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MARCH 1995— VOLUME 2, NO. 3
ARTICLES
The solitude, silence, and purity in a gentle snowfall should be enough to justify its existence. But snow has more than beauty to offer humanity. Winter storms are a good natural laboratory for examining nature's patterns on many scales, from the intricate design of a snowflake to the banded features in a snowstorm that cause accumulations to vary from hour to hour, even from minute to minute.
Water takes on many forms in its solid state (for example, snow, sleet, and graupel), but the most elegant form is the crystalline structure we call a snowflake. Sleet and graupel are formed when liquid water freezes, but the moisture in a classic snowflake goes directly from its gaseous to its frozen state.
The creation of a snowflake requires a pocket of air below 32 degrees Fahrenheit (O degrees Celsius) and so moist that it can hold no more water vapor (i.e., it has reached saturation). If the air is forced upward or otherwise cooled, it must relinquish some of its vapor, typically forming a cloud as moisture condenses onto tiny bits of dust or salt called cloud condensation nuclei. Water droplets also can become supercooled; that is, they remain liquid even below freezing because they lack nuclei on which to freeze.
The crystal nuclei in these icy clouds begin at well under 1 millimeter (0.04 inches) in diameter. They must grow far larger and heavier to reach the ground as snow. How fast they grow and their final appearance depend largely on the coldness of the surrounding air. Part of the reason for different snowflake shapes is the higher saturation vapor pressure for ice than for water. At any given temperature below freezing, the ice-water difference in vapor pressure acts to push moisture away from supercooled droplets and toward snowflakes. The strength of that push and the resulting rapidity in snowflake growth depends largely on temperature. Flakes thus form in various ways depending on whether their growth is leisurely or hurried.
As might be expected, it is the crystal shapes that tend to fall when ground temperatures are at or above freezing. Falling flakes can also clump together at these relatively warm temperatures into gigantic aggregates larger than 5 centimeters (2 inches) during their brief life. Such wet snows tend to be exceptionally dense, with more than an inch of water in every ten inches of snow. In contrast, midwinter snows are more likely to consist of hexagonal needles or plates that form and fall in very cold air. (It' s never actually "too cold to snow," although the very low moisture capacity of extremely cold air makes a big snowfall very unlikely.) The biggest accumulations tend to occur with dendrites, the classic complex crystals that are the fastest growing kind of flake.
Anyone taking a close look at snowflakes can attest that they vary widely in appearance, even though a given set forms at the same temperature, moisture level, and pressure. The old saw that no two snow crystals are identical was disproved in 1988, when National Center for Atmospheric Research scientist Nancy Knight found two that apparently were. The twin crystals were found by accident when Knight was examining samples collected at 6 kilometers (20,000 feet) over Wisconsin for a cloud-climatology study. Thick, hollow, and columnar, the crystals seem to have been Siamese twins that grew attached to each other. No satisfying explanation has yet been found.
Strike Up the Bands
Snow bands are the bane of every weather forecaster, the reason why one side
of a city can get a mere dusting of snow while the other is paralyzed. Winter
storms tend to organize in lines or bands on a scale of kilometers to tens of
kilometers (perhaps 5 to 50 miles). These result from a variety of meteorological
mechanisms, including a process called symmetric instability in which buoyantly
rising regions of air evolve into parallel rows of rising and falling air.
Another reason for snow bands is topography. When a large-scale winter storm pushes winds against an abrupt, high mountain range, such as the Front Range of the Rocky Mountains, a band of snow can form parallel to the mountains. The distance between the band and the mountain range can vary. In northeast Colorado, snow bands often develop over the lower foothills or along the immediate plains-foothills boundary. Sometimes a "barrier jet" of northerly winds near the ground can serve as a block to the upslope winds and effectively extend the Front Range eastward, causing a snow band to develop twenty miles to the east. Wherever it forms, a topographically induced snow band can sit for hours, causing huge accumulations; or it can move, causing rapid variation in snowfall intensity at a given location.
Cities along the south and east edges of the Great Lakes are particularly prone to snow bands in the early winter. Cold blasts from Canada travel over the still-warm lakes, gaining moisture and buoyant energy, and then are forced upward as they reach the gently sloping land. Cities like Buffalo, New York, have received upwards of one meter (three feet) of snow in a single day from these lake-effect snowfalls.
Frontal Assaults
Most snowstorms affecting a large area (say, several states) are related to
the passage of a cold front. In the classic case of an Arctic blast, air that
sits atop frozen ground in the weak sunlight of northern Canada gradually grows
colder and denser.
Eventually, the circulation at the jet-stream level of about 18,000 feet (5.5 kilometers) above sea level begins pushing the air south toward warmer climes. Like a spreading pool of liquid, the sheer density of the cold air propels its travel southward, helping to nose under warmer air and push it up. In turn, the rising warm air will cool; if enough moisture is present, it will condense into clouds and produce snow.
Arctic fronts sometimes hug the Front Range of the Rockies from Alberta, Canada, to New Mexico as they travel south. In the most extreme cases, the shallow surface air can reach Lubbock, Texas, before Minneapolis, Minnesota. In such a scenario, warm, moist air can easily be pulled up from the Gulf of Mexico and above the cold air, setting the stage for heavy snow. However, more than a front is needed. Some upper-level feature, usually a low-pressure center, must provide the vertical lift needed to condense and precipitate huge amounts of water vapor.
