Great examples of Mammatus Clouds!
Some of the southwest suburbs of Chicago were able to see a cloud phenomenon called mammatus clouds. Mammatus are pouch-like cloud structures and a rare example of clouds in sinking air. (Most clouds form in rising air.) Although mammatus most frequently form on the underside of a cumulonimbus, they can develop underneath cirrocumulus, altostratus, altocumulus, and stratocumulus. For a mammatus to form, the sinking air must be cooler than the air around it and have high liquid water or ice content. They derive their name from their appearance. The baglike sacs that hang beneath the cloud resemble cow's udders.
Mammatus are long lived if the sinking air contains large drops and snow crystals since larger particles require greater amounts of energy for evaporation to occur. Over time, the cloud droplets do eventually evaporate and the mammatus dissolve.
Despite popular misconception, mammatus clouds are not a sign that a tornado is about to form.
All photos taken at WFO Chicago on July 13, 2004.