Photo courtesy Bureau of Land Management
Weather's Role in Wildfires
Weather plays a major role in the birth, growth and death of a wildfire. Drought leads to extremely favorable conditions for wildfires, and winds aid a wildfire's progress -- weather can spur the fire to move faster and engulf more land. It can also make the job of fighting the fire even more difficult. There are three weather ingredients that can affect wildfires:
As mentioned before, temperature affects the sparking of wildfires, because heat is one of the three pillars of the fire triangle. The sticks, trees and underbrush on the ground receive radiant heat from the sun, which heats and dries potential fuels. Warmer temperatures allow for fuels to ignite and burn faster, adding to the rate at which a wildfire spreads. For this reason, wildfires tend to rage in the afternoon, when temperatures are at their hottest.
Wind probably has the biggest impact on a wildfire's behavior. It also the most unpredictable factor. Winds supply the fire with additional oxygen, further dry potential fuel and push the fire across the land at a faster rate.
Dr. Terry Clark, senior scientist at the National Center for Atmospheric Research, has developed a computer model that shows how winds move on a small scale. Since 1991, he's been converting that model to include wildfire characteristics, such as fuel and heat exchange between fires and the atmosphere.
"We look at what's called coupled fire atmosphere dynamics, where the fire and the atmosphere interact with each other," Clark said. "We've been looking at how fires interact with the environment and getting some of the characteristics of fire spread and fire behavior, through the modeling that we've been doing."
Clark's research has found that not only does wind affect how the fire develops, but that fires themselves can develop wind patterns. When the fire creates its own weather patterns, they can feed back into how the fire spreads. Large, violent wildfires can generate winds, called fire whirls. Fire whirls, which are like tornadoes, result from the vortices created by the fire's heat. When these vortices are tilted from horizontal to vertical, you get fire whirls. Fire whirls have been known to hurl flaming logs and burning debris over considerable distances.
"There's another way that you can tilt the vorticity. That is it can be titled without breaking into fire whirls, and basically be burst forward into what's called hairpin vortices or forward bursts," Clark said. "These are quite common in crown fires [fires at the top of trees], and so you see fires licking up hill sides." Forward bursts can be 20 meters (66 feet) wide and shoot out 100 meters (328 feet) at a speed of 100 mph (161 kph). These bursts leave a scorched region and lead to fire spread.
The stronger the wind blows, the faster the fire spreads. The fire generates winds of its own that are as many as 10 times faster than the ambient wind. It can even throw embers into the air and create additional fires, an occurrence called spotting. Wind can also change the direction of the fire, and gusts can raise the fire into the trees, creating a crown fire.
While wind can help the fire to spread, moisture works against the fire. Moisture, in the form of humidity and precipitation, can slow the fire down and reduce its intensity. Potential fuels can be hard to ignite if they have high levels of moisture, because the moisture absorbs the fire's heat. When the humidity is low, meaning that there is a low amount of water vapor in the air, wildfires are more likely to start. The higher the humidity, the less likely the fuel is to dry and ignite.
Since moisture can lower the chances of a wildfire igniting, precipitation has a direct impact on fire prevention. When the air becomes saturated with moisture, it releases the moisture in the form of rain. Rain and other precipitation raise the amount of moisture in fuels, which suppresses any potential wildfires from breaking out.