Weather

As winter takes hold across the colder parts of the country, pilots begin wishing for relief from frigid preflight inspections and continuing concerns about ice forming on their airplanes.

When a meteorologist says that a warm front is headed for your part of the country, your first reaction might be to celebrate the expected relief from continuing chill. More often than not, however, an approaching warm front can bring some of winter’s worst flying weather in the form of widespread ice storms.

The National Weather Service defines an ice storm as “damaging accumulations of ice from freezing rain that can pull down trees and utility lines, resulting in loss of power and communication. These accumulations of ice make walking and driving extremely dangerous.”

You don’t need much imagination to envision what an ice storm’s freezing rain can do to airplanes—in the air or on the ground.

What’s happening aloft. The illustration (right) provides a simplified look at what was happening at the surface and aloft over the Southeast the morning of December 15, 2005, when a major ice storm hit the region.

The map at the upper left of the illustration shows what you would see on an ordinary surface map for the morning: a warm front across the Gulf of Mexico, crossing Florida, and running northeast along the East Coast.

The rest of the illustration is a three-dimensional view of warm air sliding over the denser cold air at the surface, creating a cold/warm/cold air sandwich over the region. It shows what was happening aloft and at the surface along a line from Panama City, Florida, to Athens, Georgia, to Hickory, North Carolina, and on to Roanoke, Virginia. The front’s surface location is seen on the left, where the wedge of warm air is on the ground north of Panama City.

In this type of scenario, snow forms in the top layer of freezing air and falls into the warm middle-level air, where it begins melting. If the layer of warm air is deep enough, the snow melts into rain. If the warm layer isn’t deep enough, the raindrops cool below 32 degrees Fahrenheit, but they don’t turn into ice right away; we say the water is supercooled.

If an airplane flies into supercooled rain, drizzle, or clouds, the water instantly turns to ice when it hits any part of the airplane. Such structural icing disrupts airflow over the wings and control surfaces. It’s one of aviation’s biggest dangers. If the bottom layer of below-freezing air is thick enough, the supercooled raindrops will freeze into ice pellets, which are also called sleet. If ice pellets are falling, don’t even think about taking off—if you do, you’ll encounter freezing rain aloft.

A weather balloon that launched from Blacksburg, Virginia, at 7 a.m. that day found a cold/warm/cold air sandwich aloft, along with enough water vapor to create substantial precipitation. Surface temperature there was 30 degrees F. Below-freezing temperatures continued up to 3,860 feet, where the air was between 33 degrees and 38 degrees F. At 7,000 feet temperatures were below freezing and grew colder as the balloon ascended.

Such a layer of relatively warm air aloft is called an inversion. If freezing or near-freezing temperatures are below the inversion and the air is humid enough for precipitation to form—as it was on December 15, 2005—an inversion is also called an elevated warm layer or a melting layer. No matter what they call it, meteorologists know it’s a sign of trouble because it can create freezing rain, both aloft and at the surface.

Around noon on December 15, this was the weather at each of these cities:

Panama City: 68 degrees F, cloudy, no precipitation.

Athens: 34 degrees, rain.

Hickory: 32 degrees, freezing rain.

Roanoke: 27 degrees, ice pellets (sleet).

Warm air was riding over cold air at the surface, and the layer of cold air at the surface became thicker toward the Northeast. By Roanoke, the warm air would be 3,500 or so feet above the surface. At Athens, snow could have been falling from high clouds but it melted into raindrops in the relatively thick layer of warm air.

To the north, over Hickory, snow melted as it fell into the layer of warm air aloft, but began cooling as it fell through a thicker layer of cold air. The drops cooled below 32 degrees F and instantly froze when they hit trees, power lines, roads, and runways.

Farther north, over Roanoke, the layer of warm air was thinner and the layer of cold air thicker. Snow falling into the warm air began melting, but not completely. As the partly melted crystals fell through the cold air, they froze into ice pellets.

Quick changes. Weather like that shown in the illustration is constantly changing. For example, at Roanoke, light freezing rain fell from 5:35 a.m. until 6:23 a.m. Snow began mixing with the freezing rain, and then turned to all snow two minutes later. That snow continued until 8:56 a.m. when freezing rain began mixing with it. Ice pellets fell from 10:54 a.m. through 2:01 p.m., when freezing rain resumed and continued until 7:54 p.m. when the precipitation ended.

During that day as much as three-quarters of an inch of ice accumulated on parts of northern Georgia and South Carolina, western North Carolina, and southwestern Virginia. The ice pulled down tree limbs and power lines, causing almost 700,000 customers to lose power and numerous accidents on icy roads. Airlines and general aviation were grounded throughout a large part of the region, including at busy Hartsfield-Jackson Atlanta International Airport.

The same day, another storm was moving across the northern United States, where the air was below freezing from the surface. While snow caused some delays, most roads stayed open and instrument-rated pilots in properly equipped airplanes safely completed their flights.

The AMS Weather Book by Jack Williams, used with permission of the American Meteorological Society.