December 5, 2013
We’re in an age when scientists can detect the infinitesimally tiny particles that endow atoms with mass and are probing some of the deepest mysteries of biology, such as how experiences and memories can be passed down through an organism’s genes.
Thus, it may come as a surprise that we still don’t understand the mechanics of a relatively simple natural phenomenon: snow.
The formation of snowflakes—essentially, the idiosyncratic way that water crystallizes when suspended in our atmosphere—is an extremely complex process that still hasn’t been fully described by scientific formulas. “People think that a snowflake is just a frozen raindrop,” says Caltech physics professor Kenneth Libbrecht, who’s spent the past few decades studying the process of snowflake formation. “But that’s sleet, just little ice cubes, and not even close to what a snowflake is.”
Over the course of his research, Libbrecht’s work has grown to encompass art and science. He’s produced both scientific papers and hundreds of beautiful photos of natural snowflakes (which he’s published in several different books and had featured on U.S. postage stamps), and also devised ingenious ways to artificially grow snowflakes in a lab to study their formation in microscopic detail.
But it all started, he says, with a trip back to his childhood home of North Dakota. “I was visiting my family back there, and I went outside, and there was all this snow on the ground,” he says. “I suddenly thought, ‘Why don’t I understand more about these snowflakes?’”
That led Libbrecht to begin studying the dynamics of snowflake formation in his lab, in between researching more esoteric subjects like turnable diode lasers and the noise released by supernovae. “I realized that a lot about snowflakes is just not very well understood, and that ice is a pretty inexpensive material to work with,” he says.
The formation of even a single snowflake is a complex event at the molecular level. In nature, it begins when a cloud’s water vapor condenses into water droplets. Even at temperatures below freezing, though, most of these droplets typically stay in liquid form, because they need a particle upon which to freeze: either a dust particle or a few water molecules that have
arranged themselves into the hexagonal matrix that characterizes ice.
Once droplets begin crystallizing on a central particle, though, the process accelerates rapidly. With a crystal nucleus in place, the supercooled water molecules in the surrounding water droplets readily condense on the crystal, adding to its growth in a geometrically regular way. By the time the large crystal (which we call a snowflake) has left the cloud, Libbrecht estimates that it will have abosrbed the water from about 100,000 nearby droplets.
All that might sound simple, but as Libbrecht and other scientists have discovered, slight changes in the circumstances of these crystals—the cloud’s humidity and temperature, for starters—can lead to radically different-looking flakes. To better understand these dynamics, Libbrecht realized, he needed a way to observe the actual growth process of snowflakes. Without a way of embedding himself in a floating cloud, he decided to develop a method for artificially growing snowflakes in his California lab.
“To get an individual crystal growing in such a way that it looks like a snowflake is not easy,” he says. “If you want frost—just a bunch of crystals all growing at once—that’s pretty simple, but individual crystals are trickier.”
Libbrecht’s process, developed over the past few years, is done in a cold chamber and takes about 45 minutes in total. He starts with a completely clean piece of glass, and scatters many microscopic ice crystals onto it. With a microscope, he isolates a particular crystal, then blows slightly warmer humid air onto the glass. The water vapor condenses on the seed crystal, just like in a real cloud, eventually forming a visible snowflake.
Working with this process, Libbrecht has determined the temperature and humidity levels that lead to each particular kind of snowflake. “I call them ‘designer snowflakes,’ because you can change the conditions as you grow them and predict what they’ll will look like,” he says. Among other things, he’s found that a snowflake with a thin edge grows faster, causing the edge to sharpen even further, ultimately leading to a relatively large flake. Snowflakes that begin with blunter edges, however, grow more slowly and remain blunt, leading to blocky prisms, rather than elegant plates.
Eventually, when Libbrecht wanted to publish a book on his work, he discovered that, although they were good for their time, most of the photos of snowflakes available were out of date, like those taken by Wilson Bentley in the 1930s. In response, he began photographing them himself in high resolution, using specialized equipment and at times colored lights to give the clear flakes increased color and depth.
