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Avoiding Ice: Why Snow Pellets on the Ground Mean Danger in the Sky

Snow Pellets Wing Ice

What Causes Snow Pellets?

Snow pellets, also known as graupel, form when supercooled water droplets freeze on a falling snowflake or ice crystal. As more droplets collect and freeze, they form a small, soft ball of ice. While snow pellets may seem like small hail, they're different. Unlike hail, snow pellets freeze into fragile, oblong shapes and usually break apart when they hit the ground.

Where Do Snow Pellets Occur?

Snow pellets can form when two conditions exist:

  • Temperatures are at or slightly below 0C (32F), and
  • Supercooled water droplets and snowflakes occur at the same altitude.

Supercooled water usually exists at higher altitudes in a cloud bank and falling snowflakes usually exist at lower altitudes. At higher elevations, such as Boulder, Colorado, there is less space between supercooled water and snowflakes. This increases the chance that they will overlap. However, snow pellets can also occur at lower elevations.

What Does Supercooled Water Have to do With Ice?

As supercooled water drops hit an aircraft, they quickly freeze and build up airframe icing. Check out this video and watch how quickly supercooled water forms ice.

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