Dorothy Hall, a hydrospheric scientist at NASA's Goddard
Space Flight Center, and a team of scientists from around
the globe plan to map out the snow-covered areas of the Earth
every week. And they won't be using a plane, a bus, a car
or even running shoes to accomplish this feat. Instead, they
will employ the MODIS instrument aboard the Terra satellite
to scan the surface of the Earth every day and sends down
data that Hall can manipulate to determine where snow lies
on the globe.
As can be seen through a prism, sunlight contains all the
colors (wavelengths) of the visible region of the spectrum
from violet to red. When sunlight strikes an object, certain
wavelengths of the spectrum are absorbed and others are reflected.
These reflected wavelengths give an object its color.
White snow reflects nearly all the colors of sunlight in the
visible spectrum. While snow's brightness aids scientists
in spotting it from space, there are still many objects, such
as limestone and white water, that also reflect a broad range
of wavelengths. Often snow is duller than these materials,
because it has been muted by age or is hidden by shadows and
Fortunately, Hall's team has a more precise method for identifying
snow. Snow reflects nearly all the yellow-green light from
the sun, as do many bright objects. Yet, the porous, white
crystals also absorb mid-infrared wavelengths of light (light
to the right of red on the color spectrum). Hall and her team
differentiate snow from other objects by comparing and contrasting
pictures of these two wavelengths coming off the Earth (MODIS
bands 4 and 6). In places where the infrared pictures are
dark and the visible pictures are bright, snow is likely on
Water is one exception in that it absorbs infrared light.
To assure that water and ice are distinguished, Hall will
use MODIS to measure all of the red light (band 1) coming
from an area. Water never appears red because it absorbs most
visible red light. So if a large percentage of red light is
detected above an area, Hall will be able to separate snow
Once Hall has calculated the area of the Earth covered in
snow, the depth of the snow can be determined using microwave
satellite instruments such as Advanced Microwave Scanning
Radiometer (AMSR) aboard EOS PM-1 (scheduled to launch in
2000). Together, these measurements will allow scientists
to track changes in the volume of snow across our planet.
The increase or decrease in snow is an indicator of global
change. If researchers find that the snow is melting earlier
and earlier each year, this may indicate the Earth is warming
up. If the snow increases year after year, our Earth may be
Blankets of snow partially protect the Earth from global warming
as well. The powdery crystals reflects more sunlight per inch
than both solid earth and water. So the more snow there is,
the less sunlight is absorbed and re-radiated as heat to contribute
to global warming. Snow also insulates the ground and prevents
its heat from escaping into the atmosphere on cold winter
Many years of observation must be made before anyone can demonstrate
that our planet's surface is warming. In the immediate future,
the work of Hall's team may prove to be useful for laypersons.
Monitoring snow aids in the prediction of avalanches and floods.
Mapping snow-covered lake ice, another application of this
data product, may assist pilots in navigating large inland
lakes during the winter.