The answer, it turns out, is a math problem. If you define a snowflake as a mere ten molecules of water, then it’s possible for two different flakes to be identical at the molecular level. But for a full-size flake, he says, it’s extremely unlikely that you’d fine two identical ones that occur naturally—the same way that the odds of two identical human fingerprints are exceedingly small. “Once you start making things even slightly complicated, the number of possibilities grows astronomically, and the probability of even having two snowflakes that look remotely alike drops to zero,” he says.
August 21, 2013
Jason Ahrns, a graduate student at the University of Alaska-Fairbanks, and other scientists from the U.S. Air Force Academy and Fort Lewis College—all part of a project sponsored by the National Science Foundation—have been on a mission. This summer, the group has taken to the skies in the National Center for Atmospheric Research’s Gulfstream V research aircraft, logging a total of 30 hours over multiple flights, in search of sprites.
Sprites, also known as red lightning, are electrical discharges that appear as bursts of red light above clouds during thunderstorms.Because the weather phenomenon is so fleeting (sprites flash for just milliseconds) and for the most part not visible from the ground, they are difficult to observe and even more difficult to photograph, rather like the mischievous air spirits of the fantasy realm that they’re named for. Ahrns and his colleagues, however, have captured extremely rare photographs of the red lightning, using DSLR cameras and high speed video cameras positioned in the plane’s window. The researchers hope to learn more about the physical and chemical processes that give rise to sprites and other forms of upper atmospheric lightning.
What’s it like to capture images of some of nature’s most short-lived and erratic features? I questioned Ahrns over email, and he explained what sprites are, why they occur, how scientists find them and why he’s so interested in the elusive phenomena.
First of all, what is a sprite?
A sprite is a kind of upper atmosphere electrical discharge associated with thunderstorms. A large electric field, generated by some lightning strokes, ionizes the air high above the cloud, which then emits the light we see in the pictures. They obviously beg comparison to the regular lightning bolts we see all the time, but I like to point out that the sprites are much higher, with the tops reaching up to around 100 kilometers, and higher. A lightning bolt might stretch around 10 kilometers from the cloud to the ground, but a sprite can reach 50 kilometers tall.
Under what conditions do they occur?
They’re associated with positive lightning strokes, which is when the cloud has a buildup of positive charge and releases a bolt of lightning. Negative strokes, from a buildup of negative charge, are about 10 times more common, so sprites aren’t strongly associated with the most common kind of lightning, but it’s not really that uncommon either. More than just a positive stroke, the more charge that was moved during the stroke, the better the chances for a sprite. So we look for a large positive charge-moment-change, which is basically the positive strokes weighted by how much charge was moved. Most large thunderstorms seem to produce the conditions that lead to sprites, but some more than others. We just look for a storm with a history of lots of large positive charge-moment-change and go look at it.
What’s your scientific background? And how did you get interested in sprites?
I’m primarily an aurora researcher, that’s what I’m doing my thesis on at UAF. I got involved in sprites because one of my graduate committee members is organizing these campaigns and needed some extra help. I thought sprites were fascinating, and my advisor was supportive of me branching out a bit, so I hopped aboard the team.
From what I understand, not much is known about red lightning, discovered just 25 years or so ago. With the NSF project, what are you and the other scientists hoping to learn? What are the biggest questions you have?
With this campaign we’re focusing on three questions. First, what basic physical and chemical processes are occurring? It’s still not clear what exactly is happening in a sprite, and why there are different kinds of sprites, and what conditions give you a column sprite vs. a carrot sprite, for example. (All the sprite names just refer to their shape.) Next, do sprites have a large scale impact on the middle atmosphere? Sprites clearly represent some kind of transfer of energy, but is it on a scale that has a significant effect on the weather and climate? We can’t answer that without studying them. And, then, what can we learn about basic streamer physics? The tendrils coming off the bottom of the sprites are ‘streamers’—little balls of ionization—moving about. Streamer speed and lifetime is related to air density, so studying sprites in the very low density upper atmosphere is like looking at streamers with a magnifying glass in slow motion, though they’re still quite fast!
