Understanding Fire Danger

Fire Danger is a description of the combination of both constant and variable factors that affect the initiation, spread, and difficulty to control a wildfire on an area. Since 1974, five Adjective Fire Danger ratings have been used to describe danger levels in public information releases and fire prevention signing. They are:

Low (Green)
Fire starts are unlikely. Weather and fuel conditions will lead to slow fire spread, low intensity and relatively easy control with light mop-up. Controlled burns can usually be executed with reasonable safety.

Moderate (Blue)
Some wildfires may be expected. Expect moderate flame length and rate of spread. Control is usually not difficult and light to moderate mop-up can be expected. Although controlled burning can be done without creating a hazard, routine caution should be taken.

High (Yellow)
Wildfires are likely. Fires in heavy, continuous fuel such as mature grassland, weed fields and forest litter, will be difficult to control under windy conditions. Control through direct attack may be difficult but possible and mop-up will be required. Outdoor burning should be restricted to early morning and late evening hours.

Very High (Orange)
Fires start easily from all causes and may spread faster than suppression resources can travel. Flame lengths will be long with high intensity, making control very difficult. Both suppression and mop-up will require an extended and very thorough effort. Outdoor burning is not recommended.

Extreme (Red)
Fires will start and spread rapidly. Every fire start has the potential to become large. Expect extreme, erratic fire behavior. NO OUTDOOR BURNING SHOULD TAKE PLACE IN AREAS WITH EXTREME FIRE DANGER

Relative Humidity
Relative humidity (RH) is the ratio of the amount of moisture in the air to the amount of moisture necessary to saturate the air at the same temperature and pressure. Relative humidity is expressed in percent. RH is measured directly by automated weather stations or manually by wet and dry bulb readings taken with a psychrometer and applying the National Weather Service psychrometric tables applicable to the elevations where the reading were taken.

Relative humidity is important because dead forest fuels and the air are always exchanging moisture. Low humidity takes moisture from the fuels…fuels in turn, take moisture from the air when the humidity is high. Light fuels, such as grass and pine needles, gain and lose moisture quickly with changes in relative humidity. When the RH drops, fire behavior increases because these fine fuels become drier. Heavy fuels, on the other hand, respond to humidity changes more slowly. To see significant changes in heavy fuel moisture, there must be significant moisture, usually more than a single storm. (See Fuel Moisture.)

Fuel Moisture
Fuel moistures are measured for live Herbaceous (annual and perennial) and Woody (shrubs, branches and foliage) fuels and Dry (dead) fuels. These are calculated values representing approximate moisture content of the fuel. Fuel moisture in live fuels varies through the growing season and between different climate classes. There are 20 different fuel models, representing the variety of vegetation in the area, a manager can use when calculating fire danger. Dead fuel moisture is the moisture content of dead organic fuels, expressed as a percentage of the oven dry weight of the sample.

Dead fuel moisture is controlled solely by exposure to environmental conditions and is critical in determining fire potential. Dead fuel moistures are classed by time lag. A fuel's time lag is the time necessary for a fuel particle of a particular size to reach 63% of equilibrium between its initial moisture content and its current environment.

Dead fuels in NFDRS have four time lag classes:

10-hr: Round wood 1/4 to 1" diameters and the layer of litter that extends to 3&Mac218;4" below the surface. Computed from observation time temperature, humidity, and cloudiness, or may be a standard set of "10-Hr Fuel Sticks" that are weighed as part of the fire weather observation.

100-hr: 1 to 3" diameter. Computed from 24 hour average boundary condition composed of day length, hours of rain, and daily temperature/humidity ranges.

1000-hr: 3 to 6 " diameter. Computed from a 7-day average boundary condition composed of day length, hours of rain, and daily temperature/humidity ranges.

Greenness Maps
Greenness maps provide a visual representation of live fuel moisture. Four vegetation greenness maps are derived weekly from Normalized Difference Vegetation Index (NDVI) data observed by satellites and provided by the EROS Data Center, U.S. Geological Survey. These maps have a 1-kilometer (.6 mile) spatial resolution. Maps with historical references (RG and DA) are based on the years 1989 through 1995. The derived maps are:

Visual Greenness Maps
portray vegetation greenness compared to a very green reference such as an alfalfa field or a golf course. The resulting image is similar to what you would expect to see from the air. Normally dry areas will never show as green as normally wetter areas.

Relative Greenness Maps
portray how green the vegetation is compared to how green it has been historically (since 1989). Because each pixel is normalized to its own historical range, all areas (dry to wet) can appear fully green at some time during the growing season.

Departure from Average Greenness Maps
portray how green each pixel is compared to its average greenness for the current week of the year.

Live Moisture Maps
portray experimental live vegetation moisture with values ranging from 50 to 250 percent of dry weight.

