Weather

Pilots can’t be easygoing about ice. After all, even when temperatures on the ground are above freezing, an aircraft a few hundred feet above could fly into clouds with the potential to coat the airplane with dangerous amounts of ice. During winter, the icing dangers found in

Some clouds can descend as precipitation or form as fog that becomes a threat on the ramp or runway. To understand these dangers, you need to know a few things about ice.

WATER DOESN’T ALWAYS FREEZE AT 32 DEGREES. Understanding ice begins with forgetting what you might have been told in grammar 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—such as tiny cloud drops, drizzle, and raindrops—normally cool well below 32 degrees F without turning into ice. In fact, scientists have found tiny cloud drops in high clouds that are colder than minus 40 degrees F, and have created even colder liquid water drops in laboratories. This supercooled water is dangerous because it instantly freezes when it hits something. Ice will form on an airplane when it flies through a cloud of supercooled water drops. When supercooled raindrops fall, they coat the surfaces of trees, power lines, and airplanes on ramps with ice. When this happens, the National Weather Service calls it an ice storm.

THE MANY FORMS OF ICE. Here’s a quick look at the various forms of natural ice you might encounter and the codes used to designate them in aviation routine meteorological reports (METARs).

Frost: Ice crystals that form when water vapor in the air deposits directly onto exposed objects. Deposition means the vapor turns directly into ice without first condensing into water. No METAR code.

Snow: Precipitation of ice crystals, which can be branched dendrites such as the snowflakes you see in photos or drawings; broken pieces of dendrites; or simple, six-sided plates or columns. METAR Code: SN.

Freezing rain: Precipitation of liquid water drops more than 0.02 inches in diameter that freeze upon impact with the ground or other exposed objects. METAR code: FZRA

Freezing drizzle: Fairly uniform precipitation composed exclusively of fine drops with diameters less than 0.02 inch that fall very close together and freeze on impact. METAR code: FZDZ

Ice pellets: Small, translucent balls of ice less than 0.2 inch in diameter (about the size of a pencil point). Ice pellets often bounce when they hit a hard surface. The National Weather Service calls ice pellets “sleet,” and this is the common name used in the United States. METAR code: PL

Freezing fog: A suspension of numerous minute water droplets that freeze upon contact with the ground or other objects, such as an aircraft. Reduces visibility. METAR code: FZFG.

Hail: Balls or irregular lumps of ice (called hailstones) from roughly 0.2 to six inches in diameter. Hail falls from strong thunderstorms. Unlike other kinds of icy precipitation, hail is rare in the winter. METAR code: GR.

Snow pellets: Precipitation of white, opaque grains of ice that are spherical or sometimes cone shaped; also called small hail. METAR code: GS.

Snow grains: Very small white, opaque grains of ice. METAR code: SG.

Precipitation and fog with the word “freezing” in the name are the kind you need to worry about the most, because the drops are supercooled and will freeze when they hit your airplane. You also need to worry about frost. Several years ago researchers found that just over a hundredth of an inch of frost on an airplane’s wings can reduce lift by 25 percent while also increasing drag. Frost on wings, which can be almost invisible, has been blamed for a few takeoff crashes.

SLEET AND FREEZING RAIN. Sleet and freezing rain form when snow falls from cold, high clouds into a layer of above-freezing air and then back into frigid air near the surface. As they fall through the warmer air, snowflakes melt into raindrops. When they reach the below-freezing air close to the ground, these raindrops become supercooled. If the layer of cold air is deep enough, the drops will turn back into ice and become ice pellets (sleet).

The lesson: If ice pellets are falling on an airport you plan to take off from, or are in the METAR for your destination or any en route airport, you need to worry about ice forming on your airplane while aloft. Sleet hitting the ground means supercooled water drops in the air somewhere above the airport. In other words, there is freezing rain aloft, which can coat your airplane with ice.

SNOW: NOT ALWAYS INNOCENT. Light snow, which the National Weather Service defines as falling snow that doesn’t reduce the visibility below a half-mile, might seem harmless. Nevertheless, in the mid-1990s scientists from the National Center for Atmospheric Research (NCAR) studied 10 airline takeoff crashes caused by ice and found light snow was at least partly responsible.

They discovered that light snow, which falls when temperatures are between roughly 25 and 31 degrees F, can have drops of supercooled liquid water stuck to the snow crystals. When the snow and attached supercooled water drops hit the airplane, the water formed ice—just as supercooled drops in a cloud would.

Snow crystals in the cases studied were small, so visibility was better than it would have been with large crystals. Before the accidents, both the flight and ground crews assumed that what looked like a little snow on the wings would blow off early in the takeoff run. Instead of just snow on the wings, however, a thin layer of ice was attached.

One result of the NCAR study was that airports and air carriers began using more sophisticated systems that measure snow’s liquid content, as well as how much snow falls and the visibility. These systems enable airlines to make better decisions about deicing airplanes so that none take off with ice on them, while avoiding the expense of unnecessary deicing.

Since most general aviation pilots don’t have the option of deicing their airplanes before takeoff, ensuring that no ice is stuck to the airplane should be a regular part of all winter preflight inspections. If temperatures have been below freezing, you should brush off any snow that’s on the airplane and carefully check for frost, which might be hard to see on white wings.