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VO: What we think of snow depends a lot on where it falls.

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If you live in the eastern U.S., maybe it's fun.

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Or, maybe it's just a pain.

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Kim: But if you live in parts of the world where they

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depend on the water that's in the snow for a large

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fraction of their total water that they use for drinking, agriculture,

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for industry or for hydropower, the snow is

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a very important natural resource. For example, in the western part of the United States,

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80 to 90 percent of their renewable water comes from snow.

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VO: Snow is one part of the cryosphere that many of us

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have actually encountered before. But it also plays a critical role in

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regulating the Earth's climate. Through decades of remote sensing,

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NASA has kept a close eye on the ebb and flow of snow cover.

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Hall: We now have a 52-year record of

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snow cover in the Northern Hemisphere, and we can see

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changes in the extent of snow cover over the

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time period--particularly in the last few decades--where we can see that the snow

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cover has been retreating. It's been melting a lot earlier in the springtime.

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Osmanoglu: The extent is relatively easy to do and it has been done over the

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years. What's tricky is though how thick is that snow. And

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it's even trickier how much water is in that snow.

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VO: That tricky part is known as the snow water equivalent, or how much water would actually

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be in a layer of snow if it melted. NASA and its partners have taken

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to the air to help solve this elusive mystery. First there's the

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Airborne Snow Observatory, or ASO, a small

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plane outfitted with a couple of instruments, one of which measures snow depth using

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lidar. Lidar measures distance using light from lasers. Since

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2014. ASO has flown over basins in California and

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Colorado, taking before and after looks at snow depth.

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Scientists subtract the snow-free summer data from the snow-on winter

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data to get an idea of the snow depth. There's no single way to

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measure all types of snow across the globe, and so NASA's other airborne

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campaign, SnowEx, is testing different combinations of sensors.

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This winter, SnowEx will test a new instrument, the Snow

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Water Equivalent Synthetic Aperture Radar and Radiometer, or SWESARR.

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Osmanoglu: SWESARR consists of two main components,

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one of them being the radar and the second one being the radiometer.

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Bonds: So with the radar providing the depth of the snow and the radiometer

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providing the density of the snow, we can put those two things together and get the

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snow water equivalent. Here in the chamber we're going to measure different radiation

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patterns that are different frequencies and do some full system testing,

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in this chamber. This chamber kind of enables us to isolate

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various types of radiation and interference. In about a month,

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we're going to take the instrument and mount it on a Twin Otter in the Grand Mesa in Colorado.

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We're going to fly it over the Grand Mesa and take various different measurements.

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This is what we call our engineering flight.
VO: Making sure the sensors are calibrated

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is key in order to face the challenges nature will throw at them.

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Kim: Half of the area that gets covered by snow every winter contains trees

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and forest. And the trees make it difficult for the sensors to

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see the snow that's underneath the trees so it makes it difficult for us to measure

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how much snow there is. After the snow has had a chance to sit on the

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ground for a while, it gets denser and denser and denser over time and it changes.

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Which is another reason why snow is very challenging to remotely sense,

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it doesn't stay the same. It's constantly changing.
[shovel digging]

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Kim: One of the things that we often do in the field is go dig what we call

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a snow pit. You literally dig a pit in the snow so we can see all the different

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layers. The layering is very important.
VO: All this digging is part of ground

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truthing SnowEx, a way of matching up what the airborne instruments see and what

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is actually sitting on the surface. The ultimate

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goal of SnowEx is to figure out what the best combination of instruments

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would be for a future satellite mission in order to get a global picture

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of snow.
Hall: We need to know how much snow is in a

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snowpack. Because if we have too much snow and the snow melts

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too fast, then you can get flooding. And if you don't

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have enough snow or if the snow melts too early, that can lead

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to a longer wildfire season, a more

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intense drought, and we need to know these things for water resource

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planning.
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Parkinson: After we

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had a record that was about 15, 20 years long,

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we started noticing that the extent of

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the ice in the Arctic was getting smaller over time.

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