Atmosphere And Weather

How Sleet is Formed

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"How Sleet is Formed"
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Sleet doesn't have many redeeming qualities. Without the excitement of a hailstorm, the wondrous beauty of a snowfall or the welcome relief of a rain shower during a drought, sleet is more of an inconvenience than anything else. The only time you're likely to be troubled by sleet is during winter and the colder months of the year and it often signifies that snow is on the way.

Sleet is not quite rain and not quite snow. It's probably easiest to think of sleet as partially melted snowflakes. Usually by the time sleet hits the ground it has completely melted. However, if it falls long and hard enough and is accompanied by a driving wind, it may start building up into an icy, slushy mess similar to melting snow. While falling, sleet looks like a combination of rain and snowflakes.  

All precipitation, whether it's rain, hail, sleet or snow, is part of a continual water cycle. Water on the earth's surface is heated by the sun, causing it to evaporate whereupon it turns to water vapor and rises upwards.

Once this water vapor reaches the cooler air up in the higher realms of the atmosphere, it condenses while forming tiny droplets around the dust particles in the air. This in turn forms as clouds. If it's cold enough, 0ºC, or 32ºF or below, these droplets form into the six sided ice crystals which we know as snowflakes. And once these flakes become large and heavy enough, they start falling from the clouds as snow.  

Sleet starts its journey as snow. However, if the falling snowflakes pass through a layer of warmer air on their way to the ground, they may start to melt. If the air isn't warm enough to completely defrost the flakes, this partially melted snow falls as sleet. It's also possible for snow to fall, pass through a layer of warm air, turn to rain, then partially refreeze again if it encounters another layer of colder air before reaching the ground.

Sleet is sometimes confused with freezing rain which similarly undergoes a change of state while falling from the clouds. Freezing rain can start off as rain, or snow which may have melted while falling. Either way, freezing rain occurs if it encounters a layer of freezing air close to the ground. As soon as the rain touches the surface, whether it's the road, a car or a tree, it instantly freezes and forms a slippery glaze over everything it lands on.

Sleet, while not posing any significant threat to transport, may cause problems for drivers, especially when driving in the dark. Because sleet consists of partially frozen snow, it reflects in car headlights in the same way as snow, making it difficult to see clearly. And if it’s falling heavily for a continuous period of time, it can cause the ground to become icy.

Because sleet is a mixture of rain and snow, the only time it's likely to be encountered is during cold and wintry conditions. And while we may not give too much thought to how it's formed, it is interesting to consider the various changes in state those drops have undergone on their way down from the sky before beginning their next journey back up to the clouds.

More about this author: Caroline St Clare

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