We had been talking about the possibility for wintry precipitation for a few days and it fell, as expected, on Saturday. We received reports and pictures of ice pellets from all corners of the viewing area: Owatonna, Rochester, Austin, Charles City, Mason City, and numerous cities in between. (Picture by Dan B.)
Although a few ice pellets could have been hail, with a couple thunderstorms popping up, most fell as graupel. Graupel is not a term meteorologists get to use very frequently. It falls under the umbrella of ice pellets along with sleet and hail, but is slightly different than the other two. The graphic below explains the difference between graupel and hail.
There are some defining differences, but most important is the process in which each is formed. Hail forms as a water droplet rides the updraft and downdraft of a thunderstorm moving up and down in the atmosphere. This process causes layers of ice to form around the water droplet. When the updraft can no longer sustain the weight of the hail stone it falls to Earth. Graupel is formed as a snow flake falls in the atmosphere and ice crystals form around it. Graupel is usually small in size and typically looks like dippin dots. The easiest way to determine graupel from hail is to touch it. If it is small in size and easily breaks apart it is likely graupel, if it is harder to break apart it is likely hail.
There is very little difference between sleet and hail. Although both are small in nature, sleet is usually colorless and graupel has a milky color to it. Technically a sleet stone starts as a snow flake which melts into a water droplet and then refreezes as it falls to the ground. As defined above, a graupel stone stays as a snow flake. If you are still confused here is a simple graphic to help you out provided by @wxbrad
In addition to Saturday’s wintry precipitation, chilly temperatures settled into southeast Minnesota and north central Iowa Sunday morning. Temperatures in many locations fell below 28º for the first time this year ending the growing season and also ending all frost/freeze advisories, watches and warnings issued by the National Weather Service.
Cold air will continue all week with a few more slight chances for wintry precipitation.
Storm Tracker 6 Meteorologist