Fire Behavior and Effects

  • Weather
  • Fuel
  • Topography

When a wildfire starts, many elements can affect how it burns, how fast it moves, and how difficult it is to control.


The weather plays an important role when wildland fires burn. If it is very hot and humidity is low, the vegetation is typically very dry, so wildfires burn rapidly. If the wind is blowing, it pushes the fire into more vegetation and provides more oxygen for the fire to burn, which causes the fire to rapidly grow. Wind can also blow embers for miles, causing new fires to start. If it rains, the fire slows down or goes out. Storms can cause fire activity to increase or become completely unpredictable. Firefighters must pay close attention to the weather in order to stay safe and effectively extinguish fire.


“Fuel” means vegetation, and Idaho has diverse ecosystems with different types of vegetation. Large trees are very dense and massive, so they can burn for hours and create a lot of heat. Grass burns very easily, but it is not very dense or big, so it goes out quickly. Typically, if there is more fuel present, wildfires will burn longer and will be more intense. For example, wildland firefighters can extinguish a small grass fire in a matter of hours, but large forest fires can burn for weeks, even months, because of the dense vegetation, or fuel.


Wildfires are also affected by the terrain or topography where they burn. If a wildfire burns in a mountainous area, it can be more difficult to contain. Why? Because fire burns more rapidly up a slope, hill, or mountain. This happens for a couple of reasons; when a fire starts at the bottom of a hill, as it moves up the hill, it preheats the unburned fuel above it, so when it reaches the unburned fuel, that fuel is already hot, dry, and ready to burn. Wind also moves rapidly uphill, so it helps to push fire up mountains or slopes. This is why firefighters recommend building homes away from the tops of mountains or slopes; because if a wildfire starts below the home, the mountain or slope will serve as a funnel for the fire and can easily catch the home on fire.