Forest fires can be ignited by a variety of occurrences. In addition to lightning, human-related activities start a large number of fires every year. Unattended or out-of-control campfires, a discarded burning cigarette, arson, or even equipment use can set off a blaze.
Once a forest fire has started, many factors contribute to its spread and intensity.
- Fuel - such as leaves, needles, grass, branches, and logs
- Weather, including temperature, humidity, precipitation, and wind
- Topography or landscape of the area, as steep slopes offer greater potential for increased fire intensity and more obstacles for fire fighting
More than 83% of forest fires in 2006 were started by human activities, accounting for the burning of nearly 4.4 million acres. However, lightning-caused fires burned more total area - nearly 5.5 million acres.