What We Don’t Know about Snow

This instrument measures microwaves that are naturally emitted from Earth's surface to determine water vapor  and cloud and liquid water in the air column.

The University of Bonn ADMIRARI Radiometer deployed at the CARE site. This instrument measures microwaves that are naturally emitted from Earth's surface to determine water vapor and cloud and liquid water in the air column. Credit: NASA / Walt Petersen

From Ellen Gray NASA Goddard Space Flight Center

Predicting the future is always a tricky business — just watch a TV weather report. Weather forecasts have come a long way, but almost every season there’s a snowstorm that seems to come out of nowhere, or one that’s forecast as ‘the big one’ that turns out to be a total bust.

In the last ten years, scientists have shown that it is possible to detect falling snow and measure surface snowpack information from the vantage point of space. But there remains much that is unknown about the fluffy white stuff.

“We’re still figuring out how to measure snow from space,” says Gail Skofronick-Jackson, a specialist in the remote sensing of snow at NASA’s Goddard Space Fight Center, Greenbelt, Md. “We’re where we were with measuring rain 40 years ago.”

Skofronick-Jackson is part of a team of scientists from NASA and Environment Canada who are running a large experiment in Southern Ontario to improve snow detection. Their GPM Cold-season Precipitation Experiment (GCPEx) supports the new Global Precipitation Measurement (GPM) mission whose Core satellite is scheduled to launch in 2014.

GPM is an international satellite mission that will unify and set new standards for precipitation measurements from space, providing the next-generation observations of rain and snow worldwide every three hours.

As part of their snow detection efforts, the GCPEx science team is collecting as much data as they can to improve understanding of snow dynamics inside clouds, because they relate to how snow moves through Earth’s water and climate cycles.

Accurate snowfall measurements are important for more than just weather forecasts. Snow is one of the primary sources of water in mountainous regions. For example, the snow pack on the Sierra Nevada Mountains accounts for one-third of the water supply for all of California. The snowmelt that enters the water cycle in spring and summer provides both drinking water and irrigation for California’s $37.5 billion dollar agricultural industry. Droughts and climate change are making the snow pack a shrinking water resource, and managers have a greater need than ever to know exactly how much water is locked in snow.

But to know that, scientists first have to know just how much water snow carries as it falls to the ground.

Rain vs. Snow

Figuring out how much water comes down as snow first requires being able to tell snow from rain. On the surface, it’s obvious, rain is liquid and wet, snow is solid and frozen. But rain and snow often happen at the same time or snow can melt into rain.

To tell the difference between rain and snow from space, scientists use an instrument called a microwave radiometer. It works by measuring the microwaves that naturally radiate from Earth all the time. Different natural phenomena radiate at different frequencies. For example rain causes a response at lower frequency microwaves while falling snow affects higher frequency microwave measurements. A radiometer, like a car radio picking up different stations, picks up responses at different frequencies and thus distinguishes between rain and snow. The signal gets stronger for heavy rain or intense snow rates.

This radar scans the air column for snow falling from the clouds to the ground.

The NASA D3R radar deployed at the CARE site. This radar scans the air column for snow falling from the clouds to the ground. Credit: NASA / Walt Petersen

Currently radiometers on some satellites can tell the difference between rain and snow – but only to a point. Complications occur where the frequencies respond to both liquid rain and falling snow or even to the Earth’s surface, says Skofronick-Jackson. “There’s a mixed response in those channels, so you kind of have to know what you’re looking at.”

In those overlapping frequencies, what distinguishes rain from snow is temperature, size and shape, a current unknown in most precipitation detection.

“What we know about rain is that raindrops are spherical or slightly flattened spheres. So they all basically have one shape,” says Skofronick-Jackson. “With snow we have so many different shapes.”

This image of falling snowflakes was taken by the Snow Video Imager at one of the auxiliary ground sites, the Steamshow Fairgrounds

This image of falling snowflakes was taken by the Snow Video Imager (SVI) at one of the auxiliary ground sites, the Steamshow Fairgrounds, 5 miles (8km) south of the main CARE site, during a light snowfall on Saturday, January 21. The SVI is set up about two feet off the ground and the snowflakes are falling from top to bottom through the frame. They can be seen here in different three-dimensional orientations at 5x magnification. Credit: NASA

Needles in a Stack of Snowflakes

Snowflakes come in a wide variety of shapes and sizes. Individual flakes can be long thin needles, hollow columns, or flat plates with millions of different patterns. Their fluffy shapes and sheer variety are what make measuring snow rates tricky.

