Why that rhythmic tapping on your windshield might be more than just rain
In short: If you hear a sharp, high-pitched tap against your jacket instead of a dull splat, you are likely experiencing ice pellets. Often confused with sleet or freezing rain, these translucent balls of ice form through a specific atmospheric sandwich that forces snowflakes to melt and then refreeze before landing.
Ice pellets, known as sleet in the United States, are distinct from other winter precipitation. Unlike graupel, which is opaque and frosty, ice pellets are hard, translucent spheres that bounce upon impact. They are also fundamentally different from a slushy mix of rain and snow. While the term sleet is standard in American meteorology, it is avoided by Environment Canada, which prefers the precise term ice pellets to prevent confusion with rain-snow mixtures.
The formation of these pellets requires a precise atmospheric profile. A layer of air above freezing must exist between 1,500 and 3,000 meters above the ground, sandwiched between two sub-freezing layers. As snowflakes descend, they partially or completely melt in the warm layer. If the sub-freezing layer near the surface is deep enough, the droplets refreeze into solid pellets. If that bottom layer is too thin, the precipitation hits the ground as liquid, becoming freezing rain that coats surfaces in ice.
While ice pellets are usually brief, they can accumulate significantly in the eastern United States and southeastern Canada. When warm air from the Gulf of Mexico overrides cold surface air, accumulations of 2 to 5 centimeters can occur. These piles are deceptively dangerous; because ice pellets are denser than snow and have less surface area, they are heavier to shovel and take much longer to melt. This makes them a unique winter hazard that behaves differently than standard snowfall, requiring different management strategies for clearing and safety.
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- Summary by Intellect · source: Ice pellets
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