Most forest fires result from human carelessness or deliberate arson. Fewer fires are started by lightning. Weather conditions influence the susceptibility of an area to fire; such factors as temperature, humidity, and rainfall determine the rate and extent to which flammable material dries and, therefore, the combustibility of the forest. Wind movement tends to accelerate drying and to increase the severity of fires by speeding up combustion. By correlating the various climatic elements with the flammability of branch and leaf litter, the degree of fire hazard may be prediced for any particular day in any locality. Under conditions of extreme hazard, forests are closed to public use.

Although organizations involved with fire control have traditionally fought all fires, certain fires are a natural part of the ecosystem. Complete fire exclusion may bring about changes in vegetational patterns and may also allow dangerous accumulations of fuel, with increased potential for feeding catastrophic fires. In some parks and wilderness areas, where the goal of management is to maintain completely natural conditions, lightning-caused fires may be allowed to burn under close surveillance.