Weather Explainers

New York and Pennsylvania Thought They Saw Sleet or Hail, But Here's What Actually Happened

Not only did rain and snow fall across the Midwest and Northeast over the weekend, but so did a lesser-known precipitation type that many people confuse with freezing rain, sleet or hail: graupel.

Graupel was reported in places such as Rochester, New York, State College, Pennsylvania, Pittsburgh, and portions of Eastern Pennsylvania and Northwest New Jersey.

What is Graupel and How Does it Form?

A commonly-used term to describe graupel is "snow pellets."

Graupel begins as snowflakes up in the clouds. These snowflakes encounter supercooled water as they fall through the atmosphere.

In a process called accretion, ice crystals instantly form and accumulate on the outside of the snowflakes as they interact with the supercooled water until the original snowflake is no longer visible or distinguishable.

The coating of the ice crystals on the outside of the snowflakes is called a rime coating.

(MORE: What's the Difference Between Sleet and Freezing Rain?)

Graupel, Sleet, or Hail?

It is difficult to tell the difference between graupel, sleet, and hail. Typically, surface temperatures need to be in the mid 30s or colder for precipitation to fall as sleet and at 32 degrees or colder for freezing rain. On the other hand, hail is usually associated with thunderstorms.

Graupel generally falls when surface temperatures are 45 degrees or colder, and it normally occurs when there is very cold air aloft but some warmer air exists at the surface.

Graupel pellets normally fall apart when touched or when they hit the ground, whereas sleet and hail will typically hold their form until melted.