(WKBN) — Graupel was reported across the Valley yesterday for the second week in a row due to below-average temperatures.
Many of you sent pictures and videos to the station using the terms: graupel, hail, sleet, ice pellets, etc. All of these words have different definitions, but to the untrained eye, it might not be clear which term is applicable. Today, we are going to learn the difference between these precipitation types and how you can tell the difference.
First, the formation of graupel is a fascinating process. Graupel is a unique precipitation type because it forms during the cool season and the warm season. Graupel forms when a snowflake (ice crystal) interacts with supercooled liquid water (water that is still a liquid at temperatures below freezing).
When this interaction occurs, the supercooled water droplets freeze on contact creating an icy coating around the snowflake which is a process called riming. The result of this process is spherical pieces of ice that measure between 2-5 millimeters. The video below is a graphical representation how ice graupel forms.
Graupel can and does form during the warm season. In fact, graupel is important for the formation of lightning in thunderstorms. When graupel interacts with other snow crystals in the presence of supercooled water, then a sufficient charge is generated for a lightning strike. Often, hail and graupel coexist with each other in thunderstorm clouds.
Speaking of hail, its formation is a little different than that of graupel. Hail forms when liquid water is lofted high into the atmosphere as a result of strong thunderstorm updrafts. Eventually, the water begins to freeze as it reaches higher heights in the atmosphere. Then, the newly formed hailstone grows in size due to the accretion of supercooled water and other hailstones.
The hailstones often cycle throughout the cloud continuing to grow as the thunderstorm strengthens. Eventually, the hailstones become too heavy to be supported by the thunderstorm updraft and that is when they fall to the Earth.
Finally, there is sleet. The formation of sleet is different and more simple than the formation of graupel and hail. Sleet forms when snowflakes fall into a layer where temperatures are above freezing. The snowflake either melts partially or melts entirely, but then enters another layer of atmosphere closer to the ground where the temperature is below freezing causing it to freeze back into an ice pellet.
Sleet only occurs during the cold season and the mechanism for its formation happens as the precipitation is falling to the ground which is different from graupel and hail.
Now you have been armed with the knowledge regarding the difference between graupel, hail and sleet. Go forth and use this knowledge to help report precipitation types to the WKBN Storm Team 27 Weather Center.