Watching Wildfires from Space

NASA's Earth Probe satellite is keeping an eye on smoke from wildfires raging across the Western US.

August 4, 2000 -- Nearly 50 large wildfires are blazing throughout the western United States, consuming more than 700,000 acres and taxing resources already burdened by the worst year of fires in more than a decade. Officials in California and Idaho have enlisted the aid of the Air Force National Guard and the US Army to support their beleaguered firefighting regulars. According to the National Interagency Fire Center, the situation worsened on August 2nd when dry thunderstorms triggered hundreds of new small fires.

Above: Major western wildfires tracked by the National Interagency Fire Center. So far this year, more than 3.5 million acres have succumbed to fire. Only one other year in the last 13 saw more than 3 million acres burned by the end of July. For updated fire maps, visit the National Fire News web site.

As soot-covered firefighters face the billowing roar and crackle of the convulsing flames on Earth, NASA's Earth Probe satellite hangs in the silence of space monitoring the plumes of smoke 740 km below.

Every few hours, a global map of smoke and other aerosols detected by Earth Probe are updated at the Total Ozone Mapping Spectrometer (TOMS) web site. Anyone with a connection to the Internet can share the same "bird's eye" view enjoyed by NASA scientists.

"The smoke was thick and swirling around our house," said Dr. Tony Phillips, who lives in California's remote Eastern Sierra mountain range about 100 miles away from an on-going wildfire in the Sequoia National Forest. "We knew there had to be a big fire somewhere, but I couldn't get much information on the local radio station. So, I went to the TOMS web site and looked at their latest aerosol map. And there it was -- a big cloud of haze near the national forest."

Left: This 3-frame animation is a series of TOMS aerosol maps that shows the changing distribution of smoke from Western wildfires between July 31 and August 2, 2000. Check the TOMS Global Aerosol Hot Spots Page for updates.

"We put (data) on our Web site every day, usually within minutes of receiving it," explains Dr. Jay Herman, who works at NASA's Goddard Space Flight Center as the principle scientist for the TOMS aerosol remote-sensing instruments.

The Environmental Protection Agency sometimes uses data available on the TOMS Web site to help formulate pollution alerts, and the ability to spot smoke plumes in remote locations often aides firefighters.

"Canadian boreal fires and the fires that occur regularly in Siberia are very poorly reported, but easily seen from space," Herman said.

In addition to smoke, the TOMS instruments on Earth Probe and other satellites detect volcanic ash, pollutants, dust storms, ozone concentrations and ultraviolet light intensities. Most of these data are available online in near realtime.

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Remarkably, the sensor is able to make all of these measurements by simply observing the sunlight reflected toward it from the Earth.

"It looks at the Earth just as you would with your eye to see how bright it is in the ultraviolet (wavelengths)," Herman said. "That's all it is."

To detect the smoke from the wildfires, the TOMS instrument looks at two particular wavelengths of ultraviolet (UV) light: 331 nanometers and 360 nanometers. Some of the UV light reflecting from Earth is absorbed by the smoke, dust and other aerosols. Clouds reflect both wavelengths almost equally, while aerosols cause absorption of the shorter wavelength more than the longer one.

To be sure the instrument is detecting smoke and not clouds, the measured intensity for one wavelength is subtracted from the other. Since clouds reflect them both equally, the difference will be nearly zero. Smoke will have a difference that is not zero.

Riding aboard the Earth Probe satellite, the TOMS sensor sweeps over the planet in a polar orbit (an orbit passing over the North and South poles), completing one orbit about every 90 minutes and mapping the entire surface of the Earth once each day, Herman said.

The orbit is "sun-synchronous," meaning that if you watched from the sun, the satellite would appear to follow the same circle around the Earth all year. It also means that the satellite passes over all locations on Earth at about 11 a.m. local time every day of the year, Herman said.

Because of this special orbit, the satellite is able to detect the smoke from fires (among other things) all over the world. For example, archived images on the TOMS Web site show smoke from the burning of biomass in the Amazon region of South America, as well as from the slash-and-burn agriculture widely practiced in Africa, Herman said.

Above: TOMS isn't NASA's only "eye-in-the-sky." The SeaWiFs satellite, designed to study the role of Earth's oceans in the global carbon cycle, has recorded many striking images of smoke from global fires. This one, showing smoky plumes from wildfires on the Idaho/Montana border, was captured by SeaWiFs on August 2, 2000. [more information]

However, the TOMS sensor cannot pinpoint the location of a source of smoke within less than 40 km (about 25 miles), because that is the size of one "pixel" in the satellite’s data.

Despite having such a large "pixel" size, the satellite is able to detect the smoke from even modest-sized wildfires, Herman said.

"The smoke plumes cover huge areas, so we see them very, very clearly," he said.

TOMS home page - View the latest data on aerosols from the western wildfires.

SeaWiFs - NASA's ocean-studying SeaWiFs satellite is also recording images of smoke from global fires.

National Fire News - The latest news about US fires, from the National Interagency Fire Center.

The Ups and Downs of Ozone- This recent Science@NASA story reviews some of the realtime data products available at the TOMS web site.