Sleet is a form of wintry precipitation, but its exact definition depends on location. In some countries — for example, the United Kingdom, Canada and Australia — the term refers to snow that has partially melted on its way to the ground. In the United States, this is called slush or wet snow. To meteorologists in the USA, sleet consists of frozen raindrops that hit the surface as little ice pellets. These are quite distinct from hailstones, which form in a different way, and can occur at any time of year.
Types of Precipitation
The term precipitation refers to water in any form that falls from the sky. In wintry conditions, it may be frozen, or partly frozen. Snow consists of ice crystals that form by water vapor condensing directly into solid form, without first forming water. Hail is generally more common in summer, and forms in huge cumulonimbus clouds that often produce thunderstorms. Hailstones form by water droplets freezing onto ice crystals, often repeatedly, as strong, upward and downward winds, caused by convection, lift them up and down within the cloud.
In the USA, sleet and freezing rain often occur together, or in quick succession. Sleet consists of small pellets of ice that form by the freezing of raindrops or partially melted snowflakes as they fall through a layer of cold air. Freezing rain is made up of supercooled water droplets that freeze on contact with the ground, or other surfaces. In other countries, sleet means snow that has partly melted by falling through a layer of warmer air.
Formation
Both these phenomena are seen exclusively in the winter months, as a product of a winter storm system. They result from variations in air temperature at different altitudes. Sleet is an indicator of both cold and relatively warm air aloft in a winter storm system.
For example, in the USA, a weather system that dumps snow in the Midwest does so because the air temperature is below freezing, through all the layers of the system. This is not necessarily the case in the south, where winter temperatures are often a little above freezing point at the surface. There are alternate layers of warm and cold air, so any snow or rain that falls has a chance to melt, re-freeze and melt, possibly several times, on its way to the ground.
The south most often receives snow when a cold air mass precedes the precipitation by a day or two. When the cold air and the storm system arrive at about the same time, freezing rain and sleet are the usual result. Both tend to arise from snow melting or partially melting as it falls through a layer of warmer air, and then falling through a further layer of cold air near the ground. Freezing rain tends to occur when the warm layer is quite thick, and the lower, cold layer is relatively thin, giving the snow time to melt completely, and stay liquid until it hits the ground and freezes again. Sleet happens when the warm layer is thinner, and the cold layer thicker, so that the falling snow does not melt completely, then freezes again as it falls through the cold layer before reaching the ground.
Hazards
Sleet, in the sense of wet snow, is unpleasant as it is wet, like rain, and also very cold, but does not usually pose a substantial hazard. The ice pellet form, as defined in the USA, is not especially hazardous in itself, but it is often associated with freezing rain. In climates where snow is not common, such as the Southeastern United States, a lot of frozen precipitation, in any form, is bad news. People can drive through snow, but not on ice. Sleet is nearly always accompanied by freezing rain in these kinds of weather systems and sticks to pavements and road surfaces as an icy glaze.
Freezing rain and sleet are also the two components of the "ice storm." Snow may slow things down, but ice is destructive. It coats tree limbs and power lines, making them sag and break, due to the weight. Ice causes widespread power outages and structural damage from falling tree limbs. The best way to deal with these conditions is to stay inside if possible. Ice is treacherous and can easily cause falls or other accidents. It is also advisable to have some emergency supplies available in case the power goes out.
By Amy Pollick