“Raindrops are going to pretty much fall straight down as fairly dense liquid particles. Snowflakes wobble; they’re blown by the wind. They’re going to have all these different characteristics as to how they fall. And that makes a difference in what the satellite sees,” says Skofronick-Jackson.

The variety of snowflake shapes also complicate estimates of how much water snow holds. A “wet” snow of fluffy flakes has more water per unit volume than “dry” snow. On the ground, the same physical volume of those types of snow contain very different amounts of water, and this water, called snowmelt, is what ultimately ends up in reservoirs, rivers and other sources of freshwater.

The GPM satellite will measure global precipitation, be it heavy tropical rain, moderate rain, light rain or snow. GPM’s radar instrument, built by mission partner, the Japanese Aerospace Exploration Agency, provides essential measurements of the size of the flakes and how much water they hold. The radar works by actively sending out microwaves on two different frequencies. When the microwave pulses encounter a raindrop or snowflake, it reflects part of both pulses back to the radar’s sensors. By timing the interval between when the pulse was sent and then received the radar knows how far away the particles are in the cloud.

Add up all the particles and you get a full picture of all the rain and snow in one weather event. “It’s like a CAT-scan,” says Skofronick-Jackson. “You can actually see layer by layer what’s in the cloud.”

Atmospheric Layer Cake With Marble Swirl Frosting

The atmosphere does not lend itself to easy understanding. Temperature, humidity and winds change with altitude, creating many layers of air with different properties. The topography of land surfaces and oceans affect global weather patterns. Those conditions then combine to create both short-term weather and long-term climate. Part of resolving that picture at high latitudes is to understand snow’s dynamics inside clouds.

“We know most clouds don’t have just one classic snowflake shape, but what we don’t know is what are the mixtures of snowflake types,” says Skofronick-Jackson.

By combining the broader radiometer measurements that distinguish liquid from ice and tell how much water the clouds hold with the vertical details provided by the radar, the GPM science team may be able to see what mixture of particles are falling to the ground, or if a snowflake makes it all the way to the ground at all.

“As they’re falling, snowflakes will sometimes go through warm layers and start to melt and start to look like raindrops,” says Skofronick-Jackson. “So do you call that rain or do you call that snow?”

Finding that line is one of the goals of the cold-season experiment.

How Many Ways Can You Measure a Snowflake?

The GPM GCPEx field mission is currently underway just north of Toronto, Canada in Egbert, Ontario. Located near Lake Huron, the region is prone to both lake effect snow squalls and widespread snowstorms. NASA is working with Environment Canada to measure snow as many ways as possible to match snow on the ground with snow in the clouds and with simulated satellite passes measured from aircraft flying overhead.

Instruments on the ground at the Center for Atmospheric Research Experiments measure the quantity of snow, how fast it falls and how much water it holds. Radar and radiometers on the ground also get an up-close look at the snow as it falls from clouds to the surface. Meanwhile, two research planes, the University of North Dakota’s Citation and the Canadian National Research Council Convair 580, fly though the clouds measuring snowflake sizes and water content, temperature and cloud water. “They’ll do spirals so you can see all the way from the top of the cloud to the bottom of the cloud,” says Skofronick-Jackson.

Above the clouds at 33,000 feet, a third plane, NASA Dryden’s airborne laboratory DC-8, carries NASA Goddard-developed Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) radiometer and NASA’s Jet Propulsion Laboratory-developed Airborne Precipitation Radar-2 (APR-2). Together these two instruments simulate the instruments that the GPM satellite will carry into orbit.

The datasets will complement current measurements made by radiometers on Earth-observing satellites Aqua and Soumi NPP and the Cloud Profiling Radar on CloudSat.

“What we can do with all these measurements is learn these relationships between what the radar and the radiometer sees, what’s in the cloud, and what’s falling out,” says Skofronick-Jackson.

The GCPEx campaign, running from January 17 through February 29, is on its way to filling in that picture.

Related Links:

› GPM website
› GCPEx Campaign blog
› Conversations With Goddard – Gail Skofronick-Jackson

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35 Responses to What We Don’t Know about Snow

  1. Owen in Ga says:

    Wow, someone actually going out to the field and measuring things. How refreshing!

  2. Interstellar Bill says:

    Why are they spending money on snow measurement
    that could go to yet more doomsday climate models?
    According to the alarmists snow will very soon disappear entirely,
    so why bother measuring it?

