Weather

The last thing most people want to hear from their favorite television meteorologist on a winter evening is a forecast for the dreaded “wintry mix.” Such a forecast means that rain, freezing rain, and snow are expected. It’s dreaded because drivers face a danger of skidding on ice, if traffic is moving at all. Road crews find it harder to handle road ice than snow, which plows can push away.

Pilots have more reasons than drivers to fear a wintry mix because this combination can coat an airplane with potentially deadly ice in the air. To understand the causes of mixed precipitation we need to begin with how rain, snow, freezing rain, and ice pellets form in clouds.

Water vapor needs help. Water vapor in the air forms into clouds and precipitation with the help of particles a fraction of a millimeter in diameter. These include dust, sea salt, ash, and many other natural materials. These are:

• Cloud condensation nuclei—particles upon which water vapor condenses into liquid, which can be colder than 32 degrees Fahrenheit (supercooled).
• Freezing nuclei—particles that will cause a supercooled water drop to freeze at various temperatures. They are much less common than condensation nuclei.
• Ice nuclei—particles that will allow water vapor to deposit directly into ice on the nuclei without first condensing into liquid.

definitions

Freezing Drizzle: Supercooled, slowly falling water drops 0.01 to 0.02 inches in diameter that freeze upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less.

Freezing Rain: Falling supercooled water drops greater than 0.02 inches in diameter that freeze on impact to form a glaze of ice.

Graupel: Small pellets of ice created when supercooled water droplets coat, or rime, snow crystals. The pellets are cloudy or white—not clear like sleet—and often are mistaken for hail.

Ice pellets: Transparent or translucent falling pellets of ice that are round or irregular and have a diameter of 0.2 inch or less. They can be hard grains of ice consisting of frozen raindrops and pellets of snow encased in a thin layer of ice.

Sleet: In the United States the term generally refers to ice pellets (the term the National Weather Service uses). In some parts of the United States and other English-speaking countries, sleet refers to mixed winter precipitation.

Snow flurry: Light snow that falls for short durations. No accumulation or just a light dusting expected.

Snow shower: Snow falling at varying intensities for brief periods of time. Some accumulation is possible.

Snow squall: Intense, but of limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possible lightning.

Supercooled water: Water that is liquid at temperatures below 32 degrees F.

The differences between hail and ice pellets

Unlike ice pellets that form when falling rain freezes, hail forms in thunderstorms as strong updrafts keep icy particles, such as graupel, aloft where supercooled drops freeze onto it to form a growing ball of ice. Hail falls when the updraft weakens or the hailstone grows heavy enough to fall through the updraft. Sleet is a winter phenomenon, while hail occurs mostly during spring and summer since cold-weather thunderstorms are rarely strong enough to create hail. For more information on icing, see "Weather: Don't Take It On the Rocks," Flight Training, November 2010.