Sleet is winter precipitation, but its definition varies by location. In the US, sleet is frozen raindrops that hit the surface as tiny balls of ice. Freezing rain and sleet are dangerous and often occur together, causing power outages and structural damage.
Sleet is a form of winter precipitation, but its exact definition depends on location. In some countries, such as the UK, Canada and Australia, the term refers to snow that has partially melted on its way to the ground. In the US, this is called slush or wet snow. To meteorologists in the United States, sleet is frozen raindrops that hit the surface as tiny balls of ice. These are quite distinct from hailstones, which form differently 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 winter conditions, it can be frozen or partially frozen. Snow is made up of ice crystals which are formed by the condensation of water vapor 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 from water droplets that freeze onto ice crystals, often repeatedly, as strong up and down winds, caused by convection, lift them up and down within the cloud.
In the United States, sleet and freezing rain often occur together or in quick succession. Sleet is small balls of ice that form when partially melted raindrops or snowflakes freeze as they fall through a layer of cold air. Freezing rain is supercooled droplets of water that freeze on contact with the ground or other surfaces. In other countries, sleet means snow that has partially melted by falling through a layer of warmer air.
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Both of these phenomena are observed exclusively in the winter months, as a product of a winter storm system. They are the result of changes in air temperature at different altitudes. Sleet is an indicator of both cold and relatively warm air overhead in a winter storm system.
For example, in the United States, a weather system that dumps snow in the Midwest does so because the air temperature is below freezing, through all layers of the system. This isn’t necessarily the case in the South, where winter temperatures are often slightly above freezing at the surface. There are alternating layers of warm and cold air, so any falling snow or rain has a chance to melt, refreeze, and melt, perhaps multiple times, on its way to the ground.
The south gets snow most often when a cold air mass precedes precipitation by a day or two. When cold air and the storm system arrive at about the same time, freezing rain and sleet are the usual result. Both tend to result from melting snow or partial melting as it falls through a layer of warmer air, then falling through an additional layer of cold air near the ground. Freezing rain tends to occur when the warm layer is quite thick and the cold bottom layer is relatively thin, giving the snow time to completely melt and remain liquid until it hits the ground and freezes again. Sleet occurs when the warm layer is thinner and the cold layer is thicker, so that the falling snow doesn’t completely melt, then freezes again as it falls through the cold layer before reaching the ground.
Dangers
Sleet, in the sense of wet snow, is unpleasant as wet, like rain, and also very cold, but usually does not pose a substantial danger. The ice pellet form, as defined in the United States, is not particularly dangerous per se, but is often associated with freezing rain. In climates where snow isn’t common, like the southeastern United States, lots of frozen precipitation, in any form, is bad news. People can drive on snow, but not on ice. Sleet is almost always accompanied by freezing rain in these types of weather systems, and it sticks to sidewalks and road surfaces like a frozen icing.
Freezing rain and sleet are also the two components of the “ice storm”. Snow can slow things down, but ice is destructive. It coats tree branches and power lines, causing them to bend and snap, 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 indoors if possible. Ice is treacherous and can easily cause falls or other accidents. It is also advisable to have some emergency supplies on hand in case of a power outage.
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