Fire Risk Variables

This glossary provides an overview of key meteorological and physical variables used to assess and predict wildfire risk. These variables are essential components of fire behavior models and fire danger rating systems, helping to quantify atmospheric conditions and fuel readiness that influence a fire’s potential for ignition, intensity, and spread. The data for these variables is often collected and modeled on a grid, such as in the GRIDMET project, a source for climate data that is useful for fire management.

Source data is available:
https://www.climatologylab.org/gridmet.html

Fire Risk Variables

Burning Index (BI)
A numerical index that indicates the potential difficulty of containing a wildland fire. It is a composite of the Energy Release Component and the Spread Component. A higher BI value suggests that a fire would be more difficult to control, with a greater potential for longer flames and higher intensity.

Energy Release Component (ERC)
This value represents the total potential heat energy (measured in BTU/square foot) released per unit area at the head of a fire. The ERC is a cumulative index that reflects the moisture content of all live and dead fuels, making it a good indicator of drought conditions. As fuels dry out, the ERC value increases.

100-hr Dead Fuel Moisture
A measure of the moisture content in dead fuels with a diameter of 1 to 3 inches. These fuels take approximately 100 hours to respond to changes in atmospheric moisture. Lower moisture content in these fuels indicates a higher fire risk.

1000-hr Dead Fuel Moisture
A measure of the moisture content in large dead fuels with a diameter of 3 to 8 inches. These fuels take approximately 1000 hours (about 42 days) to respond to changes in weather. They are slow to dry out, and while they are not a major factor in fire ignition, they contribute significantly to the heat output and long-term intensity of a fire.

Maximum Relative Humidity & Minimum Relative Humidity
Relative humidity (RH) is the amount of water vapor in the air, expressed as a percentage of the maximum amount the air can hold at a specific temperature. Minimum RH, which typically occurs during the warmest part of the day, is a critical fire weather variable. Lower RH means drier air, which draws moisture from fuels and increases fire danger. Maximum RH, which occurs at night or in the early morning, can increase fuel moisture and help to moderate fire behavior.

Specific Humidity
The mass of water vapor in a unit mass of moist air. Unlike relative humidity, it is not affected by changes in temperature. While less commonly used in direct fire danger indices, it is a key variable for advanced fire weather modeling as it provides a more stable measure of atmospheric moisture.

Minimum Daily Temperature & Maximum Daily Temperature
Temperature affects fire risk by influencing fuel moisture. As the Maximum Daily Temperature increases, fuel moisture tends to decrease, making fuels more receptive to ignition. Higher temperatures also increase the rate of fire spread. The Minimum Daily Temperature often correlates with the highest relative humidity, a period of lower fire danger.

Reference evapotranspiration (ASCE) & Potential Evapotranspiration
Both are measures of the amount of water that would evaporate from the ground and transpire from plants if there was sufficient water available. They are used to gauge drought conditions and are crucial for understanding live fuel moisture content and the overall dryness of a landscape.

Precipitation Accumulation
The total amount of precipitation (rain, snow, etc.) that has fallen over a specific period. A high level of precipitation accumulation directly reduces fire risk by increasing the moisture content of both live and dead fuels. Long periods with little or no precipitation lead to dry fuels and higher fire danger.

Surface Downwelling Shortwave Radiation
Also known as global radiation, this is the amount of solar radiation reaching the Earth’s surface. High levels of solar radiation increase air and fuel temperatures and can accelerate the drying of fuels, thus increasing fire danger.

Wind Direction & Wind speed at 10 meters
Wind is one of the most critical factors in fire behavior. The Wind speed at 10 meters is a standard measurement used in fire modeling. Strong winds can significantly increase a fire’s rate of spread, carry embers for spotting, and make suppression efforts more difficult. Wind Direction is vital for predicting the path of a fire and the location of potential spot fires.

Mean Vapor Pressure Deficit (VPD)
The difference between the amount of moisture in the air and the maximum amount the air can hold. A high VPD indicates very dry air, which causes plants to lose water more rapidly through transpiration and dries out fuels. A high VPD is directly correlated with higher fire danger.