More than 170 wildfires have burned 135,000 acres in Texas over the past week. The largest fire, in Bastrop County near Austin, has destroyed 576 homes. When the fields around Austin are on fire, does the temperature rise in cities downwind (PDF) of the blaze, like Dallas and Fort Worth?
No. If anything, it cools down a little. Wildfires can reach 1,500 degrees Fahrenheit and give off nearly 11,000 kilowatts of energy per yard of fire front. That sounds like a lot, but it's not nearly enough to cause a measurable temperature difference in cities more than a few miles away. In fact, wild fires tend to cool areas downwind of the blaze. Fires launch particulate matter into the atmosphere, which acts like a giant sunshade, blocking solar radiation from reaching the ground. (The eruption of Mount Pinatubo in 1991 cooled the entire globe for the same reason.) Dallas residents shouldn't break out their parkas, though—the cooling effect is pretty small. Wildfire ash clouds usually block less than 5 percent of the sun's rays, which could probably only lower local temperatures by a degree or two at the most.
Wildfires have a dramatic effect on the weather, but only in their immediate vicinity. Updrafts send hot air and particulate matter into the atmosphere. When the air cools, the particles provide gathering points for condensing water vapor, which generates large, dark clouds. It's common to hear thunderclaps and see lightning bolts directly above a wildfire. (These fire-born clouds don't often produce significant rains.) Pressure differences around the blaze can also create a fire whirl, a sort of flaming tornado that looks like something out of the Book of Revelations.
As you move away from the fire, the effects become less dramatic and more difficult to predict. The airborne particles absorb much of the solar radiation that they block from reaching the ground, which heats up the atmosphere at a particular altitude. (Very hot fires, such as those in the Amazon, can send particles five miles high, while aerosols from smaller brush fires will top out at just a mile or so.) Under the right conditions, this temperature change can lead to cloud and storm formation. In other circumstances, the aerosols might actually prevent a storm from happening.
In any event, the effects of aerosols from the Texas wildfire are modest and localized. An online map of the world's current aerosol situation offers a sense of just how insignificant the Texas wildfires are in the grand scheme of particulate matter. Dust above North Africa and the Middle East block and absorb several times as much solar radiation as the wildfires in North America.
This field of research is in its infancy. Atmospheric scientists are just beginning to create useful mathematical models of how fires influence weather downwind. Researchers are planning to send unmanned aircraft directly above major wildfires to measure the constantly shifting changes in temperature, pressure, moisture, and wind speed and direction, which will improve weather forecasting. As of now, however, meteorologists aren't really able to incorporate the effects of the fire into the weekend weather outlook.
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Explainer thanks Georg Grell of NOAA and Eli Jacks of the National Weather Service.*
Correction, Sept. 8, 2011: This article originally misspelled the last name of Eli Jacks.