Flight Training - December 2012 - 49

Environmental temperature

10,000 ft

Deep warm layer

6,500 ft

Shallow warm layer

6,500 ft

3,000 ft Shallow cold layer Cold layer T<0⁰C Surface 0⁰C T>0⁰C
WHEN SNOW falls through an area of warm air it will sometimes turn to rain. If it drops below freezing again for a shallow period prior to reaching the ground it may turn into freezing rain (above left). That same scenario with a thicker layer of cold air could result in sleet (above right). Snow falling from clouds remains snow when it falls through below-freezing air (left).

3,000 ft Deep cold layer Cold layer T<0⁰C Surface 0⁰C T>0⁰C

Environmental temperature

10,000 ft

Deep cold layer

6,500 ft

3,000 ft

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.

T<0⁰C

sents 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

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 on places relatively close together. One should learn about lake-effect snow important lesson for a pilot thinking of and make sure none is forecast for the taking off for an instrument flight: If sleet destination. is falling to the ground, you can be sure As with any large body of water, each that freezing rain is above you. of the Great Lakes is a weather maker throughout the year because they supply SHORT DAYS CUT FLYING TIME. Since water that evaporates into the air to make almost all flight instruction occurs during clouds and precipitation. The region the day, winter’s shorter days mean even around the Great Lakes is one of the less time for instruction—even on clear cloudiest in the United States. Because days. The FAA defines night as the time water warms and cools more slowly than after the end of “evening civil twilight” land, the lakes help make winter a little until the beginning of “morning civil warmer and summer a little cooler than twilight.” Daytime flying starts with the they otherwise would be. beginning of morning civil twilight and For example, Rochester, New York, ends with the end of evening civil twilight. and Rochester, Minnesota, are on almost In Gary, Indiana, for example, on June the same latitude line; that is, they are 15 morning civil twilight begins at 4:41 pretty much the same distance from a.m. and evening civil twilight ends at the North Pole. Rochester, Minnesota, 9 p.m., for a total of 16 hours, 9 minutes with no nearby large body of water, of “daylight” flying. On December 15, in has an average daily low temperature

Surface 0⁰C T>0⁰C

DECEMBER 2012 FLIGHT TRAINING

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Flight Training - December 2012

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