What is "Fire Hazard"?

Fire hazard, fire risk, and other similar sounding terms can cause mix-ups in communication. Here is some background to help:

Fire hazard is composed of fire risk, fuel condition, weather, and topography.

Fire Risk

Risk is the chance of a fire starting based on the presence of causative agents such as humans, their equipment, and their facilities.

Past fire in a given area is the most commonly used measure of risk. The influx of humans and equipment into new wildland areas increases risk.

Fuel Condition

Fuel condition is a combination of fuel size, quantity, arrangement, and dead:live ratio. Fuel characteristics help determine how a wildfire burns. The fuel condition determined for an area makes up fuel hazard which is not to be confused with fire risk.

Fuel size is measured by the number of hours it takes the fuel to reach equilibrium with the relative humidity in the air. One hour fuels dampen or dry quickly. Dead grass is a one hour fuel. Very small twigs and branches are examples of 10-hour fuels. Branches between 1/4 and 3" in diameter are usually considered 100-hour fuels. Larger branches, logs, and stumps over 3" in diameter are 1000-hour fuels because it takes so long for them to dry out each summer.

Fuel quantity is simply the amount of all fuel sizes located on an area of land. It is usually expressed in tons per acre. Annual grasslands may have only 1/2–2 tons per acre of one-hour fuels. Hardwood brush areas with a mixture of fuels can contain 30–40 tons per acre. Heavy logging slash can contain 40+ tons per acre of mostly 1000-hour fuels.

Fuel arrangement generally refers to density and compactness of fuel. A dry grain field is densely packed as planted by the farmer; it is all standing. Cut and drying hay is lower to the ground and is somewhat compressed or pushed down. A wind-driven standing grain fire will move more quickly than one in cut hay. Ladder fuels are fuels present between the litter on the ground and the tops of trees; both living and dead. Unlopped logging slash is a ladder fuel, so is standing dead brush mixed in a forest vegetation type.

The dead:live ratio is sometimes considered, especially in standing live brush fields or where groves of trees have a significant amount of dead lower branches extending to the ground. The ratio of dead to live fuel is important because it affects the rate of wildfire spread. Dead fuel hanging on live plants above the ground can ignite easily and spread fire rapidly into the crowns of trees or brush.

Weather

This is a component of fire hazard that humans have no control over. However, we must understand it to understand the effect on fire behavior. Temperature, humidity, and wind are the major components. For example, the hot, dry, swift winds of late summer and fall have had a huge impact on driving the major high intensity destructive fires in our state’s history.

Topography

Topography is simply the lay of the land—flat, hilly, or mountainous. Canyons and valleys and their directions—north, south, east, west—have a large impact on fire spread. Rugged topography allows fire going uphill to pre-heat fuels ahead of the fire, causing explosive burning conditions. Fires generally move faster uphill than downhill or on flat terrain under the same weather conditions.