Shelf Cloud

 Arcus (Shelf cloud)  looking Northwest from HighWay 20 south of Alden, Iowa, Photo by Craig Johnson

Arcus (Shelf cloud)  looking Northwest from HighWay 20 south of Alden, Iowa, Photo by Craig Johnson

 Arcus Cloud - Looking west along Highway 20 south of Alden, Iowa,  PHOTO by CRAIG JOHNSON

Arcus Cloud - Looking west along Highway 20 south of Alden, Iowa, PHOTO by CRAIG JOHNSON

 Close-up  of Arcus Cloud Looking West along Highway 20 South of Alden, Iowa,  PHOTO by CRAIG JOHNSON

Close-up  of Arcus Cloud Looking West along Highway 20 South of Alden, Iowa, PHOTO by CRAIG JOHNSON

 Under the Shelf cloud as it moves to the east (Right to Left),  PHOTO by CRAIG JOHNSON  

Under the Shelf cloud as it moves to the east (Right to Left), PHOTO by CRAIG JOHNSON 

This magnificent shelf cloud was photographed facing west along Highway 20 south of Alden, Iowa. The date was Friday, September 9, 2016 at about 3:00 in the afternoon. Shelf clouds are part of the cloud grouping known as arcus. These clouds form along the leading edge of the outflow boundary of the thunderstorm. The outflow is the flow of cooler air descending from a thunderstorms. Outflow from above spreading out over the ground undercuts warm moist air drawn into the parent thunderstorm. Arcus clouds are not attached to the parent cloud. The fourth photo in this series is looking north underneath the shelf cloud. Winds gusted to about 45 mph with the passage of this system.