Weather: Dreaded wintry mix

We can be sure that this “dreaded wintry mix”—the description broadcast meteorologists often use for such storms—also caused general aviation pilots in the region to put off any plans to go flying.

“Mix” refers to the fact that winter storms often bring not only snow, but also sleet, freezing rain, and ordinary rain. A storm might begin with ordinary rain and then switch to freezing rain, then sleet, and finally snow. A storm might skip one or more of these kinds of precipitation, or reverse the order in which the different kinds of precipitation arrive.

Such a mix is dreaded because it affects almost everyone in a region. An ice storm can also cause widespread power outages and stop traffic on slick roads.

Location in a particular region can also make a difference in what kind of precipitation falls. Snow might be falling on higher elevations while nearby lower places are getting ordinary rain, freezing rain, or sleet.

FORGET WHAT YOU KNOW ABOUT ICE. Understanding ice begins with forgetting what you might have been told in school about water turning into ice when its temperature drops below 32 degrees Fahrenheit. This temperature is really the one at which ice begins to melt.

Small amounts of water generally cool well below 32 degrees F without turning into ice. In fact, scientists have found tiny liquid cloud drops in high clouds that are colder than minus 40 degrees F.

Supercooled water is dangerous because it instantly freezes when it hits something. Ice forms on an airplane when it flies through a cloud of supercooled water drops. When supercooled raindrops fall, they coat trees, power lines, and airplanes on ramps with ice. When this happens the National Weather Service calls it an ice storm.

WINTER’S BIGGEST DANGER. An ice storm, by definition, is a storm that brings accumulation of at least 0.25 of an inch of ice on exposed surfaces. In addition to making driving and walking difficult, an ice storm can bring down trees to block roads, and pull down powerlines.

A quarter-inch of ice might not sound like much until you try to walk or drive on it, or by far the most dangerous thing to try: take off in an aircraft covered by a quarter inch of ice. (In case you’re wondering why hail, which is a form of ice that falls from the sky, isn’t listed, it’s because hail is produced by strong thunderstorms—which are extremely rare when surface temperatures are below freezing.)

TEMPERATURE LAYERS CREATE A WINTRY MIX. The images on this page illustrate the patterns of temperature a few thousand feet above the surface that determine whether snow, sleet, freezing rain, or ordinary rain reach the surface.

In the images, blue represents air colder than 32 degrees F, 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, which can be quite different in different storms or over time in a single storm. The thickness of any layer can vary widely, and all the layers might not be present in a winter storm.

Altitudes in the images are in kilometers (km) above the surface. One km is 3,281 feet; two km is 6,562 feet; three km is 9,843 feet.

Snow falling from clouds remains snow when it falls through below-freezing air all the way to the surface. When snow falls into a relatively shallow area of warm air, it melts into rain and then becomes supercooled freezing rain when it falls through below-freezing air below a layer of warmer air.

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.

If the layer of cold air is not thick enough for sleet to form, the rain drops below 32 degrees F but does not turn into ice until it hits something; it’s freezing rain. Rain—possibly chilly but not freezing rain—falls when snow melts as it falls through a relatively thick layer of warm air with little or no below-freezing air below it.

If you imagine hills sticking up into the sleet or freezing-rain portions of the 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 into clouds or precipitation on an instrument flight: If sleet is falling to the ground, you can be sure that freezing rain is above you, and it could quickly coat your airplane with ice.

FIRST WINTER ALERTS. If you live anywhere that might see snow or freezing rain, you should keep an eye on the different kinds of alerts that the U.S. National Weather Service issues. Do you really want to wait until you’re at the airport or ready to leave for the airport to call for a preflight briefing, only to learn that the air you plan to fly through is going to be full of snow, freezing rain, or sleet?

You can keep an eye on weather alerts by signing up for text message or email alerts sent to your cell phone or computer from the NWS or one of the several private companies.

NWS offices issue a winter weather advisory when heavy snow or a “significant” ice accumulation is expected. A watch or advisory could be issued two days before the dangerous weather is expected, or maybe 12 hours before it begins.

The NWS issues warnings for dangerous winter weather that is anticipated to begin in a few hours or sooner.

Fortunately, the large storms that cause winter weather generally are well forecast two, three, or more days in advance. That is, you can expect to hear about where and when dangerous winter weather will occur in plenty of time to call off a planned flight. This is in contrast to thunderstorms. Forecasters can give the odds of the general area that is likely to have severe thunderstorms two or three days ahead. But they can’t forecast exactly where and when you’re likely to encounter a thunderstorm until the storms are actually forming.

Criteria for winter weather watches, advisories, and warnings differ from place to place. A forecast for, say, a couple of inches of snow and light ice would more or less be considered just another winter day in Buffalo, New York, while the same forecast for Atlanta would prompt advisories, watches, and warnings.