  3. Anthony Scalzi says:

    “Droughts and climate change are making the snow pack a shrinking water resource, and managers have a greater need than ever to know exactly how much water is locked in snow.”

    Mandatory reference.

  4. Bryan Short says:

    I know how freaking awful we are at forecasting snow. Just 2 weeks ago we had no snow in the forecast. Then it was upgraded to a 20% chance of very light snow. I saw the system moving across North Dakota… much too strong to not bring snow here. Well, sure enough a few hours later they upgraded the chance of snow to 80% with 1 to 2 inches possible. Then they issued a winter storm warning and warned 3 to 7 inches. Then it went up to 6 to 10 inches before a flake had fallen. Then they canceled the winter storm warning and issued a winter weather advisory and brought totals back to 1 to 3 inches, then back to 2 to 4 inches. This all happened in a period of about 5 hours. And now I know why. We got 2 inches of snow here. 10 miles north of me, they got 4 inches. Some areas saw 6 inches. Some areas got only an inch. Snow, more than any other weather phenomenon, is hard to predict. Especially in the eastern U.S. where so many dynamics can make or break a storm.

  5. Leon Brozyna says:

    The GPM GCPEx field mission is currently underway just north of Toronto, Canada in Egbert, Ontario. Located near Lake Huron, the region is prone to both lake effect snow squalls and widespread snowstorms.

    Not so much this year, but perhaps they’ll get more snow to check out during the latter half of February when we may be getting some real winter weather in the region. So far this winter in Buffalo has been amazing … November, December, and now January each had monthly temps of 5°F above average with mostly rain instead of snow. They could always run the experiment again next year when there will probably be more traditional winter weather.

  6. Hector Pascal says:

    They should send an instrument to my town in northern Japan. With a month of snowfall still to go, the snowpack in town is already greater than 2 metres. Over the past 4 nights we’ve had falls of 30, 30, 50 and 30cm. I’m averaging 4 hours per day clearing the property and dumping snow off the roof. My shoulders are mighty sore.

  7. Brian H says:

    Of course, the de rigeur obeisance to consensual physex:
    “Droughts and climate change are making the snow pack a shrinking water resource, ”

    Uh-huh.

  8. Ed Mertin says:

    What about fairly large clumps of snow, within a huge downpour of rain at the same time? About 10 years ago I was heading toward Chattanooga, Tennessee coming down Monteagle and ran into this SLOP! I call it. Had to get onto the shoulder and almost stop. It is a wonder there wasn’t a big crash. You couldn’t see a thing with windshield wipers on high. Have never seen SLOP like that before or since. But I’ll never forget it.

  9. dp says:

    A rather bright fellow in British Columbia has built what he calls an audiolumitron. This is a very simple apparatus you can duplicate. It uses a simple audio amplifier, a solar cell, and precipitation or other objects as targets. I’ve witnessed this in person and it is a very fun but highly refineable scientific tool.

    http://ixian.ca/gallery/thumbnails.php?album=24

    Listen to his recordings, build one.

  10. ISTM that the problem of discriminating between liquid water and snow is one of being able to tell the difference between the various crystaline structures. I admit that my interpretation of the subject is inadequate to justify any great leaps. So this comment is perhaps little more than my grasping at a fleeting thought for long enough to put it into writing.

    My understanding is that liquid water forms (amongst other shapes) concentric spherical crystals each with up to around 80 molecules. Solid water structure (snow/ice) appears to be substantially different because the structure is more vacant – lower density overall. If the sensors can tell the difference between the molecular response to excitation, because every crystaline (and molecular) structure has different characteristic harmonics (resonance), then at least liquid and solid water can be identified.

    I understand that such structures appear to be short-lived (fractions of a second), much to the chagrin of the pseudo-science of homeopathy. Molecules don’t appear to be tied into a specific crystal, even if the conditions favour a particular configuration.This reticence to “hold still” may be because the structure mutates to respond to available energy and the molecular inter-molecular forces diminish sharply once a molecule is “in place”.

  11. observa says:

    Don’t get much snow in the Mediterranean climate of Adelaide, South Australia but I sure don’t like the idea judging by the international news headlines-

    ‘HEAVY snow that has blanketed northern Japan for weeks, triggering avalanches and affecting transport networks, has left at least 55 people dead.
    In one of the country’s coldest winters in recent years, the toll includes more than 40 people who died as they removed snow from roofs or roads, seven crushed by snow falling from buildings and four killed in avalanches.’

