Fires in Alaska

Fires Across Alaska

25 August 2005

In the third week of August 2005, an area of high atmospheric pressure built up over Alaska. Large areas of high pressure often lead to calm weather, with light (or absent) surface winds. Unfortunately for Alaska residents, the high pressure system that parked over the state coincided with a period of significant fire activity, with more than a hundred forest fires churning out thick smoke. For several days the smoke piled up over the Interior leading to hazardous air quality warnings for many areas.

August 2005

Large images:

This pair of images from the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite shows smoke measurements over Alaska and western Canada on 15 August (top) and 21 August (bottom). (The background for the image is NASA’s Blue Marble.) Increasing amounts of smoke are shown as an aerosol index with shades of blue (little or no smoke) to dull red (thick smoke). On 15 August, a large mass of smoke had drifted westward over the Interior and spread out over the Bering Sea toward Russia. Less than a week later, the weather patterns shifted and the smoke blew to the east and north, over Yukon Territory in western Canada and over Victoria Island toward the Arctic Ocean.

Smoke contains many substances, including carbon dioxide, carbon monoxide, water vapor, and particulate matter. OMI measures smoke by tracking black carbon particles, or soot, that absorb ultraviolet (UV) radiation, the wavelengths of sunlight that cause sunburns. By measuring how much UV radiation the soot absorbs, OMI provides estimates of the amount of black carbon aerosol in the smoke layer. This method of detecting aerosols based on their interaction with UV rather than visible (rainbow) light allows OMI to measure absorption by black carbon in smoke even if the smoke is mixed with or floating above clouds. Measurements of how much radiation aerosols absorb are important for scientists trying to calculate the net effect of aerosols on Earth’s energy budget and climate.

(Source: Earth Observatory)

Forest Fire Smoke Surrounding Mt. McKinley: 

This view of Mt McKinley (Denali)—the highest point in North America (6,194 meters; 20,230 feet)—looks as if it were taken from an aircraft. In fact, an astronaut onboard the International Space Station took advantage of cloud-free skies and a powerful 800-millimeter lens to photograph this peak while the spacecraft was over the Gulf of Alaska, 800 miles to the south of the mountain. The powerful lenses are difficult to use, requiring motion compensation by the astronaut, so these kinds of detailed images of horizon detail are seldom taken. The rising sun casts long shadows across the Kahiltna Glacier that angles down from Denali (left).

click on image to enlarge

In addition to the blueness inherent in all images taken at great distance (the atmosphere scatters blue light more than it does other colors), this image also shows unusually dense atmospheric haze at lower altitudes: all the valleys in the foreground appear murky. The explanation is dramatically portrayed in a Moderate Resolution Imaging Spectroradiometer (MODIS) image taken on the same day, Sunday, 14 August , from the Terra satellite (see below). On that day, an enormous smoke pall hung over central Alaska; all the major mountain ranges protruded above the smoke layer, which was held close to the surface by high atmospheric pressure.

click on image to enlarge

The smoke came from more than 100 forest fires burning in the summer heat of Alaska. The MODIS image shows that the smoke on 14 August was far thicker to the north of the Alaska Range where Denali is. The Space Station image shows this denser smoke settled between the Alaska Range and the distant horizon of the Kuskokwim Mountains, 80 miles to the north.

(Source: Earth Observatory)

For background information on the Fire Situation in Alaska see:

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