They'll go on about the storm that covered their driveways with 15 inches of snow in a single day while the office, only 10 miles away had less than a half an inch of snow. In such a case you can only hope the boss is a lake-effect snow veteran who'll believe a story about 15 inches of snow only 10 miles from the office's snow-dusted parking lot.
A pilot making a winter trip to the Great Lakes, however, isn't likely to hear such tales about lake effect snow. He (or she) might fly to a city such as Rochester, New York, or Muskegon, Michigan, on a relatively mild December day and tie down the airplane for the night. The next morning the pilot could awake to find a bitter wind blowing around the 15 inches of snow that's covered the airport's ramp and the airplane, overnight.
That is, if the pilot is lucky, the lake-effect snow will blow in after the airplane is tied down. On the other hand, an instrument-rated pilot might discover that the smooth, stratus clouds he has been flying in have grown as bumpy as a thunderstorm - and ice is forming on the airplane.
Besides the Great Lakes, the Great Salt Lake in Utah can also create lake-effect snow.
Like any large body of water, each of the Great Lakes is a weather maker any time of the year. First, each is a source of water that evaporates into the air to make clouds and precipitation, making the region cloudier, say, than most of the West. Also, because water warms and cools slower than land, the lakes help make winter a little warmer and summer a little cooler than they would otherwise be.
Rochester, New York, and Rochester, Minnesota, share almost the same latitude as well as the same name. In January, Minnesota's Rochester, which has nothing but a few fences between it and the North Pole, has an average daily low temperature of 2?F and an average high of 20?F. New York's Rochester, on the southern shore of Lake Ontario, averages a low of 15?F and a high of 26?F in January.
But the lake's water, which warms the wind blowing in from the Arctic parts of Canada, also adds moisture to the air. This is why the eastern Rochester averages 90 inches of snow in a winter while its Minnesota namesake averages only 46 inches. The 44 extra inches of snow in New York don't come nicely spread out over the winter, or even over the area. Instead, much of it comes from lake-effect snow bands.
Frigid air blowing over the warmer lakes creates lake-effect snow. Researchers and forecasters have found that heavy 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 or warmer while Arctic air blowing in from the north can easily be in the teens or colder.
Wind speeds of 10 to around 25 mph allow the cold air to stay over a lake long enough to pick up moisture. As you can imagine, the wind's direction is a key. Southwest winds blowing the lengths of lakes Erie or Ontario produce heavy lake-effect snow for Buffalo, at the eastern end of Lake Erie, or the Tug Hill Plateau, south of Watertown, New York, at the eastern end of Lake Ontario.
You might think that winds from the southwest would be warm. But, often the southwest winds will be air blown counterclockwise around a low-pressure center to the north over Canada. The air has warmed somewhat 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 snowy.
Cold, dry air blows across the lake, water evaporates into the air. The warmer water vapor, along with heat from the lake warms the bottom layer of air. As the air warms, it rises and the added water vapor begins to condense into cloud droplets. The wind carries the clouds inland, where hills force them to rise, cooling them even more. Heavy snow falls from these clouds along the shore and far inland.
Warming the air from below makes it unstable, just as the sun warming the ground and the air next to the ground on a summer's day makes the air unstable. Unstable air, in turn, leads to up and down air motions - convection - which causes turbulence in the clouds.
Wind blowing across the lakes forms 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 land. The bands of clouds explain why one place can get 15 inches of snow while another only 10 miles away gets only a dusting.
Slight changes in wind direction can affect where heavy snow falls. Forecasters can predict in a general way where and when lake effect snow will occur, but it's difficult to pin down exactly which airports in a region are likely to have near-zero visibility as a snow band moves overhead.
Among the biggest hazards pilots face are the quickly changing ceilings and visibility that lake-effect snow can bring. 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 of a mile in minutes as the snow moves in. The heavy snow with its low ceilings and almost-zero visibility can last for hours.
Lake effect snow clouds contain another hazard - icing. As air quickly rises in the convective clouds water droplets can cool well below freezing, but remain liquid. They become "supercooled." When such supercooled droplets hit something, such as the wings, tail, and windshield of your airplane they turn instantly into ice.
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.
Lake effect snow is usually worse in the early winter because the lakes are warmer and also mostly ice free. Shallow Lake Erie freezes over most winters, which cuts off lake effect snow on its eastern and southern sides. But the other four lakes rarely freeze completely, which means lake effect snow can last all winter. It does tend to ease off, however, as the water cools.
If you plan to fly toward the Great Lakes in winter, study weather maps and forecasts closely. Lake effect snow usually begins a few hours after a cold front has crossed the lakes and cold air is pouring in behind the front.
On satellite photos you'll see the fingers of clouds extending from around the middle of the lake onto the land when lake-effect snow has started or is on the way. Obviously, you should study area forecasts and terminal forecasts for mentions of snow along with low ceilings and visibility.
If you and your airplane don't meet the requirements for flying in instrument-flight conditions, lowering clouds and even light snow should be a signal to turn around and land at the nearest airport in clear air. Having enough fuel on board to allow for detours is an obvious precaution.
Even if you and your airplane are certificated for IFR, you have to be aware of the danger of icing in the clouds. You should be prepared to ask air traffic control to divert you to an airport that hasn't yet been closed by heavy snow as soon as you see any indication that ice is forming on your airplane.
The days after a lake-effect snow storm can be beautiful with the sun making the snow sparkle. Ski areas around the Great Lakes love the lake-effect snow that piles up on their slopes. But getting to a ski resort or anywhere else during a lake-effect snow storm can be hazardous whether you're in a car or an airplane, especially in an airplane. If you're flying when a lake effect snow storm occurs, you can't just pull off the road until a snow plow comes to the rescue.
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