Natural Ignition Sources

Lightning ignition

Annually, approximately 30 million cloud-to-ground lightning flashes strike the United States, of which 1% to 4% ignite a fire. Ignition requires a sustained current from ground to cloud (the return stroke) of milliseconds to hundreds of milliseconds – characteristics exhibited by only 30% of lightning strikes. Return ground flashes can be either positive (where positive energy travels from ground to cloud) or negative (where negative energy travels from ground to cloud). Although positive strikes are rare, almost all positive discharges have a continuing current, while only about 20% of negative discharges have one (Pyne et al. 1996; Johnson and Miyanishi 2001).

Approximately 14,000 lightning fires are reported each year in the United States (Arno and Alison-Bunnell 2002). The actual number is likely higher since many lightning fires die out before they are detected by humans, usually due to accompanying rain. In the southeast, most of these occur during the summer months between May and September, with a peak in July (see the Florida Forest Protection Bureau website).

Ignition probabilities from lightning can be determined for different landscapes using ignition models. Ignition depends on the duration of the current and the kind of fuel the lightning hits, varying with fuel type, depth, moisture content, and bulk density. The Wildland Fire Assessment System produces daily maps of lightning ignition efficiency based on these ignition models.

“Spontaneous” ignition

Fires can spontaneously ignite in accumulations of organic matter where decomposition is occurring. As decomposition processes break carbon bonds in composite sugars, heat is released. If heat is produced at a rate greater than it can dissipate, ignition can occur. In wildland fire situations, this would most likely occur in large piles of sawdust or wood chips, but it is a rare event.