Q&A: Unusual Cloud Formation

Question: Thank you for allowing me to ask about a strange but fascinating cloud formation. I took these photos over my house in Dracut Massachusetts in the month of June about late afternoon. I am trying to find out more about this. The circles on the ends are very strange. The cloud dissipated quickly as shown in the 3rd photo. Any information on what kind of cloud this is with the circle shapes on the ends would be appreciated. I tried searching online but cannot seem to find anything on it. — Ann Pergakis, Dracut, MA

[Additional info from my followup questions: Temperature at ground level was probably around 60°, and there was some light rain later in the evening.]

Answer: My first impression is that it’s related to a strong downdraft, like what happens in fallstreak holes. For some background on that phenomenon see:

https://sky-lights.org/2015/05/11/fallstreak-holes/
https://sky-lights.org/2015/02/02/qa-burning-a-hole-through-a-cloud/

Fallstreaks are rare. And they tend to be vertically oriented. Your photos are oriented at several different camera angles, so it’s hard to tell, but the cloud structure looks reasonably vertically oriented to me. There could be a crosswind tilting it slightly off vertical.

Fallstreaks can be created when a large pocket of air in a layer of clouds is below freezing temperature and still contains water in liquid form — a state called supersaturation. This is an unstable state and the water can be triggered to freeze by any number of disturbances, including the passage of an aircraft that creates pressure variations, wind induced turbulence, or a rising plume of dust or smog (which provides nucleating sites for ice crystals).

Once crystalized the ice falls from the sky in a visible “streak” sometimes leaving a “hole” in the cloud where the water has been removed. Hence the name “fallstreak hole.” The ice can reach the ground intact as snow, or melt on the way down and arrive as rain. If it reaches a layer of air where temperatures are higher than the dew point it will re-evaporate and never reach the ground — a type of rain known as virga.

What you saw was the aftermath of a fallstreak that became virga. There are no plumes of rain or ice crystals streaming downward in your photos, but I suspect there was a brief fallstreak that happened shortly before you noticed the cloud. Fallstreaks drag air along with it as they fall. That created the narrow channel of cloud between the top and bottom structures. It’s a “streak” but it’s not rain or ice crystals — at the time of your photos it’s just the residual cloud. Some of that cloud was dragged down from above by the virga, and the rest of it formed from evaporated virga that re-condensed into microdroplets.

Could the air be cold enough to form ice in Dracut in June? Here’s the NWS chart for your area:

The average mid-June temperature is 57°, but you say it was warmer, so let’s go with 60° at ground level. Still, it would have been freezing temperatures (32°F) not that far overhead. Air temperature drops by 5.4°F per 1000 ft, so freezing temperatures will be reached at (60° − 32°) / 5.4°/1000 ft = 5185 ft AGL. It’s hard to estimate altitude from your photos, but 5185 ft seems reasonable for the top cloud. Lots of assumptions and estimates, I know, but stay with me …

As the pocket of air fell, it eventually reached a lower layer of air with greater density. This was likely air from the typical afternoon ocean breezes — Dracut is only 20 miles from the coast. The cool sea air pushes under the less-dense warm air at the surface (which has been warming since sunrise). When you have warmer air above cooler air it’s called an inversion, and in this case there’s enough of a temperature difference to create a strong discontinuity in density. That why the lowest layer of air acts like a barrier to the air falling from above.

When the pocket of falling air hit the discontinuity it spread out radially, as any fluid does when encountering a barrier. The collision caused it compress and warm to the point where the clouds evaporated again. As the ring expanded and cooled, the cloud reformed creating the hole at the bottom. There’s no fallstreak “hole” at the top because there was no continuous cloud cover to begin with. The fallstreak came from an individual isolated cloud, and the disturbance that created it wasn’t intense enough to crystalize all the water in the cloud.

Here’s the process in graphic form:

Rain falls at a speed that depends on the size of the drop. The largest fastest drops fall at around 10 m/s (33 ft/s), so if my altitude estimates are correct, the fallstreak would have formed in:  (5185 ft − 1000 ft) / 33 ft/s = 127 s, or around 2 minutes from start of fall to bottom ring expansion. It would have been fascinating to see a video of the entire event unfolding. Alas, it seems you missed that opportunity by a few minutes.

I suspect this variation of the fallstreak phenomenon is pretty rare. Thanks for sending the photos and question, and keep looking up!

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