Jul 26, 2000

A Lot Less Snow




NASA's Terra satellite saw less snow than usual over parts of North America during the winter of 1999-2000.


Terra snow cover map
July 27, 2000 -- This spring if you thought there was less snow than usual in parts of the Midwest and western United States, Terra satellite data agree with you. Early results from Terra's Moderate Resolution Imaging Spectroradiometer (MODIS) clearly observed a lot less snow cover than normal.


"The winter of 1999-2000 brought relatively little snow cover to parts of the North American continent, and the snow melted early as compared to normal years. Low snow cover can result in drier soil conditions, affect crop production, and lead to wildfires," said Dr. Dorothy K. Hall of NASA's Goddard Space Flight Center.

Right: In this image, derived from data collected over an 8-day period between March 5 and 12, 2000, the areas covered by snow are colored white, the non-snow covered land surface is colored green, those regions obstructed by clouds appear as grey, and water is blue. The red line represents the "average" March snow line, and the yellow line represents the "average" February snow line, as determined from NOAA/NESDIS snow maps (1966-present). Note that the snow line in March of 2000 is considerably farther north than the average February or March snow lines. [more information from NASA's Earth Observatory]

Using data from MODIS and other satellites, scientists can determine the extent of spring snow cover which can be a harbinger of flood or drought conditions. Hall presented the results from MODIS at the International Geoscience and Remote Sensing Symposium (IGARSS) yesterday in Hawaii.




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The MODIS composite snowcover map (above), derived from data taken over an 8-day period between March 5 and 12, depicts the snow line into Canada, in the provinces of Saskatchewan and Manitoba. Only scattered snowcover existed over parts of the northern United States, though the mountains were still snow covered. According to the National Oceanic and Atmospheric Administration's National Environmental Satellite, Data, and Information Service (NOAA/NESDIS) the average March snow line would normally extend from New England through the Midwest including southern Wisconsin, to southern portions of North Dakota. The snow line then normally continues farther south in the western states including the Rocky Mountains and west into the Cascades and the Sierras.

NOAA/NESDIS has been producing weekly snow maps of the Northern Hemisphere land surfaces since 1966 using visible-band satellite imagery. Because snow has such a high reflectivity compared to other surfaces on Earth, snow covered areas appear much brighter in satellite imagery than most other surface types.

Terra image of snow cover around lake Tahoe
However, Dr. Hall noted that the key difference between the MODIS-produced snow maps and the images produced by NOAA/NESDIS is that "MODIS has a higher resolution and an improved ability to discriminate between snow and clouds."


Left: A MODIS image of Lake Tahoe surrounded by snow in March, 2000. A gallery of such images is located at the MODIS Snow and Ice Products web site.

Typically, more than 40 percent of the Earth's land surface in the Northern Hemisphere can be covered with snow during the winter months. The highly reflective nature of snow combined with its large surface cover make it an important factor in the Earth's radiation balance, which includes incoming solar energy and energy reflected back into space. Because the Earth is in a steady-state balance of incoming and outgoing energy, its temperature undergoes small change, but the mean temperature stays nearly the same. According to the National Snow and Ice Data Center, snow may reflect up to 80 and 90 percent of incoming solar energy, whereas a surface without snow would only reflect 10-20 percent. Retained solar energy means increased warmth.

Many areas of the world rely on the snowmelt for irrigation and drinking water. In the western U.S, mountain snowpack contributes up to 75 percent of all year-round surface water supplies. Therefore, it is necessary to monitor snowpacks closely throughout the winter and spring for assessment of water supply and flooding potential, and MODIS data will prove useful in this capacity. Dr. Hall said that the lesser snowpack in March hinted at possible drought conditions from the Midwest to the Rockies this summer. However, recent rains have alleviated dry conditions in the Midwest.



As an instrument on NASA's Terra satellite, MODIS continuously observes the Earth's surface in a sweeping motion, every 1-2 days with a scanning imaging radiometer. Its wide field of view (over 2,300 kilometers or over 1,429 miles) provides images of daylight-reflected solar radiation and daytime and nighttime thermal emissions over the entire globe. Sample MODIS imagery is available at: .


MODIS Direct Broadcast image of the Great Lakes

Above: MODIS constantly broadcasts data to receivers on Earth. This is a snippet from a sample image showing the Great Lakes region North America on July 26, 2000. Direct Broadcast data like these are provided by the Code 935 Direct Readout Laboratory at the Goddard Space Flight Center. Learn more about MODIS Direct Broadcast imagery.

Terra was launched on December 18, 1999 and began collecting data on February 24, 2000, part of a 15-year global data set on which to base scientific investigations about the Earth.

Snow and ice products generated from MODIS data will include daily and 8-day composite snow-cover maps, including lake ice on large inland lakes, daily and 8-day composite sea ice-cover maps, and sea ice-surface temperature maps will be produced. There will also be 8-day composite map products available at a high resolution for climate modeling. These products will be archived at the National Snow and Ice Data Center (NSIDC) in Boulder, Colorado.

NASA's MODIS Instrument Science Team expects to release the first snow and sea ice products this fall.

Web Links - Find out what NASA scientists are learning about the 3rd planet from the Sun.

MODIS Direct Broadcast images - See the latest MODIS pictures at this NASA/GSFC web site.

MODIS image archive -- sample pictures of many familiar spots around the globe. Hosted by the National Snow and Ice Data Center.


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