    Have you northerners been bunging up too many windmills and solar panels or what? Stick to rain dancing sort of-
    http://www.news.com.au/breaking-news/airlifting-underway-in-flooded-north-nsw/story-e6frfku0-1226260951977

  12. DirkH says:

    Interstellar Bill says:
    February 1, 2012 at 7:12 pm
    “According to the alarmists snow will very soon disappear entirely,
    so why bother measuring it?”

    Oh, that’s an outdated prediction. The new one is that CO2 will lead to more weather extremes, and especially to cold and snow-rich winters. Meet the new CAGW, now with snow. Google Judah Cohen.
    Or amuse yourself by watching a NYT writer trying to get to grips with it. Oops, the writer is Mr. Cohen himself.
    http://www.nytimes.com/2010/12/26/opinion/26cohen.html

  13. Is this why Eskimos have a hundred different words for snow?

  14. Garrett says:

    Why do WUWT articles sometimes look like original articles for the site when they’re not? Did Ellen Gray send Mr. Watts this article without asking him to reference where she originally published it (http://www.nasa.gov/topics/earth/features/know-snow.html) or did Mr. Watts simply copy it from that webpage without linking back to it?
    An very interesting article all the same.

  15. observa says:

    Of course we only started doing rain dances Down Under because of the usual suspect’s predictions-
    http://www.heraldsun.com.au/opinion/it-pays-to-check-out-flannerys-predictions-about-climate-change-says-andrew-bolt/story-e6frfhqf-1226004644818
    You lot up north been making sacrifices to the Snow Gods due to similar predictions from the usual suspects?

  16. Old Goat says:

    Withoutat all wishing to be pedantic, I had to laugh at this in one of the earlier paragraphs:
    “…NASA’s Goddard Space Fight Center…” One imagines all sorts of goings on with that Freudian slip!!

  17. Urederra says:

    The sun is shining outside my place right now, and some snow dust is falling at the same time. The grass is turning white but the snow cannot cake on the roads.

  18. Richards in Vancouver says:

    Charles Gerard Nelson says:
    February 2, 2012 at 1:05 am

    Is this why Eskimos have a hundred different words for snow?

    Charles, this is a scientific blog. Can you cite a reference for your statement?

  19. Kelvin Vaughan says:

    Hail Ceasar!
    What about hail?

  20. wayne Job says:

    I have always considered the weather stick to be the most accurate instrument for all weathers.
    If it is wet it is or has been raining, if it is throwing a good shadow the sun is shining. Set up correctly it can also show the time of day and the date. Graduations on the stick show both the depth of flood and or snow. Some thousands of weather sticks in strategic locations and a few staff could monitor snow depth over large areas. Graduated buckets filled with snow when reading the sticks when melted would give water content. I must be be missing some thing as snow is obviously trickier than I imagined.

  21. oMan says:

    Very informative. It sounds as if these people are trying to do good science, not just getting their instruments calibrated before they ship them into orbit, but doing new and fine-grained characterization of structure and motion of different “water molecular aggregates” in the air column.

  22. Jon Hutto says:

    I hate to be a killjoy, but why is NASA studying any of this? Don’t we have a NOAA for our Atmosphere? How about NASA get back to space exploration? If they are not going to do space exploration, maybe they don’t need much of a budget?

  23. observa says:

    And while you’re all tucked up by the crackling log fire with a hot toddy in hand here’s a wee bit of light entertainment from the not too distant past to really lift the spirits-
    http://www.independent.co.uk/news/science/expect-more-extreme-winters-thanks-to-global-warming-say-scientists-2168418.html

  24. Owen in GA says:

    Wayne: It is hard to drop a bucket in from the satellite then haul it back up to orbit to melt though.

    Jon: NASA tends to be the ones to put the satellites together and manage the sensors thereon and the data therefrom (which is how they got in the climate science business in the first place – confusing sensor quality control with climate analysis.) NOAA is an end user of the data provided from the NASA sensors. Though at this point one could argue for a bureaucratic food fight as both agencies have interest in atmospheric research (eg Air is the second word of NASA).

  25. More Soylent Green! says:

    Richards in Vancouver says:
    February 2, 2012 at 2:33 am
    Charles Gerard Nelson says:
    February 2, 2012 at 1:05 am

    Is this why Eskimos have a hundred different words for snow?

