Weather refers to the conditions at the time of a fire and largely affects the behavior of the fire. The primary weather variable that most affects fires is wind. At low wind speed, fuel structure plays a critical role in fire spread. Live foliage dissipates heat by losing water and often will not combust until it has been heated sufficiently to drive off water. In the absence of wind this typically requires the presence of dead fuels, which combust more readily and will heat living fuels sufficiently to combust. Fire spread commonly depends upon a sufficient mixture of live and dead fuels in order to sustain the fire. However, under windy conditions this equation changes and wind is capable of carrying heat to living foliage and rapidly spreading fire through mostly living foliage. Wind increases combustion by mixing of oxygen and altering the flame angle so that there is increased heating of fuels ahead of the flaming front. In general, as wind speed increases the role of fuels diminishes. Weather patterns that lead to high winds are a major determinant of fire size and essentially all large fires on contemporary landscapes are the result of severe fire weather conditions coupled with high temperature and low humidity.
On some landscapes weather may be of such overriding importance that it obscures climate signals that influence fire regimes in many parts of North America. For example, predictable annual autumn foehn winds in southern California are the primary determinant of large fires, and consequently droughts show little or no relationship to annual variation in area burned. However, outside the autumn foehn wind season droughts are associated with a lengthening of the fire season.
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