MISSOULA — You've probably seen some of the graupel showers over the past few days. But...what is graupel? Is it hail? Is it snow? How is it different than hail? How does it form? And how do you pronounce it?
All great questions! Let's dive in!
Graupel is not hail, although it can look like it.
Both hail and graupel are balls of ice, but they form in very different ways.
Hail usually forms from intense thunderstorm updrafts (rising air). Ice crystals high in the clouds are caught in the updraft of a thunderstorm and will continue to grow by colliding with each other and sticking together.
They will do this until they become to large and heavy for the updraft to support. Once this happens they fall as balls of ice or hail.
Graupel starts out as a snowflake high in the atmosphere. As it falls, it falls through a layer of the atmosphere with super-cooled water droplets.
Super-cooled water droplets are drops of water that are remaining liquid even though their temperature is below 32° or freezing.
As the snow flake falls into this zone, the water droplets attach to it and immediately freeze. This creates a thin layer of ice around the snow flake creating what we know as graupel!
As for pronouncing it: GRAW-pul or GROP-pul are both commonly accepted.
So next time you see those dip and dots falling from the sky, you have a better idea of what they are!
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From the National Severe Storm Laboratory:
The primary difference between frozen precipitation is how the different types grow and the maximum sizes of the individual particles.
Snow forms mainly when water vapor turns to ice without going through the liquid stage. This process is called deposition. Snow can form in the gentle updrafts of stratus clouds or at high altitudes in very cold regions of a thunderstorm. Snowflakes that most of us are used to seeing are not individual snow crystals, but are actually aggregates, or collections, of snow crystals that stick or otherwise attach to each other. Aggregates can grow to very large sizes compared to individual snow crystals.
Graupel are soft, small pellets formed when supercooled water droplets (at a temperature below 32°F) freeze onto a snow crystal, a process called riming. If the riming is particularly intense, the rimed snow crystal can grow to an appreciable size, but remain less than 0.2 inches. Graupel is also called snow pellets or soft hail, as the graupel particles are particularly fragile and generally disintegrate when handled.
Sleet are small ice particles that form from the freezing of liquid water drops, such as raindrops. At ground level, sleet is only common during winter storms when snow melts as it falls and the resulting water refreezes into sleet prior to hitting the ground. In thunderstorms, sleet is possible above the melting level where cloud droplets become supercooled and may instantaneously freeze when making contact with other cloud particles or debris, such as dust particles. Sleet is also called ice pellets.
Hail is frozen precipitation that can grow to very large sizes through the collection of water that freezes onto the hailstone’s surface. Hailstones begin as embryos, which include graupel or sleet, and then grow in size. Hailstones can have a variety of shapes and include lumps and bumps that may even take the shape of small spikes. Hailstones must be at least 0.2 inches in size.
