A layer of graupel on top of snow. Graupel is also sometimes referred to as “snow pellets.” (NOAA)

When you think of wintry precipitation, you probably think of snow first. Then sleet, and maybe freezing rain. But it’s likely that the word “graupel” doesn’t come to mind.

Graupel, which is a kind of hybrid frozen precipitation, is sometimes referred to as “snow pellets.” The National Weather Service defines graupel as small pellets of ice created when super-cooled water droplets coat, or rime, a snowflake.

A capped snowflake under a microscope, which shows rime on both ends. (U.S. Department of Agriculture) A capped snowflake under a microscope, which shows rime on both ends. (U.S. Department of Agriculture)

Graupel pellets are cloudy or white — not clear like sleet — and often are mistaken for small hail. The most critical necessity for the formation of graupel is extremely cold air at the cloud level. This creates the super-cooled water (liquid water that exists below the freezing point), which adheres to the snowflakes.

Such must have been the case on a chilly Friday morning, Nov. 28, when I went to play tennis at some local outdoor courts. Though I noticed some unexpected flurries as I left home, I didn’t think they would interfere with our tennis game, especially since weather radar was showing just an isolated patch of precipitation over lower Montgomery County that I thought would move off.

A chart and explanation of wintry precipitation types (plus hail). (NWS Pittsburgh)

Well, the flurries did move off, but it took about an hour and a half to do so. During that time we were able to play an energetic game in the snow, which came down lightly for the most part, with occasional moderate bursts.

Interestingly, the courts barely became damp, and certainly not what I would call wet. A similar period of light rain would have rendered the courts unplayable. I attributed this to the very low moisture content of the snow, even though temperatures were right around the freezing point.

Toward the end of the snow showers, a solid period of graupel suddenly began to coat the courts. It only lasted about five minutes, but during that short time the graupel did what an hour and a half of light snow showers couldn’t do: it rendered the courts unplayable, due to its higher moisture content.

Unfortunately, I wasn’t quick enough to capture an image of the graupel. But having seen it before, I did know it was graupel — not sleet — for these reasons: it was opaque and white, not clear, as sleet is; and it crumbled in places upon hitting the pavement, which graupel is known to do.