Flight Training - December 2012 - 50

WEATHER»

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of 8 degrees F and an average high of 24 degrees F in January and an average winter snowfall of 49 inches. For the same month, New York’s Rochester, which is on the southern shore of Lake Ontario, averages a low of 18 degrees F and a high of 35 degrees F. That water, which helps warm the city, also adds enough moisture to the air to give Rochester, New York, a winter average of 92 inches of snow.
HOW THE LAKES MAKE SNOW. As frigid

DEFINING WINTER WEATHER
BLIZZARD: Winds more than 35 mph with snow and blowing snow, reducing visibility to one-quarter mile or less for at least three hours. BLOWING SNOW: Wind-driven snow that reduces visibility and causes significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind. FREEZING RAIN: Supercooled rain drops that freeze when they hit something. SLEET: Frozen raindrops that bounce when they hit. SNOW FLURRIES: Light snow falling for short durations. No accumulation or a light dusting is all that is expected. SNOW SHOWERS: Snow falling at varying intensities for brief periods of time. Some accumulation is possible. SNOW SQUALLS: Brief, intense snow showers accompanied by strong, gusty winds. Accumulation may be significant. THUNDERSNOW: Rare snow that falls from a thunderstorm, often as snow squalls.

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air blows across the much warmer lake, water evaporates into the air to create lake-effect snow as the air blows inland and over hills. Lake-effect snow is most likely when the air from the surface up to about 5,000 feet is more than 20 degrees colder than the water. Early in the winter, the water of any of the Great Lakes might be 35 to 40 degrees F, while Arctic air blowing in from the north can easily be in the teens or colder. Wind direction determines how much lake-effect snow falls. For example, southwest winds blowing the lengths of lakes Erie or Ontario produce heavy lakeeffect snow for Buffalo and other areas at the eastern end of Lake Erie—or the Tug Hill Plateau, south of Watertown, New York, at the eastern end of Lake Ontario. By the way, southwest winds blowing across Lake Erie aren’t warm breezes from Arizona. In winter these winds are blowing counterclockwise around a low-pressure center to the north over Canada. The air warms a little as it swings around over Ohio, but it can still be more than 20 degrees colder than the water of lakes Erie and Ontario. West-northwest winds blowing at angles across the two lakes dump snow on the southern shores. Northwest winds across Lake Michigan make western Michigan especially snowy.
BANDS OF THICK CLOUDS. Lake-effect snow falls from bands of thick clouds that might be 10 or 20 miles wide, with thinner clouds or sometimes even clear air between them. On satellite photos you see these bands of clouds pointing like fingers from over the lake onto the

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land. The bands explain why one place can get 15 inches of snow while another 10 miles away gets only a dusting. Lake-effect clouds are especially notorious for icing on the western slopes of the Appalachian Mountains, where the winds push the clouds upward. Air-mail pilots of the 1920s dreaded flying over these mountains in the winter because icing brought so many of them down. Quick changes in ceiling and visibility are among the dangers of lake-effect snow. An airport where the bottom layer of clouds is only 2,000 feet above the runway—the ceiling—with a visibility of 10 miles can change in minutes to snow that’s so heavy you can’t tell where the bottom of the cloud is. Visibility can drop from 10 miles to a quarter-mile in minutes as the snow moves in.
Jack Williams is an instrument-rated private pilot. His latest book is The AMS Weather Book: The Ultimate Guide to America’s Weather.

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Flight Training - December 2012

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