth to cover the Indian subcontinent, according to Silivia Kloster of the Max Planck Institute for Meteorology. Some fires are set by humans by accident or for agricultural purposes, but many are natural phenomena caused by the right combination of dry timber, hot weather and lightning. Once a wildfire starts, the speed at which it spreads depends on a number of factors, but it can burn miles of forest in one day.
How Fires Start
It takes three elements to make a forest fire: fuel, oxygen and a source of heat. The heat source may be a smoldering campfire or carelessly discarded cigarette, but most often, it's one of the 100 random strokes of lightning that occur on Earth each second. Once the fire starts, light, fast-burning fuel, such as dried leaves and small branches, cause it to spread. Heavier fuels, such tree trunks and logs, burn longer and hotter and increase the overall heat output of the fire. Winds supply oxygen to the hottest parts of the flames and carry embers to other parts of the forest.
Fanning the Flames
Among the factors that determine how fast a fire spreads are the availability of fuel, the weather conditions and the topography of the land. Fire spreads quickly into areas that have large quantities of fuel, in the form of dry trees, logs and deadwood, and it spreads even more quickly if the weather is hot and dry and the fuel has been pre-heated by the sun. Fires spread most quickly when moving in the crowns of the trees, and accelerate when going upslope or when fanned by a strong tailwind. Most fires maintain their crown run only for short periods.
According to Richard C. Rothermel of the United States Forest Service, running crown fires can spread at the rate of 1 to 7 miles per hour, reaching their maximum rates of spread when wind-driven and moving upslope. Another type of crown fire, which occurs in low wind conditions as a plume of hot air collects over the burning trees, can spread as fast as a wind-driven crown fire, or approximately 7 miles per hour. These conditions are exceptional, however. Most surface wildfires, which spread closer to ground level, move more slowly.
Average Spread Rate
The many factors that influence the spread of wildfire make it difficult to predict the speed of a given fire at a given time. For example a fire tornado may develop, and these can spread as quickly as 19 miles per hour (30 kilometers per hour). California's Rim Fire, which threatened Yosemite Park in August, 2013, sustained a crown run during which it spread quickly, but it took 10 days for this fire, which was one of California's biggest, to cover an area with a mean radius of 7 1/2 miles. That's an average rate of spread of less than a mile a day.
- A. Hartfield Ltd: Facts and Figures about Lightning
- British Columbia Wildfire Management Branch: Fire Behaviour
- NBC News: Yosemite Blaze Sparks Burning Questions about Fast-Moving Fires
- Wildfire Today: Documented Fire Tornado
- United States Department of Agriculture: Predicting Behavior and Size of Crown Fires in the Northern Rocky Mountains
- Deutsche Welle: Climate Change Fuels Wildfires Around the World
- Photo Credit Creatas Images/Creatas/Getty Images