Drought Maps
Keetch-Byram Drought Index (KBDI) - This can be used to measure the affects of seasonal drought on fire potential. The actual numeric value of the index is an estimate of the amount of precipitation (in 100ths of inches) needed to bring soil back to saturation (a value of 0 being saturated). The index deals with the top 8 inches of soil profile so the maximum KBDI value is 800 (8 inches), the amount of precipitation needed to bring the soil back to saturation. The index's relationship to fire is that as the index values increase, the vegetation is subjected to greater stress because of moisture deficiency. At higher values living plants die and become fuel, and the duff/litter layer becomes more susceptible to fire.

KBDI = 0–200
Soil moisture and large class fuel moistures are high and do not contribute much to fire intensity. This is typical of spring dormant season following winter precipitation.

KBDI = 200–400
Typical of late spring, early growing season. Lower litter and duff layers are drying and beginning to contribute to fire intensity.

KBDI = 400–600
Typical of late summer, early fall. Lower litter and duff layers actively contribute to fire intensity and will burn actively.

KBDI = 600–800
Often associated with more severe drought with increased wildfire occurrence. Intense, deep burning fires with significant downwind spotting can be expected. Live fuels can also be expected to burn actively at these levels.

Lower Atmosphere Stability Index
The Lower Atmosphere Stability Index, or Haines Index, is computed from the morning (12Zulu) soundings from Radiosonde Observation (RAOB) stations across North America. The index is composed of a stability term and a moisture term. The stability term is derived from the temperature difference at two atmosphere levels. The moisture term is derived from the dew point depression at a single atmosphere level. This index has been shown to correlate with large fire growth on initiating and existing fires where surface winds do not dominate fire behavior. Haines Indexes range from 2 to 6 for indicating potential for large fire growth:

2 Very Low Potential (Moist Stable Lower Atmosphere)
3 Very Low Potential
4 Low Potential
5 Moderate Potential
6 High Potential (Dry Unstable Lower Atmosphere)

Lightning Activity Levels
Lightning Activity Level (LAL) is a measurement of the cloud-to-ground lightning activity observed (or forecasted to occur) within 30 mile radius of the observation site. The National Fire Danger Rating System requires two inputs for lightning activity level. The first covers the period from when the previous day's observation was taken until midnight (commonly referred to as Yesterday's Lightning) and the second covers the period from midnight until today's observation time (commonly referred to as Morning Lightning). The Fire Weather Forecaster of the National Weather Service use the same scale when forecasting lightning activity levels.

Lightning Activity Level is rated on a scale of 1 to 6 as described below:

No thunderstorm or building cumulus clouds observed.

A single or few building cumulus clouds with only an occasional one reaching thunderstorm intensity observed. Thunderstorms or lightning need not be observed for this activity level to be assigned; however at least one large cumulus cloud must be present.

Occasional lightning (an average of 1 to 2 cloud -to-ground strikes per minute) is observed. Building cumulus clouds are common; thunderstorms are widely scattered.

Frequent lighting (an average of 2 to 3 cloud-to-ground strikes per minute) is observed. Thunderstorms are common and cover 10 to 30 percent of the sky. Lightning is primarily of the cloud-to-cloud type but cloud-to-ground lightning may be observed.

Frequent and intense lightning (cloud-to-ground strikes greater than 3 per minute) is observed. Thunderstorms are common, occasionally obscuring the sky. Moderate to heavy rain usually precede and follow the lightning activity. Lightning of all kinds (cloud-to-cloud, in-cloud and cloud-to-ground) is characteristically persistent during the storm period.

A dry lightning situation. Low lightning flash rate observed (less than one to three cloud-to-ground strikes per 5-minute period per storm cell passage). Scattered towering clouds with a few thunderstorms. Bases of the clouds are high. Virga is the predominate form of precipitation.

Ignition Component (IC)
The Ignition Component is a number that relates the probability that a fire will result if a firebrand is introduced into a fine fuel complex. The ignition component can range from 0 when conditions are cool and damp, to 100 on days when the weather is dry and windy. Theoretically, on a day when the ignition component registers a 60 approximately 60% of all firebrands that come into contact with wildland fuels will require suppression action.

Ignition normally takes place in the dead component of the fine fuels. Three distinct steps must be considered in this ignition process. They are:

Living material in the fine fuel complex reduces the efficiency of ignition. Therefore, an adjustment to the ignition component is made based on the percentage of live fuel (herbaceous vegetation) in the fine fuel complex.

The moisture content of the dead component of the fine fuel (1-hr. timelag fuel moisture) is determined by the state of the weather (sunny or cloudy), air temperature, and relative humidity at the time of the 2 p.m. fire weather observation.

The condition of the herbaceous (live) vegetation and the 1-hr. timelag fuel moisture are then integrated in the calculation the fine fuel moisture (FFM) which expresses the effective moisture content of the fine fuels.

The closer the initial temperature of the fuel is to the ignition temperature, the more likely a fire will result when a firebrand in introduced into the fine fuel complex, since not a much energy is required to raise the fuel particle to its ignition temperature.