A Lot Less
Snow
NASA
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NASA's Terra satellite saw less
snow than usual over parts of North America during the
winter of 1999-2000.
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.
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. 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: http://nsidc.org/NASA/MODIS/
.
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.
Related Links
Earthobservatory.nasa.gov
- 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.
