Like topography, weather conditions have a large influence on fire behavior. Weather conditions significantly impact the rate, intensity, and direction in which fires burn. Temperature, humidity and especially wind are the more important weather variables used to predict fire behavior.
Marin County has warm, dry summers with a fire season that typically extends from May to October. Cool, moist winters are characteristic of the fog belt area, with spring rainfall that accelerates light, flashy fuel growth.
Summer temperatures are often quite warm, with some days over 100 degrees. It is common for coastal fog to form during summer evenings and mornings, while the steep ridge to the west that blocks the fog from moving inland. According to weather data at the Marin County Fire Department Wooacre Fire Station, the average maximum temperatures in June through September exceed 80 degrees daily. Hot days associated with stagnant air and high pressures aloft pose times of extreme fire danger. During this time, periods of continuous high temperatures and low relative humidities dry fuels to a point where the National Fire Danger Rating System (NFDRS) Burn Index offten indicates critical fire weather, indicating that a vegetation fire starting at that time would exhibit extreme resistance to control.
"Diablo," or "Santa-Ana" winds occur infrequently, but can influence fire behavior greatly. These offshore wind patterns create extremely low humidities and enhance the possibility of ignition and extreme fire behavior. Once a fire escapes initial attack, teh fire itself can generate its own wind, creating strong updrafts that entrains additional wind into the burning environment.