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Design Concept



MODIS Data Product Non-Technical Description - MOD 10 & MOD 33

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 trees.

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 the ground.

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 and water.

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 cooling.

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 days.

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.


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