Weather seems to be the most important factor dictating the severity of forest fires. Hot, dry weather lowers the moisture content of fuel. Once a fire has started, wind would become influential concerning oxygen supply and the rate of spread. Other important factors include forest types and forest conditions. For example, fire is more likely to occur in conifers than in hardwoods; slash-covered or brushy areas are especially hazardous because the rate at which combustion consumes fuel is related to fuelvolume or surface area. Some fires are caused by lightening; others are caused by people. Those caused by people may be accidental or intentional. Regional differences exist regarding the causes of fire. For example, fires in Rocky Mountains present a different picture from the fires in the South. 70 percent of the fires in the former area are caused by lightening, whereas intentional fires take up to 40 percent in the later.