In addition to the three ways wildfires spread, there are three primary conditions that affect how quickly and with what intensity a wildfire spreads. They are:

• Fuel
• Weather
• Topography

Click on the links to learn about each of these conditions that affect wildfires.

Fuel conditions refer to the amount, density, and flammability of fuel. Fuels are anything that will burn, including:

• Vegetation—Whether a tree in the woods or a shrub in a garden, vegetation can fuel a wildfire.

Because dead plants burn very easily, the presence of dead vegetation increases the likelihood of a more intense and faster spreading wildfire.

Live, green, wet plant life does not burn easily and may slow a wildfire’s progress. However, in a wildfire, all vegetation can eventually act as fuel. The density of vegetation, or how close plants are to one another, can also impact the ability of a wildfire to spread.

• Structures—Both the contents and the building materials used can greatly impact the spread of a wildfire. For example, a cedar-shake or wood-sided home will burn more quickly than a brick home.

The greater the structural density, or how close structures are to one another, the faster the wildfire will spread.

Weather has an impact on the spread of a wildfire. High temperatures, low humidity, and high winds increase the likelihood that a wildfire will spread from wildlands to inhabited areas. In contrast, cold, humid, and calm conditions inhibit a wildfire’s spread. (Photo shows a wooded area with storm clouds overhead.)

Even topography affects the speed at which a wildfire spreads. Wildfires move more quickly up a hill than down.

A wildfire moving up a slope causes hot gases to rise in front of it. The hot gases pre-heat and dry vegetation ahead of the wildfire, causing it to catch fire more rapidly. A grass fire can advance four times faster moving up a slope than on level ground. (Illustration shows wildfire moving up a hill).