Fire Weather

Fire behavior is dramatically influenced by weather conditions. Large costly fires are frequently associated with severe fire weather conditions. High temperatures, low humidity, and strong surface winds typify fire weather.

Weather is a very dynamic event in California. There are several weather influences that are in constant conflict. Factors such as a marine influence, solar radiation on numerous slopes, and inversion layers, alter the weather in different ways. In Marin, strong north or northeast winds drive severe fire weather. Under these circumstances, the entire county is at risk. These winds may happen several times a fire season, or not at all. To capture this event in an assessment would be difficult at best.

Instead, the department developed an assessment based on events that are less dynamic than wind, but play a very big part of the daily fire weather potential. There are four components of the weather index: the inversion component, the weather station component, the sheltering component, and the aspect component. Adding the scores of all four components derives the final weather rank.

Inversion Component

In fire season an inversion layer is constantly in place. The inversion layer, or warm body of air, acts as a lid and traps cool moist air below it. The cool moist air is commonly called fog. Above the inversion, temperatures are normally warmer and relative humidity is normally lower. When areas are fogged in, this combination of warmer air and lower humidity allows fires to burn. The inversion layer is situated at approximately 1100 feet elevation.

The ranking is simple:

Weather Station Component

There are six fire weather stations within Marin County. Five of the stations were used to create a Fire Weather Index (FWI). The sixth station is a Remote Automated Weather Station and data was not available. This index combines air temperature, relative humidity, and wind speed into a single index value. This index is calculated from daily weather readings collected at the five weather stations. Appendix C includes all values for the FWI.

Once the indices were developed, ranks of low through very high were placed on each weather station. Each Quad 81^st was assigned a weather station. The weather stations were assigned based on areas with similar weather influences. The five fire weather stations used are:

Weather Station	Index Value	Rank	Station Value
Civic Center	1.97	Very High	3
Rock Springs	1.58	High	2
Woodacre	1.50	High	2
Bear Valley	1.02	Moderate	1
Muir Woods	.40	Moderate	1

Sheltering Component

Tree canopies shelter or alter the underlying environment in different ways. As the fog rolls in, the tree canopies collect the moisture and deposit it on the soil below. This keeps soil moisture high, slowing fire behavior. The canopies also shelter the soil from the sun. This, too, allows fuel moistures to remain higher and prevent vegetation from growing altogether. Lastly, the tree canopies alter wind speed and direction. In fully sheltered locations, wind speeds may be decreased by a factor of .1(5 mph wind x .1 = .5 mph wind speed).

The fuel model assessment was utilized to identify sheltered areas. The following areas shelter wind and sun: fuel model 8 (oak, bay, madrone), model 9 (moderate conifer), or model 10 (heavy conifer). Sheltering is a positive factor relating to fire weather. As the sheltering slows wind, blocks sunlight, or deposits dew, it helps slow a fire. For this reason, a negative value is given.

Aspect Component

The direction a slope faces is called aspect. As the earth rotates on its axis, certain aspects receive more or less sunlight and solar heating. The north- northeast slopes receive the least amount of solar radiation whereas the south-southwest slopes receive the most.

Fires react differently on distinct aspects, depending on the time of day. This is primarily due to the fuel variety found on different slopes combined with the fuel temperature. Southern slopes are dryer with lighter fuels such as annual grass and brush. Fires burn very quickly on south-southwest slopes. Northern slopes are moister, supporting heavier fuel types such as conifers. Fires normally burn slower in these areas.

By adding the Inversion Value, Weather Station Value, Sheltering Value, and Aspect Value, a Total Weather rank is derived:

The following maps indicate the process and ranking for fuel in each Quad 81^st:

• Weather Station Assignment Map - Under Construction
• Sheltering Map - Under Construction
• Inversion Map - Under Construction
• Aspect Map - Under Construction
• Weather Process Map - Under Construction
• Weather Rank Map - Under Construction

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