How many sprite-hunting missions have you been on?
Personally, this is my second aerial campaign. The first, in 2011, flew a total of 40 airborne hours, and this campaign did another 30 hours. It’s probably around 15ish total flights. The same crew, minus me, did one other aerial campaign in 2009.
What conditions, times of the day, areas of the country and altitudes are ideal for these flights?
The midwest is productive, mostly because it gets these powerful thunderstorms that last all night. Obviously, we need it to be dark, but other than that the time of night doesn’t seem to matter much, only how strong the storm is and how much powerful positive lightning it’s producing. We do notice that when the storm is going good it produces the column sprites and carrot sprites, but as it dies off it seems to switch over to less frequent, but bigger and brighter, jellyfish sprites. We fly as high as we can get, usually between 41,000 and 45,000 feet, but that’s simply to get a view over the clouds. We’re still below the sprites.
The lightning lasts just milliseconds, so I’m especially curious about how you photograph it. What equipment do you use?
For the still photographs, I just set my camera (a Nikon D7000 and a fast lens) facing out the window and set an intervalometer so the camera just constantly snaps pictures. Then I go through later and delete everything that doesn’t have a sprite in it. It’s the same principle as lightning photography; it seems like you’d have to get the timing just right but it’s really just statistical, if you snap a bunch of pictures one of them is going to get something sooner or later. I probably snap on the order of 1,000 pictures for every sprite I come away with.
For the high speed video cameras, the camera has a buffer that constantly cycles through the previous however many frames of video, and when I see a sprite I hit a trigger that tells the camera to stop and save whatever it just recorded. When we’re running at 10,000 frames per second, the buffer fills up in about a second, so that’s how long I have to recognize a sprite and hit the button. This can be pretty taxing on a slow night when you have to watch nothing happen for 45 minutes straight and still be ready with that less than one second reaction time.
Can you describe the setup? How do you actually take photographs from the plane window?
A picture is worth a thousand words, right?
And for the high speed video…
We have an internet connection aboard the plane so we can watch weather conditions in real time. We just point the above cameras at the most productive looking part of the storm and wait for sprites.
How rare are photos like these that you have taken?
As far as I can tell, they’re pretty rare. There are some sprite images taken with meteor cameras and webcams out there, but they’re usually low resolution due to being very far away and using a wide angle lens. I’ve seen two or three sprite images taken with a DSLR, but they’re still from the ground and a good distance away, and usually shots of something else that got lucky with a sprite in the background. I have the advantage of being up in the air, close to the sprite producing region, with a good guess of where the sprites will appear, so I can use a lens with a narrower field of view to capture the sprite up close.
As for the images I got of blue jets, as far as I can tell they’re actually the first images of jets taken with a DSLR. That makes some sense, because the jets are a lot closer to the top of the clouds than sprites so much harder to see from the ground. Being in the air is a major advantage.
What do you find artful about the images, if anything?
I think there’s a really otherwordly starkness about them. Take this one (above), for example. You’ve got this nice serene starfield, and some cool, calming blue light coming up from the lightning below. Then BLAM! This weird, menacing, totally alien looking sprite just takes over the whole scene, like ‘I’m here, what are you gonna do about it?’
Hans Nielsen, the principal investigator on the campaign (and my previously mentioned committee member), says this one (below) reminds him of the classic Dutch paintings, with its sepia tones and slight blurring from the atmospheric haze.
What have you learned thus far about sprites by participating in this project?
Personally? When I joined the 2011 campaign I knew nothing about sprites beyond the Wikipedia entry. I learn more every night of the campaigns, listening to the others talk about conditions beforehand, what we’re seeing during the flights and our ‘what we did right, what we did wrong’ discussions over post-flight beer. I’m still a newbie compared to the other guys, but I’m now at the point where I can field most general public questions about sprites and sprite hunting.
Where and when are you flying next?
Nothing is set in stone, but we’d really like to fly again next summer. Hopefully we can make that happen.