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Role of Weather in Wildfires

Weather plays a major role in the birth, growth and ability to control 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 and homes. Wind can also make the job of fighting the fire even more difficult. There are three weather ingredients that can affect wildfires:

• Air Temperature
• Wind
• Moisture

Air Temperature

Air temperature has a direct influence on fire behavior because of the heat requirements for ignition and continuing the combustion process. This heat warms up the surface of the earth, and the atmosphere close to the surface is in turn warmed by heat reflecting from the surface. For this reason, wildfires tend to rage in the afternoon, when temperatures are at their hottest.

Forest fuels, which include homes in the Wildland-Urban Interface (WUI), receive heat by radiation from the sun, and less heat is required for ignition. The differential heating of the earth’s surface is the driving force behind most of the influences on the atmosphere. When sunlight strikes a solid object such as trees, grass or your home, the fuel is warmed. The surface absorbs some of the heat and reflects some in long-wave radiation that is absorbed by the water vapor in the air, thus raising its temperature as well.

Fuel temperatures also affect a wildfire’s rate of spread. Warm fuels will ignite and burn faster because less heat energy is used to raise the fuels to their ignition temperature. Fuels exposed to sunlight will be warmer than the fuels in shade. They will also be drier. Look to the foothills immediately west of Ken-Caryl to recognize this effect. The fuels on the north-facing exposure are cooler. A hike up Massey Draw on a hot day provides an immediate lesson on the cooling effects of the shaded areas.

Fires also burn more intensely in the afternoon. The temperature is the highest at that time resulting in higher fuel temperatures. Consequently, less heat is needed to raise the fuel to its ignition temperature. At the same time, rising temperatures result in decreasing relative humidity and fuel moisture.

Wind

Wind has a strong effect on fire behavior due to the fanning effect on the fire. Wind can change direction and intensity throughout the day. Abrupt changes generally occur during the afternoon when atmospheric conditions are most unstable, further increasing safety risks associated with wildfire.
Wind is important to homeowners and firefighters alike because of three influences it has on fire behavior:

• Supplying oxygen for the combustion process.
• Reducing fuel moisture by increasing evaporation.
• Exerting pressure to physically move the fire and heat produced closer to fuel in the path of the fire and carrying burning embers, firebrands, ahead of the fire.

Wind increases the supply of oxygen, which results in the fire burning more rapidly. It also removes the surface fuel moisture, which increases the drying of the fuel. Air pressure will push flames, sparks and firebrands into new fuel. By pushing the flames closer to the fuel in front of the fire, the fuel is preheated quicker because of the increased radiant heat. More of the fuel becomes available for combustion since it is drier and can reach ignition temperature quicker.

Wind may cause the fire to crown into the top of the trees and to jump barriers that would normally stop a fire. Wind can carry sparks and firebrands ahead of the main fire. Wind generally increases evaporation from damp surfaces by carrying away moist air and bringing in drier air. The major fires in Colorado in 2012 – Waldo Canyon and High Park – were driven by extreme winds.

Moisture

Relative humidity (moisture) is the ratio of the amount of moisture in the air (in the form of water vapor) compared to the amount that the air can hold at the same temperature and pressure if it were saturated. A relative humidity of 95 percent (not often seen in Colorado) indicates that the air is nearly saturated with almost all of the water vapor it can hold. The air is much drier when the relative humidity is 30 percent, as compared to 100 percent.

Moisture is exchanged between the air and nearby objects, including dead and live fuels. For example, at low relative humidity the moisture moves out of fuels to the air, and thus, drying occurs. At high relative humidity the dead fuels retain most moisture, i.e., less moisture moves out of the fuel into the air. When a fuel has more moisture, it is harder to ignite and burn. The dry climate of Colorado can dry out fuels on hot, windy days to the level of kiln-dried lumber purchased at a lumber yard.

The relative humidity of the air changes faster than an entire fuel particle can exchange moisture with the air. Fine fuels (such as grasses) can experience significant changes in moisture levels within an hour or two of large changes in relative humidity, whereas coarse fuels (large trees) require longer exposure to change moisture levels.

This article appeared in the June 15, 2016 issue of Life at Ken-Caryl.

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