Weather

Short days, cold weather, snow, ice, and winter storms combine to challenge student pilots and their instructors this time of the year across the northern states, and occasionally in the South and on the West Coast.

Shorter days mean fewer hours in a day for ordinary flying lessons. A winter storm can ground airplanes for days with snow—or a mixture of snow, sleet, and freezing rain. Big storms aren’t needed to reduce prime instructional time at many northern airports, especially those near a large body of water such as the Great Lakes—which help supply moisture for low clouds and poor visibility in fog or snow.

For example, a National Weather Service report of monthly average ceiling and visibility over a 35-year period at the Gary/Chicago International Airport Indiana (GYY) shows that on the average in December the weather met visual flight rules (VFR) requirements only 51 percent of the time. January was almost as bad, with VFR ceiling and visibility 52 percent of the time. June, in contrast, has VFR weather 75 percent of the time.

LAYERS CREATE WINTER PRECIPITATION. The most troublesome winter storms bring combinations of precipitation. For example, a storm might begin with cold rain that gives way to freezing rain, followed by sleet, and finally snow. Such weather is common north of advancing warm fronts in the winter. In the images shown above, blue represents air colder than 32 degrees Fahrenheit, while red represents air warmer than 32 degrees F. (The altitudes in the drawings are illustrative of the relative thickness of the layers of above-freezing and below-freezing air.)

The images show us that snow falling from clouds remains snow when it falls through below-freezing air. When snow falls into a relatively shallow area of warm air, it melts into rain and then becomes supercooled freezing rain. If the layer of cold air near the ground is thick enough, this rain freezes into the tiny pieces of ice known as sleet as it falls. Rain—possibly chilly, but not freezing rain—falls when snow melts as it falls through a relatively thick layer of warm air with no below-freezing air beneath it.

If you imagine hills sticking up into the sleet or freezing rain images, you can see how elevation makes a difference in the kinds of precipitation that can fall on places relatively close together. One important lesson for a pilot thinking of taking off for an instrument flight: If sleet is falling to the ground, you can be sure that freezing rain is above you.

SHORT DAYS CUT FLYING TIME. Since almost all flight instruction occurs during the day, winter’s shorter days mean even less time for instruction—even on clear days. The FAA defines night as the time after the end of “evening civil twilight” until the beginning of “morning civil twilight.” Daytime flying starts with the beginning of morning civil twilight and ends with the end of evening civil twilight.

In Gary, Indiana, for example, on June 15 morning civil twilight begins at 4:41 a.m. and evening civil twilight ends at 9 p.m., for a total of 16 hours, 9 minutes of “daylight” flying. On December 15, in contrast, morning civil twilight begins at 6:38 a.m. and evening civil twilight ends at 4:52 p.m. for a total of 10 hours, 14 minutes of “daylight” time.

One of the few advantages of winter darkness is that an instructor doesn’t have to extend the working day past 10 p.m. to give students the night flying hours needed for a private certificate.

GREAT LAKES GENERATE SNOW. Nature has another way of keeping students and instructors who live around the Great Lakes on the ground in winter: lake-effect snow. Anyone planning to fly toward the Great Lakes in winter should learn about lake-effect snow and make sure none is forecast for the destination.

As with any large body of water, each of the Great Lakes is a weather maker throughout the year because they supply water that evaporates into the air to make clouds and precipitation. The region around the Great Lakes is one of the cloudiest in the United States. Because water warms and cools more slowly than land, the lakes help make winter a little warmer and summer a little cooler than they otherwise would be.

For example, Rochester, New York, and Rochester, Minnesota, are on almost the same latitude line; that is, they are pretty much the same distance from the North Pole. Rochester, Minnesota, with no nearby large body of water, has an average daily low temperature 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 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 lake-effect 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 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.