Ice pellets

Bouncing from the ground like tiny hailstones, and seldom falling for more than a few minutes at a time, ice pellets are a type of precipitation that is basically freezing rain refrozen. Ice pellets only fall in the colder months, since they need layers of air below freezing to be created.

Ice pellets are tiny grains of ice about the same size as individual grains of the millet that people put into their bird feeders every winter. Although they are made up of ice and are translucent, they are much too small to cause any damage when they hit -- unlike hail -- and differ from snow mainly in their weight, since they are quite a bit heavier. Ice pellets seldom accumulate to any depth, except in the eastern parts of the United States and Canada, where exceptionally favorable air flow patterns sometimes cause an inch or two to build up during a storm.

How ice pellets form

Ice pellets are an interesting example of how the atmosphere's action often depends on different layers of air. Just as clouds form at the point where one air mass or air layer touches another, so ice pellets form when snow falls through a layer of warmer air, then back into colder air again. The depth of the cold air near the ground determines whether the final result is ice pellets or freezing rain.

The warm air layer must be at least a mile above the ground for ice pellets to form. Snowflakes fall into this zone of air, which is itself around a mile thick on average, and melt as they pass through it. If there is a mile of cold air beneath the warm layer, then there's enough time for the melted snowflakes to re-freeze into ice pellets. If the layer of below-freezing air hugging the ground is less than a mile, then the melted snowflakes probably won't freeze again and the ground will be coated with a slick layer of freezing rain instead.

Ice pellets sometimes form when regular raindrops fall into a thick layer of below-freezing air before reaching the ground. Again, around a mile of cold air is needed to freeze the drops into pellets, and if the layer isn't thick enough, freezing rain is the result.

What is the significance of ice pellets?

Ice pellets usually don't do much more at ground level than wet snow would -- that is, they are moist, they make roads a bit slippery, and if there are enough of them then they will form a layer that will have to be cleared away. Meteorologicaly speaking, they are the sign of a winter warm front -- and thus, a herald of possibly warmer days and even a brief thaw. They are also likely to show up at the end of winter when the warming process has begun but hasn't fully taken hold.