The biggest snows in northeast Colorado occur when a "cut-off low" -- an upper-level center of low pressure divorced from the polar jet stream-drifts slowly across southern Colorado. The counter-clockwise circulation around such a system produces a deep layer of easterly wind moving upslope against the Continental Divide. At the same time, spokes of energy traveling around the upper part of the low-pressure system provide periodic enhancement of the snow and often add moisture from the Pacific Ocean, causing large accumulations. Silver Lake, in the mountains northwest of Denver, Colorado, set the nation's 24-hour snowfall record with 195 centimeters (76 inches) on 14-15 April 1921. Before the storm was over, 49 more centimeters (19 inches) fell.
Studies of snow at NCAR have ranged in scale from the microscopic (micrometers) to the synoptic (many thousands of meters). Microscale meteorologists analyze the structure of crystals and determine the conditions needed to produce them. Synoptic meteorologists investigate the large-scale cyclones that travel across continents, leaving snow in their wake. The most active area of NCAR's recent winter-weather research has been in the mesoscale-the zone extending from a few to a few hundred kilometers, a scale which standard weather observations cannot probe in detail. Remote-sensing instruments (radar, satellite, profilers, lidar, and radiometers) all have been used to study the structure of snow bands.
Since 1990, the Winter Icing and Storms Program (WISP) has evaluated the synoptic, mesoscale, and microscale elements of northeast Colorado storms, along with the icing hazard that central U.S. storms present to aircraft. The Stormscale Operational and Research Meteorology program, whose Fronts Experiment Systems Test took place in the winter of 1992, studied the mesoscale aspects of winter weather across the central states and worked toward development of accurate snowfall predictions.
While research works to better define the ways in which snow bands and related phenomena develop, a new network of remote sensors will go into place by the year 2000 to aid the National Weather Service in issuing public forecasts. Computer modelers will improve their models so that the best ones may predict where and when snow bands might form. In time, a local forecast might pinpoint the time when snow will begin and end, and which parts of a county will be the hardest hit. Until then, the beauty of falling snow will be accompanied by the mystery of just how much might fall.
ACTIVITY
On March 12,1993, a low-pressure center now called "The Storm of the Century" struck the eastern seaboard. The intense cyclone, with wind gusts as high as 100 mph and record low pressures, spawned deadly tornadoes in Florida and dumped huge amounts of snow from Gulf Coast states northeastward through New England and into the Canadian Maritime Provinces.
Birmingham, Alabama received 13 inches of snow, setting a daily snow record, a single-storm snowfall record, and even a new total seasonal snowfall record all at once. The eastern seaboard from Florida to Maine received snow; Mt. Mitchell, North Carolina recorded 50 inches, and Washington D.C., New York City, and Boston received nearly a foot of snow.
To get a sense of how much snow fell and the location of snow bands as the storm moved northeastward, plot isolines of snow-depth on a Surface Snow Depth map for March 14,1993.
Materials:
Colored pens or pencils and a Surface Snow Depth map for March 14, 1993. Write
to Project LEARN, P.O. Box 3000, Boulder, Colorado 80307 or e-mail a request
for copies of the maps to chanson@ncar.ucar.edu.
Procedure:
1. Make isolines of snow depth by connecting all stations with 1 to 4 inches
of snow with a colored pen or pencil.
2. Using a different color, connect all areas with 5 to 10 inches of snow.
3. Continue to use a different color for each of the following categories: 11
to 19 inches of snow; 20 to 29 inches of snow; 30 to 39 inches of snow; 40 inches
of snow.
4. Examine your map and determine what geographical feature received the most
snow. (The Appalachian Mountains).
5. Determine which state had the highest total snowfall from the storm (Pennsylvania).
Extensions:
Research the impact of the March 1993 storm. How many people lost power? How
many people died? How was transportation affected? Was the storm predicted by
weathercasters?
RESOURCES
Have you ever wondered why meteorologists tell you that there is a 70% chance of rain and it turns out to be sunny all day? Or how they know a mild snowstorm is coming instead of a dangerous blizzard? How difficult is it to predict a storm? Is the equipment being used outdated and what progress has been made in weather forecasting by the National Weather Service? Find the answers to these questions and more using SIRS Researcher CD-ROM.
Available for PCs and Macs, the database provides thousands of full-text, indexed articles relating to social issues; scientific developments and issues within the disciplines of earth, life, medical, and applied sciences; and global events and issues of historic, economic and political note.
Search the database by using one of three methods: Subject Headings, Topic Browse, and Keyword Search. Many articles are accompanied by grapahics, including charts, maps, diagrams, and illustrations. The users can locate and print or download articles along with corresponding bibliographic citations.
The SIRS Researcher initial subscription price is $1,650. Annual updates are $1,250 per year. Annual subscribers receive three discs per year. Call to request a 60-day, no-obligation preview by contacting SIRS Customer Service at 1-800-232-SIRS or via e-mail at custserv@sirs.com.
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Now
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Corporation for Atmospheric Research
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Editor:
Caroline Hanson
Scientific
Editor:
Pat Kennedy
Contributors:
Bob Henson, NCAR Outreach and Information Staff;
Marcia Politovich, Research Applications Program
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© COPYRIGHT 1995 UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH. ALL RIGHTS RESERVED.
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