Skip to main content
University of Wisconsin–Madison
  • Home
  • News
  • Weekly Weather Event -Week of Aug. 5

Weekly Weather Event -Week of Aug. 5

August 7, 2019

As part of a new weekly series, the AOS department will be covering a major weather event that happened during the week, explaining both the events that took place and the science behind it all.

Strong winds and large hail damaged houses, cars, and golf courses alike as thunderstorms battered parts of Minnesota and the upper Midwest earlier this week. The National Weather Service reported several instances of severe weather, including 78 mph gusts in Michigan’s upper peninsula and 4-inch hail in Delano, Minnesota, a town about an hour west of Minneapolis.

Hail generally forms in thunderstorms when updrafts move water droplets high into cumulonimbus clouds. Some of the water droplets freeze into small pellets of ice, but some water droplets remain liquid after dropping below freezing, resulting in supercooled water droplets. At a certain height within the cloud, the air temperature falls below freezing, and these supercooled water droplets begin to condense on particles such as dust, ash, or ice pellets.

Depending on the storm, hailstones can accumulate ice by traveling up and down through the cloud, resulting in the hailstone having layers like an onion. Alternatively, a hailstone can remain aloft in the updraft, which can produce a hailstone with little to no layers at all.

The hailstone continues to grow until the hailstone is large enough that updrafts can no longer overcome the force of gravity. Once this happens, the hailstone falls to the ground. Depending how strong the updrafts were, the resulting stone could be a small as a pea (¼ inch diameter) to as large as a grapefruit (4 ½ inch diameter).

The storms were caused by a cold front moving south across Minnesota combining with high heat and humidity in the area, said Tyler Hasenstein, a meteorologist for the National Weather Service in Duluth.