Shelf Cloud (Arcus) Approaching Cedar Falls, Iowa

Shelf clouds are one of two types of arcus clouds that sometimes form on the leading edge of thunderstorms. The other type is the Roll Cloud. Arcus clouds mark the leading edge of cool air descending from the storm. The cooler air plows into warm air flowing into the storm. This shelf cloud approached Cedar Falls, Iowa just before Noon on Wednesday, July 17th. Wind speeds at the surface often reach 40 to 70 mph with extreme speeds over 100 mph possible. However, in this case winds were less than 40 mph and there was no damage reported.