The Goods on Graupel

Ski conditions in the Wasatch backcountry continue to be excellent with cold weather and cloudy skies enabling pretty good snow preservation despite it being late March.

Over the past couple of days, a lot of the snow that has fallen has been in the form of graupel, which is an opaque, white snow pellet that sometimes takes a lump, hexagonal, or conical form. Graupel often causes the snow surface to have a dippin' dots appearance.  

Catch some on your sleeve and take a close look you'll find that graupel particles are aggregates of a bazillion tiny frozen cloud droplets.  

Graupel forms in strong updrafts when supercooled cloud droplets freeze on falling snowflakes.  Supercooled means that the cloud droplets are unfrozen, despite being colder than 0˚C.  These droplets freeze on contact.  This process is sometimes called riming or accretion.  If the riming is light, you can still distinguish the original snowflake.  However, at the extreme, the flake is completely coated and you get graupel.  

One of the reasons why we have had so much graupel the past few days is that it's been very unstable.  Graupel requires strong updrafts to suspect a snow crystal until it is fully rimed.  Snowflakes typically have a fall speed of about 1 meter per second.  Graupel is about 3 meters per second.  So, if you want big graupel, you typically need updrafts of at least 3 meters per second.  

Graupel is important for thunderstorm electrification.  In thunderstorm updrafts, smaller ice crystals are often carried upward by updrafts, whereas the larger graupel particles can fall out.  This contributes to the charge separation process within the cloud and in some cases lightning and thunder.  This happens even in summer thunderstorms, which extend well above the freezing level, although in those small hail can also form and be a contributor to electrification).  This has been happening some in Utah the past couple of days.  Snowbasin closed early yesterday due to lightning.  

People are often surprised by thundersnow, but the process is the same as in summer thunderstorms. It's just less common because you need strong updrafts and those are less common in many winter storms. Lake effect snowbelts see thundersnow more frequently because lake-effect storms often have strong updrafts.  In northern Utah, the daytime heating if post-frontal cold airmasses in the spring can also lead to strong updrafts and thundersnow.  

Because graupel is dense and has a higher fall speed, it can penetrate farther below the melting level than most snowflakes.  Thus, sometimes you see graupel falling at higher temperatures than snowflakes.  

Graupel should not be confused with sleet.  Sleet is a translucent ice pellet.  Unlike graupel, which is opaque and typically a bit pliable if you squeeze it, sleet is hard.  Sleet also forms through a different pathway.  Sleet forms when snow falls into a warm layer aloft with temperatures above 0˚C and melts into rain or droplets that are predominantly water with perhaps a small particle of ice in it.  It then falls into a colder layer near the surface that is below 0˚C and freezes.  

Sleet is basically a frozen raindrop.  Graupel is a snow particle formed by riming.  Very different processes.  Graupel skis much better than sleet.  You would definitely notice the difference.  Sleet is exceptionally rare in northern Utah (I'm not sure if I've ever seen it here). 

Enjoy the graupel skiing while it lasts.