For those of you wondering, graupel is a certain form of precipitation which is commonly mistaked for sleet by the untrained eye. Even though graupel and sleet may look-alike, the process in which they are formed is a whole lot different. Graupel falls from rain showers, or thunderstorms, similar to the way hail does. Usually these storm systems aren’t associated with lightning. The one way you can tell the difference between the two is that graupel is quite softer than hail, along with the fact that it graupel rarely accumulates. Hail usually falls as much larger stones and can quickly accumulate into multiple inches on the ground. The quote below is from the NWS which explains a little more information about graupel.
GRAUPEL TENDS TO FORM IN SITUATIONS WHERE VERY COLD TEMPERATURES
ALOFT RESULT IN A RAPID DROP OFF IN TEMPERATURE AS YOU INCREASE IN
ELEVATION. GRAUPEL IS PRODUCED BY THUNDERSTORMS OR SHOWERS THAT
ARE NOT QUITE INTENSE ENOUGH TO PRODUCE LIGHTNING. GRAUPEL FORMS
AS SNOW FLAKES HIGH IN THE CLOUDS ENCOUNTER SUPERCOOLED WATER
DROPLETS…LIQUID WATER THAT HAS A TEMPERATURE BELOW FREEZING BUT
IS STILL LIQUID…CAUSING THE WATER TO INSTANTLY FREEZE INTO SMALL
ICE PELLETS WHICH AS SOMETIMES REFEREED TO AS SNOW PELLETS.
TYPICALLY…GRAUPEL FORMS WHEN TEMPERATURES AT THE SURFACE ARE
ABOVE FREEZING AND IT GENERALLY DOES NOT CAUSE SIGNIFICANT TRAVEL
PROBLEMS DUE TO MELTING.
Now when sleet falls, there must be a layer of cold temperatures associated with snow up in the higher levels of the atmosphere. To sum it up, sleet occurs when snowflakes fall into a warmer layer of air, which in turn causes them to partially melt. They then re-freeze in a colder layer closer to the ground and fall as what would appear to be large rain drops that look like wet snowflakes when they hit the ground. Sleet can accumulate as well, but not nearly as the rate of snow. This is mainly because the water content in sleet is so much higher than the water content seen in snow. Sleet typically occurs when the temperatures are slightly above freezing. If there is anything we left out, or if there is something you don’t understand, then don’t hesitate to ask us a question in the comments. ~Alec M & Alec WxForecaster