"Fire is a natural environmental phenomenon and has been an integral part of our ecosystem for millennia. The population and development of North America has repeatedly brought humans into contact with fire in all manner of circumstances including wildland fires. Over the past 400 years, Americans as a society have grown to fear all forms of fire and have sought ways to suppress it as completely as possible. Wildland fires, however, pose unique challenges to the fire service and require vastly different approaches to its prevention, mitigation, and suppression. As more people choose to leave the cities and build their homes in the 'wildland/ urban' interface, it is critical that these concerns are addressed. Natural wildland fires are generally caused by lightning, which strikes the earth an average of 100 times each second or 3 billion times every year1 and has caused some of the most notable wildland fires in the United States (e.g., Yellowstone in 1988). Other natural causes include sparks from falling rocks and volcanic activity."
U.S. Fire Administration: http://www.usfa.dhs.gov/
Topical Fire Report Series (October 2000 (Rev. December 2001)), v.1 no.3