Graupel - not Hail - Photo by Don Collins

 

In April, 2022, winter does not want to "give-up".  Many folks at Warren Wilson College noticed this "wintry mix" on April 9, 2022.  This wintry precipitation resembles Styrofoam beads, but on close inspection these "beads" are icy and some have melted to form liquid water.  Many said that they resembled hail because they consist of ice.  They are not hail - hail is formed by the strong convection updrafts in thunderstorms that form only in the summer season.  Graupel forms in a winter snowstorm under special conditions that only form in the winter.  Graupel is actually a form of sleet. 

 The graupel "beads" actually began as snow flakes from a snow forming cloud.  A snow-forming cloud requires a dew point within the cloud that is below the freezing point (0 deg C or 32 deg F).  The moisture in the cloud sublimates directly from the solid into frozen snow crystals.  As more of the snow crystals form, they fall randomly and often meet other small snow crystals and coalesce into larger groupings that fall as familiar snow flakes - multiple amassed snow crystals.

Sometimes these growing snowflakes encounter a super-cooled or else form in the super-cooled cloud.  A super-cooled cloud is below the freezing point of water.  The tiny water droplets of the super-cooled cloud remain liquid - even though the ambient temperature is below freezing.  However, if the super-cooled cloud droplets encounter a nucleation site - a solid particle such as dust spec, pollen grain, or an ice crystal - the nucleation site will immediately cause the super-cooled droplets to immediately freeze.     Snow crystals that were already formed in the same or higher cloud then grow in size due to the rapid freezing super-cooled vapor.  This process is termed accretion and causes the pellets to grow in size.  The accretion process is very similar to the formation of rime ice on trees and structures (See PPOW post at an archive blog).  The individual graupel pellets are quite delicate and soft and are easily crushed.  Hail stones, on the other hand, are hard ice that were formed from raindrops carried upwards many thousands of feet thunderstorm updrafts where those raindrops become frozen into  hard solid ice spheres at the very high altitudes and low temperatures before falling to Earth.

Other common names for graupel are "hominy snow", "snow pellets", and "soft hail".

Much of the information for this post came from an excellent Wikipedia article (https://en.wikipedia.org/wiki/Graupel) 

 

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Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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