Thunder-Stormy Summer is the Time to See Mammatus Clouds

On several recent days, Spaceweather.com's author, Dr. Tony Phillips, has been reporting viewer sightings of mammatus clouds (June 22 and 26 are still archived). Mammatus clouds are found gracing the underbelly of spent thunderstorms, so summer is the time to find them.

Mammatus clouds are collections of smooth, rounded lobes that extend below the underside of thunderstorm anvil clouds. Some say that they resemble a group of cow's udders - some say not so much....

Ian McDonald of Manitoba, Canada, a Spaceweather.com enthusiast, sent a photo image of mammatus clouds to Dr. Phillips, which is featured on Spaceweather.com for June 22, 2007, with the comment, "They were absolutely amazing; the 'pouches' were in constant motion." So, for those of us who have never seen them, or very rarely seen them, we now know that mammatus clouds' lobes churn and boil just like storm clouds of other types.

Kevin Black, who sent in photos of the same mammatus event as seen from Winnipeg, comments to Dr. Phillips that "The clouds we saw on June 22 appeared just after an F4 tornado hit the town of Elie, Manitoba." Black's comment points out that mammatus clouds appear when storms are exhausted and are breaking up. It has been commonly thought that mammatus clouds herald the coming of bad weather but new research indicates they follow instead of precede bad weather.

New research provides a clearer picture of what mammatus clouds really are and how they form. Previously, according to Dr. D. M. Schultz, mammatus clouds were investigated only as incidental chance encounters while other meteorological studies were on-going. Mammatus clouds most commonly occur on the underside of cumulonimbus clouds but also occur under other types, like cirrocumulus and altocumulus.

USAToday.com/Weather has a good summary of the conclusions of the newest research. Sinking cold air that meets rising warm air is required for the formation of mammatus clouds, this is why the appearance of mammatus indicates a weakening of a storm system. Mammatus clouds form on the underside of the anvil of the storm cloud. These storm systems contain tremendous updrafts of air, so much so that the anvil (the flat spreading top of a cumulonimbus) may drive into the wind shear instead of being blown back by it, which is called a "back-sheared anvil."

Storm cloud anvils consist of ice crystals which drop out of the anvil head and cool as they pass through the clear air below the anvil before hitting the storm clouds underneath the spreading head of the anvil, as explained by USAToday.com.

It is for this reason that it is following the weakening of the storm that mammatus clouds form. It is also true that mammatus clouds sometimes form on the upwind, or backside, of severe storms with high winds and hail, but the storm precedes the mammatus clouds. In general, though, mammatus clouds signal the end of a stormy ordeal. They do not herald the beginning of severe weather.

Dr. Tony Phillips, NASA, "Mammatus Clouds." Spaceweather.com. URL: http://www.spaceweather.com/

Chris Cappella, "The true meaning of mammatus clouds." USATODAY.com/weather. URL: http://www.usatoday.com/weather/tg/wmamatus/wmamatus.htm

D. M. Schultz et al, "The Mysteries of Mammatus Clouds." Journal of the Atmospheric Sciences, Volume 63, Issue 10 (October 2006). URL: http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2FJAS3758.1