Hole Punch Clouds and Making Rain
Chris - First of all, tell us what actually is the cloud phenomenon that you've been looking at? How would people recognise it?
Andy - This started back in the late '50s and early '60s but with more and more aircraft flying, these features have become more apparent. The most obvious examples are very thin cloud layers, mid-level clouds like 5 kilometres to 7 or 8 kilometres, and what they appear to look like, from photographs and people observing on the ground, is basically just like a circular hole inside of this thin cloud. So the cloud is surrounding the hole and often, almost always, snow or even rain is observed falling out of the layers.
Chris - Was there debate over whether aeroplanes were actually causing this phenomenon or was that always a given and we just didn't understand why it was happening?
Andy - The thought was early on that these were either meteorites, burning up cloud, or maybe they were rockets. Even more recently, there were observations that look like maybe they were flying saucers in the clouds! A spectacular example of that occurred a year and a half ago in Moscow, and it looked basically like a flying saucer, but what I believe it was, was precipitation falling out of a deeper cloud layer, so you couldn't see the hole because it was covered over the top by cloud, but had this feature that was almost like a flying saucer.
Chris - So how did you step-in in trying to understand what was actually going on in these clouds with these funny features?
Andy - Well, first thing that we just coincidentally noticed was four years ago, we had a project in Colorado flying our organisation's research aircraft through clouds to study their properties, and we made the observation that we inadvertently flew through a snow band which looked like it was produced from one of these holes. And it turned out, we went back and identified the two aircraft which actually flew through that cloud layer and they turned out to be two turbo props. Then we actually can measure the contents of this precipitation and that led us to extend the study to what looks like possibly not only turbo prop or turbine or propeller aircraft were inducing this effect, but jet aircrafts are as well. So we had a spectacular example in this study we did in the last year of almost 100 aircraft which were generating little lines, but some narrow lines may be 2 or 3 kilometres across, some were reaching 200 kilometres across of holes in clouds over Texas.
Chris - So what you were effectively doing was marrying up flight data where the aeroplanes were with observations of these features, presumably from satellites, in order to say, "Look, the planes are present at the scene of the crime every time, so we can be reasonably sure that planes are triggering this thing to happen."
Andy - That's right. Possibly not every time, but the circular holes are just giveaways. And so, what we did was we went to this terrific satellite image with a hundred of these holes with lines in them, identified the aircraft as both jet aircraft and propeller aircraft, turbo props, every aircraft you can imagine, military... So we identified aircraft, and then the next question we had is, "how long do these features last?" We think of an aircraft as just being a little pencil line through a cloud, and yet, these holes are visible in satellite imagery and elsewhere. And so, we decided, "okay, we would like to study this effect." So we used probably the most sophisticated weather research forecast model which is developed at my organisation, the National Centre for Atmospheric Research, and use that model with the same type of conditions, thermodynamic atmospheric conditions that were present in that terrific case that I just referred to and we were able to actually model the mechanism by which these holes were produced.
Chris - So what is the mechanism behind this then? What actually causes the aeroplane to make this phenomena occur?
Andy - So around the tips of propeller blades for propeller aircraft and for turbo prop aircraft, there is expansion of the air, that's the engines pushing back behind it, and the expansion process causes cooling. Cooling then can cause cloud droplets, which are at the ambient temperature anytime they're below about -10 degrees centigrade, to cool to the point where they spontaneously freeze at -40. And this is a well-known phenomenon of spontaneous freezing. It's just that the aircraft, just like a weather modification activities, act as a nucleus that provide ice crystals and these crystals then seed the cloud.
Chris - So does this only apply to a subset of clouds because I would think that clouds that are quite high up in the air will already be at that much lower temperature? So will this be clouds closer to the ground then?
Andy - It could be clouds in the middle atmosphere in the summer. They're at the temperatures where this process can occur, which is where almost all of the sightings of holes in clouds have occurred. But in the winter time where there is thicker cloud, often near the surface in a large percentage of cases in mid-latitudes then this effect can occur at low altitudes and you wouldn't necessarily see a hole but there is precipitation that will be formed when temperature is below -10 degrees.
Chris - So the plane goes into the cloud. In the case of propeller driven aircraft, you get zones of low pressure around the tips of the propellers. Do you get the same phenomenon caused by jet planes because of the wing tips which have their associated low pressure regions around them?
Andy - That's absolutely right and so again, that low pressure is due to expansion of air and that then has the effect of cooling cloud drops at their ambient temperature down to as low as minus 40 degrees where they spontaneously freeze. So it really opens up almost any type of aircraft to this process and we have documented this for just about any type of aeroplane.
Chris - So what actually happens once the hole has got seeded? So the aircraft has gone through, it's created the optimum conditions to make one of these holes on its way through, does the hole then grow because as you recruit more particles in and you get downdrafts as the particles fall out, do you then get more stuff coming in to replace it that then itself turns into more ice particles that drop out?
Andy - What happens is that the process of going from cloud droplets to ice crystals liberates heat and if those crystals continued to liberate heat, they generate, just like a cloud, this buoyancy. That causes rising in the centre and cooling, or subsidence; subsiding air, and evaporation of cloud drops on the edge. Even though it's just maybe one burst of particles, this process is self-sustaining. We've documented 2 hours or more following passage of an aircraft that these holes are still observable.
Chris - And how big do they become?
Andy - We have documented 6 to 10 kilometres from a little pencil slice through a cloud from an aeroplane.
Chris - So given that you've got this phenomenon, you can put the aircraft at the scene of the crime - we know they're doing it. Is it actually a meteorologically significant impact? In other words, how much water is the cloud robbed of in terms of its precipitation potential and if you summed all of the impacts of this around the world, is it important?
Andy - It's not important on a global scale, but where it might be important is in the winter time, in mid-latitudes with busy airports like Heathrow, Frankfurt, De Gaul, O'Hare in the US, those airports which have considerable amount of traffic, those clouds can be continuously seeded in the winter where there's low cloud.
Chris - And what sort of volume of precipitation are we talking about? Is it significant over background?
Andy - We have documented a case at an airport in Colorado where the precipitation rate is up to 5 centimetres per hour. And if it lasts over that spot, if there are lots of aeroplanes flying through, we're talking about 5-centimetre per hour raise under conditions where there's a lot of aircraft traffic especially on climb out.
Chris - And are there any other things that we can learn about cloud science or atmospheric science on the basis of what you've discovered?
Andy - The most interesting thing is that we are able to document what's called dynamic seeding and in many of the earlier cloud seeding attempts have tried to invigorate clouds by dynamic seeding where they put ice in the cloud at a certain point, and they hope that the ice grows and closes these updrafts and downdrafts that are around the edges or circulations. This is a good example of this process and we're able to model it.