First off, my apologies to Lee Corso for stealing his phrase made famous through his appearances on Saturday College Gameday. Did you spot some hail mixed in with snow with our windy/cold spring day this past Wednesday? Don’t feel bad if you answer yes, because even meteorologists mistake what actually fell with the snow fairly often as well.
As defined by the Glossary of Meteorology published by the American Meteorological Society : “Graupel - Heavily rimed snow particles, often called snow pellets; often indistinguishable from very small soft hail except for the convention that hail must have a diameter greater than 5mm. Sometimes distinguished by shape into conical, hexagonal, and lump (irregular) graupel.”
If graupel were human, I would bet that it would have quite an emotional complex. It’s probably the most mistaken precipitation type, usually identified as hail. Now you may be saying to yourself, “But the definition above states that graupel is very similar to hail!” While it may be similar, the process that goes into creating graupel and hail are very different.
Let’s visually compare the two.
- Posted by Arvay at 4/15/2010 12:32:00 PM Via the website: http://silicon2tanana.blogspot.com/2010_04_01_archive.html
- Via website: http://www.billcasselman.com/unpub_four/graupel_chionablepsia.htm
- Photo of graupel in Port Angeles on Feb. 26, 2012, courtesy of Colin Bradley
- Via website:http://www.cocorahs.org/media/images/stone6_7102002.jpg
- Courtesy WGN-TV Photo: Amanda Pickett
First off, as noted in the glossary definition graupel is usually very small compared to hail. The other most noticeable difference comes when comparing the hail picture that shows the interior of the hail stone. Notice the concentric nature of the hail (the pattern of rings, like you find in the rings of a tree trunk). The presence of rings in hailstones marks the big difference in how these two similar, yet different precip types are created.
Graupel forms within our winter weather events. It’s just another precip type that is present in nearly all wintry events but doesn’t fall to the ground on every occasion. In the winter season, a snow storm begins by a small grain of ice way up in altitude within a cloud. As this fine ice crystal moves around in the upper regions of the cloud it encounters other small cystals and they begin to stick together. When enough of these crystals stick and beome heavy enough, they fall. If they fall all the way to the ground, this is what we find as snow. In situations where graupel is formed, the snowflake (gathering of fine ice crystals) falls through an area of the cloud where supercooled water is found.
Hang on a minute… what the heck is supercooled water?!
Sidebar: supercooled water is water that has a temperature of less than 32° F. This most commonly occurs when water doesn’t have an impurity found within. For ice to form, it needs a little tiny particle called a “condensation nuleus”. This is just a fancy pants way of saying a particle of dust/dirt or salt that is the foundation for an ice crystal pattern to begin forming.
So back to our falling snowflake that encounters this supercooled water. Since the water is below 32° F and hits our snowflake/ice party, the wather instantly freezes to ice but begins to spread out as a “sheen” on the snowflake. More and more supercooled water then starts to stick and turn to this sheen of ice on the snowflake and you get a little ball of irregular ice. Yay! That’s graupel!
This is a pretty close diagram of the process I just described:
- Image created by Judy Stanley of The Herald in Everett, WA
Hail, on the other hand, is formed by a much more vigerous environment. Hail forms within thunderstorms. Thunderstorms are formed by currents of rapidly rising air that we call “updrafts”. These currents of vertically rising air can move up to and beyond 100 mph! In a thunderstorm, fine particles of ice again form at the top of the cloud. Again, these particles begin to stick together and eventually begin to fall. Similar to the graupel situation, the snowflakes/ice groupings can encounter supercooled water and form a small graupel pellet. Other times the ice groupings/snowflakes can encounter regular water and then melt and continue to fall. But here comes the key part in the life cycle of a hail stone: eventually it encounters the updraft. When the small ice/water particles meet the updraft, they get carried back to the top of the storm. Each time these ice/water balls make a ciruit of falling then rising, a ring of ice is formed (remember the rings found in the hail picture I showed you before?). In the largest hailstones, these ice balls encounter updrafts of 100 mph or more and become the size of softballs/grapefruit!
This is a pretty good diagram of how this process works:
- Via website: http://www.weathermod-bg.eu/pages/obr_en.php
In fact, the largest hailstone ever recorded just recently broke the record of a hailstone that held it for many years found in Whita Falls, TX. This is our current record holder:
- 8" hailstone from Vivian, SD © 2010 Associated Press. All rights reserved.
As you see, hail and graupel share some common ground but also display some very unique differences! The big tip for the next time you need to decide whehter you’ve seen hail or graupel: if there’s snow or sleet also falling or being reported, it’s most likely graupel. If you’re seeing flashes of lightning and severe thunderstorms are being reported, then the ball of ice you encounter is most likely hail!