    Charles, this is a scientific blog. Can you cite a reference for your statement?

    I’m curious about how we really no for sure that no two snowflakes are alike, and whether NASA now has the technology to verify that or not.

  26. RCase says:

    I fully understand and appreciate the difficulty in predicting snow accumulations. What bothers me is the incessant promotion by local television stations about all of their superior technology and how much better that makes them at forecast accuracy. A few years ago, I lived within a quarter mile of a local TV station (in a major metro market). One evening, the meteorologist was telling the audience that snow was going to bypass our metro area that night, and not to worry about driving conditions. Meanwhile, it was snowing fairly heavily outside my window and had already accumulated nearly an inch (and ended up getting about 2 more inches on top of that). I called the station and told them that their renowned chief meteorologist might want to stick his head out of the window before going on live TV and making a fool of himself. And that they might want to back off all the Doppler/ACCU-cast/Futurecast/Tru-point crap that they push on the viewing public.

  27. Michael Schaefer says:

    From the article:

    “Droughts and climate change are making the snow pack a shrinking water resource…”

    AAAARRRRRGGGHHHH!

  28. mkelly says:

    Richards in Vancouver says:
    February 2, 2012 at 2:33 am
    Charles Gerard Nelson says:
    February 2, 2012 at 1:05 am

    Is this why Eskimos have a hundred different words for snow?

    Charles, this is a scientific blog. Can you cite a reference for your statement?

    Eskimos have no words for robin. reference Sen. McCain. OOPS. They have several.

  29. Michael Schaefer says:

    Jon Hutto says:
    February 2, 2012 at 5:16 am

    I hate to be a killjoy, but why is NASA studying any of this? Don’t we have a NOAA for our Atmosphere? How about NASA get back to space exploration? If they are not going to do space exploration, maybe they don’t need much of a budget?
    ——————————————————————————————————
    NASA ran out of Space Shuttles last year. So they went for the next-best thing they had in the inventory, to at least do something, anything to justify further funding: An ageing DC-8…

  30. RHS says:

    The other thing which is hard to measure, esp, from space is the moisture content of snow. According to our local TV station (9News) the moisture content can range from 6in snow (nearly slush) to 60 inches (light fluffy stuff for skiing) to an inch of precipitation.
    Granted it is much easier to measure on a sunny day after the snow is done falling.

  31. woodNfish says:

    “Droughts and climate change are making the snow pack a shrinking water resource”

    It is way past time to shut down NASA and begin anew with real science.

  32. M.A.Vukcevic says:

    This is Mediterranean seaside town 1 & 2 today under one foot of snow

  33. Rhys Jaggar says:

    ‘Droughts and climate change are making the snow pack a shrinking water resource, and managers have a greater need than ever to know exactly how much water is locked in snow.’

    I’m not sure I agree with this statement, given that last year the snow pack at Mammoth Mountain was the highest on record (since 1969). Sure, this year is a bust so far, but it’s happened before and will happen again.

    I’d like to see quantitative data on that statement, because the only thing I know about California is that it’s precipitation levels in the mountains vary quite a bit from year to year, between El Nino and La Nina and from Northern California to SoCal.

    I’m sure the research will have benefits, but simply saying: ‘automation of water accumulation within snowpacks using satellites will be extremely cost-effective in the long-term’ would appear to me to be a more honest statement………

  34. polistra says:

    Seems to me that the forecasts are getting worse every year.

    A network of careful observers would be better than any satellite or radar. We actually had such a network in the late 1800′s when every small town had a railroad station with a telegrapher. Now we have stations only at airports, and forecasters who won’t look out of their own windows.

  35. KJKearney says:

    Richards in Vancouver says:
    February 2, 2012 at 2:33 am
    Charles Gerard Nelson says:
    February 2, 2012 at 1:05 am

    Is this why Eskimos have a hundred different words for snow?

    Charles, this is a scientific blog. Can you cite a reference for your statement?

    At most 2 dozen words for snow.

    http://www.uaf.edu/anlc/snow/
    Kaplan, Lawrence. 2003. Inuit Snow Terms: How Many and What Does It Mean? In: Building Capacity in Arctic Societies: Dynamics and shifting perspectives. Proceedings from the 2nd IPSSAS Seminar. Iqaluit, Nunavut, Canada: May 26-June 6, 2003, ed. by François Trudel. Montreal: CIÉRA — Faculté des sciences sociales Université Laval.

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