A couple of months ago, BAbloggee Henrik Magnus Ulriksen sent me a link to a video of a cloud that frankly left me -- har har -- twisting in the wind. When I saw it, I had the strong feeling it was authentic; taken with a hand-held camera, it doesn't look obviously faked. But what it shows is very odd indeed. The video is of a cumulonimbus cloud, a puffy white cauliflower-shaped cloud that forms when warm air rises rapidly. The camera view is between two buildings, and you can see the puffy cloud top just below the center of the frame. Keep your eye on the little wisp of cloud just above the cumulonimbus, right in the middle of the frame.
[embed width="610"]http://www.youtube.com/watch?v=E4sY98zsBH0[/embed]
Did you see it? If it helps, the picture here shows you where to look. Starting at 9 seconds in, that little wisp suddenly snaps into a new shape, as if someone had stopped the video, waited for the cloud to change, then started up the video again. But it's clear that's not what's going on; the video is smooth with no transitions. Assuming the video is real, I had a sneaky suspicion it had to do with the electric currents generated inside the cloud, the same currents that create lightning. Clouds can carry huge electric potentials -- essentially, the ability to move charges around -- and that stored energy can be suddenly released, creating lightning. When that happens, the electric field resets itself, and starts to store up energy again. But I had no clue how that would make the cloud appear to dance like that! As it happens, by coincidence, I met Joel Gratz at the TEDxBoulder talk
in September. Joel is a meteorologist who runs websites like Open Snow
. I sent him the video, and sure enough a short while later he had a reply for me. Joel had sent the video to Walter Lyons, a meteorologist from WeatherVideoHD
, who was able to identify this phenomenon. Here's his reply:
The answer lies in this: ice crystals, especially long needles, tend to become aligned with the ambient electric field. So what you are seeing is sunlight reflecting off ice crystal faces that are constantly being oriented by the developing electric field just above the [cumulonimbus] top. Then there is a discharge in the cloud, and the field collapses momentarily, and the crystals begin to realign again. Then this just keeps happening over and over.
Aha! I hadn't thought of that. The outer surface of ice crystals can hold a static electric charge, similar to what happens when you rub a glass rod with a cloth, or rub a balloon on your hair and stick it to the wall. When placed in an electric field, the charges feel a force on them, and align themselves along the field. So all the ice crystals above that cloud top are aligned one way in the field. Then the field snaps (maybe due to lightning releasing the energy) and then reforms. The ice crystals change their orientation suddenly when that happens. So why does it look like the entire shape is changing? That's because ice crystals can act like little prisms, bending light when it passes through them (or they can act like mirrors, with light reflecting off their flat surfaces). When they float in the air you get all sorts of astounding and beautiful formations like sun dogs
, halos
, and more
. These all depend on the angle between you, the Sun, and the orientation of the crystal in between.
[UPDATE: Bill Beatty has more videos of this phenomenon. Thanks to @MattTheTube for the tip.]
In this case, some of the ice crystals are bending light toward the camera, and when the field snaps they rearrange themselves. Think of them like little flashlights, some pointed at you, some away. When the field changes, they all turn, so different ones are pointed at you, and ones that were pointed at you are now turned away. That's what happens when the field suddenly changes. The overall shape of the cloud hasn't changed; you're just seeing the light coming from it differently! I want to take a moment here and relay the fact that this is one of the coolest things I have ever heard. That video is truly bizarre (I've seen more than one website -- the kind that talks about "chemtrails"
seriously -- touting it as some sort of government conspiracy), It shows something I had never seen before, or even dreamed of. Mind you, I'm pretty familiar with the sky; I spend a lot of time looking at it. So this was pretty weird. Then to get an explanation that is not only, to me, obviously correct, but also so simple, so clean, and so wonderful... well, that's one of the things I love most about science. Some people think that science takes away the beauty and wonder of the world, but it doesn't. It adds to it. It takes something we don't understand and turns it into something we do. Instead of spinning conspiracy theories, we can use science to help us construct a more accurate, more complete, and more true view of the Universe. And yes, more beautiful as well. I'll take the beauty of truth any day of the week. Even a cloudy one.
Related posts: - Science Fare - The pressure of living on a spinning planet - The fist of an angry cloud - What I learned from Carl Sagan
