There have been a coupla comments on that aircraft effect in clouds blog of a coupla weeks ago and so I thought I would follow up with this sequence from the Atmos Sci Building rooftop at the University of Washington where I spent most of my time instead of at my desk.1
Here is a rarely photographed sequence of the effect of an aircraft on a supercooled cloud. The first photo, right after a contrail-like feature was seen in these Altocumulus clouds.
In the minutes after this first photo, the aircraft trail seems to disappear as it widens and the shadow lessens. This stage is not shown because I didn’t realize what was going to happen until minutes later. This second stage is almost impossible to pick up visually because there are no ice trails yet, nor is the cloud opening up at this time. This “invisible” stage might last 5 minutes before you see the hair-like signs of a fallout of ice crystals.
Ice grows rapidly in the presence of the supercooled drops. Ice represents something of a low pressure center in the middle of all those droplets and that attracts the vapor from them, causing them to evaporate. That vapor deposits as ice on the newly present ice “germs”/crystals created by the aircraft. Since the drops are disappearing, before long, you get a hole or ice canal in the cloud where the droplet cloud used to be.
The ice crystals shown above are clearly falling out (ever-so gradually because they are so small still, perhaps a few hundred microns in width). Becasue they are so small, they usually evaporate well before any precip reaches the ground. However, recently it has been shown that in deeper clouds and more moist conditions, that an aircraft can actually produce a bit of rain/snow at the ground due to this effect.
Here is last photo I took that day.
The ice crystal induced hole in the Altocumulus layer has gotten closer to exiting the liquid cloud (has moved to the edge of it) as well as expanding some. This suggests that the ice cloud was moving faster than the droplet cloud, something that happens when waves in the atmosphere are producing the droplet cloud. It was also getting closer to the observer, however.
Sometimes, if the cloud layer is lifting enough, the original Altocumulus clouds will gradually fill back in because all of the ice has settled below the liquid cloud layer.
For history buffs, holes in clouds with ice in the center, or ice canals were seen in the 1930 and 1940s, but as you can see, unless the observer saw the original trail (which they usually didn’t) no one knew what caused them. Eventually an ice canal was was photographed in 1946 that was so convoluted it was realized that ONLY an aircraft could have done it.
Furthermore, that report in 1946 preceded the discovery of modern cloud seeding with dry ice by Vincent Schaefer in 1947 who performed his most convincing, and could be seen as ironic, demonstration of seeding with a similar convoluted ice canal as was seen in 1946 in a supercooled Altocumulus cloud layer Its interesting in retrospect, as so many things are, that Schaefer did not have to drop dry ice on his clouds that day in 1947 in which he made history, but rather only had to fly his prop aircraft through them and likely would have gotten the same effect!
BTW, there is a lot of new interest in this topic, a new article recently appearing in Science mag.
I mention this cloud seeding benchmark since these aircraft events represent inadvertent cloud seeding, and in a sense they demonstrate that you CAN get something in the way of precip to fall out of a previously non-precipitating or barely precipitating cloud by seeding. When clouds are already naturally precipitating, what happens when you do cloud seeding is subject to question; the science domain in this murky world is highly polarized.
The “Story of APIPs”–Aircraft-produced Ice Particles) is told (in the usual “style” you will often find here) in the gigantic powerpoint “show” on this website under Sci Talks toward the middle of the show, around slide #472 (hahaha).
This ppt “show”, BTW, is WAY overdone, but, what the HECK! Why tell only “the whole 9 yards” when you can tell 12 or 14?
1Hahaha, sort of. Sometimes, looking at those several thousand film shots from the rooftop of the Atmos Sci Building, I do wonder about that. But then again, since I used 1/100 of a second exposures with my film cameras, these photos would only PROVE that I had been on the roof, maybe 30 seconds in 30 years there at the UW.
Feeling better now.