The Weather Never Sleeps

How You Can Tell The Difference

Many people, including some pilots, are confused by the differences between sleet and hail. Both are ice that falls from the sky, but the conditions that create them - and the dangers they present to pilots - are very different.

Make a comment like, "Yesterday's snow and hail made it the worst February day we've had in years," and you're likely to hear a lecture from your neighborhood weather geek. A pilot who says something like this or comments on the sleet from a May thunderstorm probably shouldn't be allowed back into the air until after sitting through the weather geek's lecture.

An understanding of what causes the various kinds of freezing or frozen precipitation is needed to make sound aeronautical decisions about weather.

Let's start with snow and sleet.

The details of how snow forms can get very complicated, but the basic mechanism is water vapor in the air that sublimates - that is, it goes directly from vapor to ice - to create ice crystals.

Snow crystals, by the way, don't all look like the six-sided "snowflakes" with elaborate designs that you see in illustrations. Some are needles of ice; some are six-sided flat plates; and some are just tiny blobs of ice. The temperature at which they form determines the shapes of snow crystals. Snow crystals that stick together can form huge "snowflakes."

Sleet is not snow. In the United States, the term sleet is used to describe frozen raindrops. You know that the raindrops are frozen because they bounce when they hit the ground or your car or airplane. In other parts of the world, and sometimes in the United States, sleet can refer to a mix of rain and frozen raindrops or other kinds of mixtures such as rain and snow or even rain and hail.

To make it even more complicated, you will never see the word sleet in a National Weather Service report of current weather or in a forecast. When a television weather person says the local airport is reporting "sleet," he or she is referring to an observation of "ice pellets," or "PL" in the METAR code.

The National Weather Service defines ice pellets as: "Precipitation of transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch (5 mm), or less."

Obviously ice pellets are pretty small, about the size of the tip of a pencil, which is as large as raindrops get before they break into smaller drops.

The important thing for pilots to know about sleet, or ice pellets, is that when they are hitting the ground there is freezing rain aloft. Freezing rain can coat an airplane with deadly ice because even though the raindrops are colder than 32 degrees Fahrenheit (0 degrees Celsius) they are still liquid, but they turn into ice as soon as they hit anything.

Ice pellets and freezing rain are created by a layer of warm air above a layer of air that's colder than 32 degrees F, which means you are going to see sleet during the winter, not on a warm spring, summer, or fall day. Since sleet and freezing rain occur during the winter, the precipitation usually begins as snow falling from a higher level of cold air. As the snow falls into the warmer air it melts to become raindrops.

One of the most common places for this to happen is ahead of an advancing warm front at the surface.

The thickness of the layers of warm and cold air - and how warm and cold they are - determines what happens next. If the warm layer is warm and thick enough to melt the snow into raindrops, and the cold layer is cold enough and thick enough for the raindrops to completely freeze, ice pellets hit the ground.

A thicker or warmer warm layer and thinner or warmer freezing layer means that the raindrops don't turn to ice, but will cool to below 32 degrees F before hitting the ground. In this case trees, roads, sidewalks, and parked cars and airplanes are covered with a glaze of ice. This is an ice storm.