Lumpy Sky

The underside of an approaching cumulonimbus anvil contained this formation of mammatus clouds just after 6:00 PM on July 24, 2009. The spectacle preceded a thunderstorm by about an hour. Mammatus usually form in a very moist and unstable middle or upper level of the atmosphere, overlying a drier layer below the anvil. The elements of this type of cloud represent inverse convection--instability in the downward direction rather than upward. This view looks west toward Bowman Woods Park from Brentwood Drive NE in Cedar Rapids, Iowa. The very pronounced formations lasted for about 15 minutes.