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Caroline Johnson, age 11, of Wichita, Kansas, for her question:

How is sleet caused?

Meteorology is one of our most neglected sciences. This explains why so many ordinary everyday weather events cannot be explained in exact detail. It also explains why we still are at the mercy of droughts and deluges, floods and hurricanes plus an endless series of other unexpected disasters. Students who dream of benefiting the world might prepare themselves for research in meteorology.

First let's clarify what we mean by sleet and what the meteorologists say it is. There is a difference. Most ordinary folk assume that sleet is a soggy shower of ice cold rain mixed with partly melted snow. In England, the weathermen agree with that point of view. Our American meteorologists reserve the term sleet for a downfall of ice crystals or small pellets of transparent ice.

They are smaller than hailstones, which appear frosty white because they are formed from different layers of partly melted and refrozen moisture. It seems that the formation of hail and sleet also varies, though at present meteorologists cannot prove exactly how the atmosphere creates them. For this reason we can only suggest a theory. A theory, as we know, is an educated guess that may be proved right or wrong when more information has been gathered.

We know that sleet and all other forms of precipitation occur when moisture in the atmosphere cools and condenses. As a rule, the mysterious process begins at cloud level, perhaps several miles above the ground. The air up there usually is cooler, often very much cooler than it is at ground level. The creation of sleet most likely begins when water vapor condenses onto fragments of salt, dust or other small solid nuclei. This produces a cloud of misty droplets of liquid moisture.

Between the cloud and the ground there are layers of different weather conditions, subject to change without notice. When cloud droplets congeal in drops of rain, they begin to fall. If they chance to fall through a lower layer of cold freezing air, liquid raindrops may be changed to solid pellets of ice.

This is the form of precipitation that the weathermen call sleet. It may strike the ground as small glassy beads of ice, or several beads may clog together in odd shaped bundles. This sleet is not snow or rain, not hail or a mixture of these things.

Apparently it forms only when there is a layer of freezing air just above a warmish layer at ground level.

Sometimes the air on the way down is just warm enough to keep moisture wet  and the ground below is freezing cold. When the cool raindrops touch the surface on or near the chilly ground, they suddenly turn to ice. This produces a glaze that adds icy coats on the stones and encases every twig in an icy stocking. If sunshine follows such an ice storm, the whole world glistens and glitters with a million sparkling mirrors.

 

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