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How do wildfires start and spread?

Wildfires often occur during a region’s driest season and are most commonly started by lightning strikes; by human accidents, such as a downed power line, a tossed cigarette, or an escaped campfire; or deliberately by arson. Once a fire is burning, drought, heat, and wind all increase its intensity. Topography also affects wildfires, which spread quickly uphill and slowly downhill.