Lake-effect snow is special stuff
Picking the wrong winter day to fly along the southern or eastern shores of the Great Lakes could teach you more than you want to know about lake-effect snow.
Never heard of lake-effect snow? That's not surprising; it's most common around the Great Lakes, especially on the U.S. sides of the lakes, but is unknown or rare elsewhere. A couple of flight scenarios will give you an idea of what we're talking about.
Imagine you are flying to Buffalo, New York, in December on business and to spend a weekend with friends in nearby Niagara Falls. When you land it's cold, but the sky is clear. A cold front has moved through the area and an Arctic air mass is settling in, promising at least a couple of cold but clear days.
Niagara Falls gets two or three inches of snowfall overnight, but the sky is clear in the morning, and you suggest driving to the airport so you can take your friends on a sightseeing flight.
"Can't do it," they tell you. Ten inches of snow has fallen on the airport overnight and it's still coming down, even though the sky is clear where you are-only 20 miles away.
In this case the worst that happens is you spend the day seeing Niagara Falls by car instead of from the air, while snowplows clear the roads to the airport and the ramp where your airplane is sitting, covered with snow.
Here's another way you could be surprised. You run into clouds as you near Buffalo, but you are instrument-rated and your airplane is well-equipped for flying in clouds. You know which alternate airport you will divert to if conditions at Buffalo drop below your personal minimums.
Soon after flying into clouds that look like the snow clouds you've flown in before, the ride begins to get bumpy, reminding you of cumulus clouds you've flown through. But, you tell yourself, these are snow clouds, not thunderstorms.
You see the first signs that ice could be forming on your aircraft.
Just then the brightest flash you think you've ever seen occurs along with the loudest bang you've ever heard. You're sure lightning has hit your airplane, but everything seems to be OK-except your state of mind. You've never heard of lightning, thunder, and snow occurring together. What's going to happen next?
These scenarios illustrate a few things that make lake-effect snow different from other types of snow.
Lake-effect snow occurs in bands, often maybe 20 miles wide and 10 to 70 miles long, with much lighter snow-or even clear sky-between the bands of heavy snow. But when conditions are right, lake-effect snow can stretch from Lake Erie to southern Pennsylvania, and even into the Washington, D.C., metropolitan area. In addition, some locations are favored targets for lake-effect snow while places only a few miles away don't see nearly as much.
You can see widespread bands of lake-effect snow in a satellite image from December 5, 2000 (facing page).
As it happens, the southern side of Buffalo is in a lake-effect zone that averages 80 to 100 inches of snow a year. Buffalo Niagara International (BUF) averages 90 inches a year. Niagara Falls is in an area that averages 50 to 60 inches of snow a year.
Lightning and thunder occurring with snow surprises most people because it's rare. In the United States only 0.07 percent of weather observations of snow are associated with reports of thunder. When this does happen it's known as thundersnow, and it is more common in lake-effect snow than in other kinds.
The lightning tells you that lake-effect snow clouds have more convection - up-and-down air movements-than ordinary snow clouds. Such convection, in addition to giving airplanes a rough ride, helps to separate positive and negative electric charges on the ice crystals and water drops in a cloud, setting the stage for lightning.
Convection also increases the danger of airframe icing. Air can quickly carry water droplets upward, causing them to cool below 32 degrees but not to freeze-they become supercooled drops that turn into ice when they hit your airplane.
A look at how lake-effect snow forms will help you understand why the snow occurs in bands, why it can be heavy, and why it sometimes falls to the rumble of thunder.
Frigid air blowing over relatively warm bodies of water creates lake-effect snow. 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 temperature of any of the Great Lakes might be at 35 to 40 degrees F, perhaps warmer. But, even in the early winter, Arctic air blowing across the lakes can easily be in the teens or colder.
Wind speeds of 10 to 25 mph help to create snow by allowing the cold air to stay over a lake long enough to pick up moisture. The air has to blow across a relatively long "fetch" of warmer water to pick up enough moisture to create heavy snow.
In the satellite image, you can see bands of clouds lined up with winds blowing from the northwest. If you look at Lake Superior, in the upper left corner of the image, you see bands of snow begin after the cold air has blown across a portion of the lake. They stretch downwind almost to Lake Michigan, where more bands of snow clouds are forming over the middle of the lake.
The clouds form because as the cold air flows over the lake, some of its warm water evaporates into the air, which helps to warm the lowest layer of air. The warm, now-humid air right over the water begins rising and cooling, causing clouds and snow to form. The resulting convection can be strong because the great temperature contrast between the air right above the water and air higher up makes the atmosphere very unstable.
As the wind and the clouds it's carrying with it reach the shore, they encounter more friction than over water and begin slowing down. But, the air from over the lake keeps on coming; the air "piles up" over the land-meteorologists say it converges. This convergence pushes the air higher, which cools it more; thus enhancing the amount of snow being created. If the air runs into hills as it continues inland, it's pushed up farther, making the snow even heavier.
The figure above shows annual average snowfall around Lake Ontario and the eastern end of Lake Erie. The areas south of Buffalo and east of Lake Ontario with the heaviest snow averages are hilly. But, that's only part of the reason that these areas encounter such heavy snow.
When the wind is blowing from the west or west-southwest it has the full length of Lake Erie or Ontario to pick up moisture for snow. In contrast, northwest winds blow across the western end of Lake Ontario into the Niagara Falls area. But, the cold air doesn't blow across enough water to pick up huge amounts of moisture.
While lake-effect snow is most common and heaviest around the Great Lakes, it also occurs near the Great Salt Lake in Utah. Similar "ocean effect" snow can occur on the New England coast, and even near the Chesapeake and Delaware bays. Lake-effect snow has fallen even in Arkansas..
If you are planning to fly to the Great Lakes region in the winter, however, you should pay close attention to the weather forecasts. Familiarize yourself with the characteristics of lake-effect snow, and be able to recognize the condition that could cause it-cold air blowing across warm lakes.
You might want to look at the public weather forecasts, not just those for aviation. National Weather Service offices around the Great Lakes put a lot of effort into understanding and forecasting lake-effect snow. Their messages for the general public are likely to sound a stronger alarm than the symbol for "heavy snow" you would see in a terminal forecast.
Jack Williams is weather editor of usatoday.com. A meteorologist and pilot, he wrote The USAToday Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and is co-author of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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