A Storm Over Sippewissett

The storm cell above Little Sippewissett Marsh. A dramatic downdraft sits under the anvil.
This evening a line of thunderstorms crossed the Cape, ending a sunny spring afternoon with the perfect dessert. The storms brought only a bit of rain and thunder to Falmouth, but plenty of beautiful cloud forms. I watched these clouds from Little Sippewissett Marsh, my favorite local wetland. The storms were vigorous enough to produce thunder and lightning during their transit, yet were fairly low-topped: GOES-16 satellite measurements showed that the cloud tops were somewhere between 20,000 to 25,000 feet, which is only about 2/3 of the height of a typical thunderstorm. Their squat stature was visible to the naked eye: rather than towering castles, these storms appeared as long, variable creatures that lumbered across the sky.

I focused on one particular storm cell that passed over the marsh. The cell was topped by a mature-looking anvil (the flat, outward-spreading top portion of the cloud) and no updraft was visible, so it seemed that it was toward the end of its lifecycle. Beneath the center of the anvil were dense wisps of cloud slowly falling and evaporating in a downdraft; I was literally witnessing a waterfall!. This feature resembled a wall cloud, which is the central region where a thunderstorm is taking in air from below. But because the cloud in this region was falling, it was associated with a downdraft rather than an updraft. Perhaps it had been a wall cloud earlier in its lifetime and was now collapsing?

The trailing end of the storm was continually roiling with a patch of mammatus clouds: uncannily smooth lumps of cloud that form in groups at the bottom of a cloud. (See the bottom left cloud in seconds 15-20 of the video above.) These clouds are more often seen hanging from the anvil cloud of a strong thunderstorm, but in this case they were embedded in the lower portion of this weak storm. They are mesmerizing and uncommon, so getting to see them today was a great treat. The mechanism by which mammatus clouds form is still under debate by scientists, but it seems that turbulence, phase change of water, and the sharp gradient at the base of a storm cloud are all important factors.

The cloud was also decorated by a rainbow, which at first sat below the center of the cloud as if it were a colorful tornado, but later grew all the way across the sky to frame the cloud. There was even a faint double rainbow visible. What a special treat of a storm cell!