17 June 2022
Severe thunderstorms were predicted for 6:00pm yesterday in the upper Ohio Valley. By 2:00pm the Severe Thunderstorm Watch called for an inch of rain in 1 hour — definitely flash flood material — but at 5:00pm the storm line split. Some went north toward I-80, the rest went south to West Virginia. Pittsburgh had no lightning, no strong winds, no rain. Nothing happened.
But the sky got weird. At sunset the last of the storm clouds left our area with a flourish of rare mammatus clouds, dramatically lit from below. Their name is derived from the Latin word for breast or udder.
As Wikipedia explains, mammatus are formations that hang from the base of rain clouds. The distinct lumpy undersides are formed by cold air sinking down to form pockets. Usually composed of ice, each lobe averages 1/2 to 2 miles across and 0.3 mile deep. Alone a lobe can last 10 minutes but a cluster may last several hours.
Mammatus are an indication of a severe thunderstorm but in my experience in Pittsburgh they do not predict the storm. Instead they show up after the storm has passed.
The clouds started out as lines and gave way to stratus clouds and a gleam at sunset.
Next time you see these weird clouds, remember their name describes their shape.
p.s. Steve Tirone left a comment with a link to his video of the clouds.
(photos by Kate St. John)