Aircraft can cause precipitation by "seeding" clouds

In a paper that is about to be published in the Bulletin of the American Meteorological Society, researchers Andrew Heymsfield, Patrick Kennedy, Steve Massie, Crl Schmitt, Zhien Wang, Samuel Haimov and Art Rangno make the claim that "The production of holes and channels in altocumulus clouds by two commercial turboprop aircraft is documented for the first time. ... Holes and channels in supercooled altocumulus clouds can be the result of homogeneous ice nucleation induced by turboprop and jet aircraft at temperatures warmer than previously accepted for commercial aviation influences on clouds."

In short, it is possible that when an air plane flies through a cloud that is just floating there minding its own business, the plane can cause the cloud to start precipitating, letting out rain or snow. The conditions have to be just right, so it does not happen often, but it can happen. So this is, perhaps, one of those interesting physical phenomena that is theoretically possible but pragmatically insignificant that can be the seed of argument and confusion, which is probably (I'm guessing here) why this study was done: I imagine a bunch of meteorologists sitting around arguing over whether or not such a thing can happen, and finally one of the goes, "OK, yooz guys stay here on the ground, and us guyz, we'll fly through the cloud, and you watch and see what happens, K?" ... And then they do it and get a paper out of it.

Well, to be more exact, the observation was made by accident, then further investigated. According to one report (source:

In 2007 Andrew Heymsfield ... was a on a research flight west of Denver International Airport when he and his team flew right below a big [cloud that had been flown through by an airplane]. When they went back and looked at footage from a ground-looking camera on the plane, they found the area directly beneath the hole had been coated in two inches of fresh snow.

source

Here's the science behind the story. Water vapor is a more or less invisible component of the atmosphere (it does affect optical conditions of the air, but it is not something we see and point to). Under certain conditions having to do with how much vapor there is, air pressure, and temperature, some of that vapor can condense into very tiny droplets of water or ice. Clouds. When an aircaft passes through the atmosphere, it can affect conditions in number of ways. It can add water vapor, it can cause a vortex which, by definition, has a pressure gradient with a low pressure center, it can add heat (especially a jet engine) and it can cause air that varies smoothly across three-dimensional space to get mixed up, so, for instance, colder dryer air and warmer moister air can mix to cause a new set of conditions to emerge micro-locally.

Sometimes, an air craft will fly through a cloud in such a way that the conditions that cause cloud formation ... the transformation of water gas (vapor) into tiny flying bits of water-solid (water droplets or ice crystals) get altered locally. This can cause a "hole-punch" or "punch-hole" cloud. This is a big hole where the cloud that used to be there isn't any more. I'm not sure why this vernacular term is constructed as it is .... it should be "hole-punch non-cloud," but whatever. Here is a nice YouTube video showing some excellent examples of holes-punched-in-clouds:

(Gotta love the cloud music that goes along with that video!)

Well, just as a cloud can locally go away because of the effects of an aircraft flying through it, a cloud can go from not precipitating to precipitating. Indeed, in the example cited above, the hole punch cloud seems to have represented a place where condensed H₂O fell out of the sky as precip (snow, in this case). The findings of the paper coming out indicate that this happens at an altitude lower than the level at which contrails are generally formed. I get the impression that this surprised meteorologists. Apparently it was assumed that because we see contrails at high altitudes, that aircraft were having more of an effect at those altitudes than at lower altitudes. Thus, the discovery of an interesting and rather dramatic effect at a lower altitude is a bit of a shock. Perhaps more of a giggle than a shock, but something new and interesting, anyway.

The specific mechanism that appears to happen in the case of the present study is that water droplets aloft as cloud are converted by the effects of the aircraft into ice. This happens when the water is very close to be converted to ice anyway, as super-cooled water droplets (way below freezing but still liquid). Then the aircraft comes along, and primarily through the effects of the vortex, ice crystals are formed. Heat added by an aircraft is probably antithetical to the process, so this effect may be more likely to occur with turboprop planes than with jets. (Turboprops also fly at a lower altitude than jets, where this phenomenon is more likely to occur.)

The ice crystals are caused by swirling air at the propeller tips of turbo props' props at warmer temperatures (just below freezing) or by similar pressure changes over wing surfaces at colder temperatures (below about negative 20 degrees C). Essentially, the very local lowering of pressure chills the already super-cooled water to -37 degrees C or lower. That freezes the droplets.

The ice crystals become a surface on which neighboring super cooled water droplets can glom, and they then become part of the condensed ice. And snow is born. Well, it's a little more complicated than that, actually. What happens is the liquid and ice together at the same temperature cause a disequilibrium, causing the liquid to evaporate back into vapor and then re-condense on the ice crystals. Yes, dear reader, meteorology can be nearly as spooky as quantum physics.

In any event, the snow then falls to earth. If it melts on the way down, that's rain. (That's true of much rain, by the way. Those whopping big and freezing cold drops of water from a thunderstorm started out as globs of ice at high altitude.)

Some small version of this effect probably occurs fairy often, but it is when the effect continues after the aircraft has passed through that it is notable. The exhaust trail of the aircraft may spread over time, the evaporation/condensation process can propagate itself, ice-9 like (as in Cat's Cradle), across a large area, or heat caused by the growth of ice crystals causing a local thermal rise, thus mixing layers and causing more condensation of supercooled liquid droplets.

Sometimes the "hole" in the clouds is a big blobby thing, and that's the "hole-punch" or "punch-hole." Sometimes the "hole" is long and thin, and that's called a "canal."

The conditions for this effect to occur are somewhat rare. What is needed is a midlevel liquid-layer topped stratiform cloud. That gives the right combination of super-cooled droplets, pressure, etc. for the aircraft to "seed" the cloud. The authors note that about 7.8% of the earth's surface is covered by this sort of cloud, on average.

You can get a preliminary copy of the paper here, and there's a press release here.

Heymsfield, A., Kennedy, P., Massie, S., Schmitt, C., Wang, Z., Haimov, S., & Rangno, A. (2008). Aircraft-Induced Hole Punch and Canal Clouds: Inadvertent Cloud Seeding Bulletin of the American Meteorological Society, preprint (2009) DOI: 10.1175/2009BAMS2905.1