mammatus clouds over Colorado

Mammatus Mia!

Contributed by
May 31, 2016

Here in the Boulder, Colorado, area, we get odd weather. In the summer the wind blows mostly from the west, and in the afternoon moisture-laden clouds stream over the Rocky Mountains and toward the plains to our east. Mornings in the summer are typically clear, but then a few hours later the skies turn ominous and we can get some pretty rollicking storms.

Usually the clouds clear out, and the low afternoon Sun then creates fantastic rainbows to the east. Over this past weekend we had a pretty good cumulonimbus cloud blow over us, heading east. I kept my eye out, hoping for a rainbow, but it never cleared up enough to get one while the rain was coming down.

But I’m OK with that, because what I did get was way, way better: mammatus clouds.

mammatus clouds over Colorado

Mammatus clouds hanging ominously over Colorado. Credit: Phil Plait

Mammatus clouds are bulb-like appendages that sometimes hang down from the bottoms of clouds. The name comes from the Latin mamma, which means udder or breast, and when you see a field of them stretched across the sky you can see why they’re named that. They are very peculiar-looking, and relatively rare. I commonly see them trying to form sometimes on the western edges of storms, but they generally don’t organize themselves well. I’ve only seen well-defined examples of them maybe three or four times in my life, and I look up at clouds a lot.

I’m fascinated by clouds, and how they form. And given their unusual shape, you’d think mammatus clouds would be well-understood … but they’re not. At all. It’s unclear how and why they form, and the Wikipedia page for them has a list of ten possible formation mechanisms.

The first time I saw them I was living in Maryland, and a hurricane south of us had a wide-flung feeder band (the spiral arms of the system around the outside), and it left behind a field of mammatus lit red by the setting Sun. It was astonishing, and may have been what cemented my passion for weird clouds.

mammatus clouds over Colorado

Why are they forming along those rows? Credit: Phil Plait

But this time it was different. The bulbs weren’t just scattered around; in two different spots there were clear linear features, the bulbs lined up. To the north of me the lines continued westward in the cloud even when the mammatus bulbs faded out. My first thought was that these were from gravity waves, ripples in the air flow as a stream of air bobs up and down like waves on the ocean surface. But the pattern didn’t look sinusoidal so much as like rolls, something like cloud streets. The lines looked like where the rolls were pushed together, like two paper towel rolls lying side by side, touching along their edge. It was very peculiar. Wikipedia has an excellent example of what I mean, with mammatus clouds seen in Nepal. It’s hard to say from the photos what’s happening, especially when three-dimensional structure is difficult to tease out.

The mammatus clouds hung out for quite some time, over an hour. I had enough time to go outside and take the photos you see here, and also do a live Periscope of them. If I see more (or a good rainbow) I’ll probably do another quick livestream.

mammatus clouds over Colorado

You can see the lighter colored lines moving off to the west (upper left) even though there are no obvious mammatus formations there. Credit: Phil Plait

With these, iridescent clouds, lenticulars, a tornado, and other weird things, to my knowledge I now have only three more cloud formations I need to see for myself: undulatus asperatusroll (or arcus) clouds, and noctilucent clouds. None of those is common here in Colorado, so I just hope they’ll appear someday when I’m on travel.

There’s so much to see, so much to experience on our planet! I’ll never ever get tired of it.

mammatus clouds over Colorado

Just as I took this shot, a red-winged blackbird flew in front of me, across the clouds. Credit: Phil Plait