Rolling Over Wisconsin

Although they may look sinister, roll clouds like this one are no tornado. These unusual clouds form near advancing cold fronts when downdrafts cause warm, moist air to rise, cool below the dew point, and condense into a cloud. Air in the cloud can circulate around its long horizontal axis, but the clouds won’t transform into a tornado. Roll clouds are also known as Morning Glory clouds because they often form early in the day along the Queensland coast, where springtime breezes off the water promote their growth. The clouds do form elsewhere, though; this example is from Wisconsin in 2007. (Image credit: M. Hanrahan; via APOD)