Observing ice crystals

Mon, 04 Apr 2011 12:59:39 +0000

d ~ 241'843'200 km: day 94

In my previous post I mentioned that clouds of snow crystals are above us all year round, thousands of metres above us in the atmosphere. In this post I want to show you how we can continuously observe these ice crystal clouds using radar. Radar works by transmitting a short burst of radio waves (a 'pulse'), then listening to see if part of that pulse is reflected back. By timing how long it takes for the reflections to come back to the radar, we can work out how far away the reflecting object is. Most people are probably more familiar with radar being used to detect aircraft. But just as aircraft reflect radio waves, so too do snowflakes - the only difference is that the reflections are a lot weaker.

So if we want to monitor snow crystals above us, we need a sensitive radar. Fortunately the UK has one of the best facilities for this in the world: the Chilbolton Observatory in Hampshire. Founded in the 1960s for radio astronomy work, since then a number of different radars, remote sensing and meteorological instruments have been installed there making it perfect for cloud and precipitation research. Crucially, a significant number of instruments are now operated 24 hours a day, 365 days a year, letting us continuously monitor the properties of clouds. Here's an example of a cirrus cloud:

On the left here you can see the signal we measured using our 'Copernicus' cloud radar - the colours are the reflections from the snow crystals. The warmer the colours, the bigger the crystals are. On the right you can see a photograph taken by an automated sky camera - this is what the same cloud looks like to the naked eye. In spite of how tenuous the cloud looks in the photograph, the radar demonstrates this is a big mass of ice: the cloud is 3km (just under 2 miles) deep. The streaky character of the cloud is very obvious in both images - this is caused as ice particles are formed initially in narrow regions where the air is rising near the top of the cloud. The crystals then grow, and fall, and as they do so they form these streaks. The streaks are curved because the wind is strong up at 8000 metres, and this blows them off-course as they fall.

We can measure how fast the crystals fall by making use of the 'Doppler effect' where the reflected pulse is shifted in frequency (this is the same as when a police car siren is high pitched coming towards you and lower pitched as it goes away from you). In this case the crystals were falling at about 1m/s, so it would have taken them a bit less than an hour to fall from top to bottom of the cloud. You'll see when they get to 6000m the crystals seem to disappear. This is because the air beneath is too dry, and the crystals evaporate before they get anywhere near the ground. This saga goes on all the time above our heads when there is cirrus around, it is incredible. Cirrus clouds actually warm the planet slightly (a greenhouse effect), and because they are so common across the globe, it is very important to understand how they work so we can get them right in climate models.

The second kind of cloud I'd like to show you is another common one - but one which we are now discovering is much more common than was previously thought. These clouds contain 'supercooled' water droplets. Amazingly these water droplets do not freeze, even at temperatures far below 0 degrees C. Because the droplets themselves are so minute (1/10th the width of a human hair) normal radar can barely detect them. But if we exploit a different kind of radar, which uses infrared light rather than radio waves, we can see exactly where these droplets are. It's called 'lidar'. Here's an example from back in December last year when things were very chilly and we were getting some snow:

The top picture is the lidar - you can see that red stripe at the top of the cloud - that's the supercooled water - it's really reflective to infrared light. The green stuff faling underneath is snow crystals, formed as some of the droplets freeze. The bottom picture shows the radar image - this just detects the snow crystals. The top of this cloud is -13 degrees C, and it's amazing that the liquid can be this cold without all freezing. Actually our work has shown that even as cold as -25C, around half the snow crystal clouds in the atmosphere have supercooled water at the top of them! Exactly how this water persists at top of these cold clouds is still not clear: even though the droplets do not all freeze, the presence of other ice crystals around them should in theory make them evaporate - but they don't. This is important, because the droplets reflect a lot of sunlight (just the same as they do to our lidar pulse), cooling the planet.

That's a small sample of some of the things we can observe at Chilbolton. If you want to see what's above your head right now, check out the realtime images.

What determines the shape of a snowflake?

Thu, 20 Jan 2011 10:00:56 +0000

d ~ 51'456'000 km: day 20 of Earth's orbit

The UK snowfall this past winter gave us a great opportunity to see what snowflakes really look like, rather than just admire them as patterns on Christmas cards. It's often surprisingly easy to observe the shapes of the ice crystals by eye as they fall onto your coat. About a week before Christmas it was snowing heavily in my garden and I took some photos of the crystals which were landing:

Chris Westbrook

I caught the crystals on an old black tin which I keep in my shed (so the surface of it was nice and cold). Most digital compact cameras have a 'macro' mode now, and this is what I used here. The first thing that struck me from these pictures was how much the shape varies from flake to flake. So why is that?

Well, the biggest influence on a snowflake as it grows is temperature. This was first unravelled by the Japanese physicist Ukichiro Nakaya in the 1930s, who was the first to grow snowflakes artificially in a lab. The incredible fact that he discovered is that ice grows in completely different ways depending on what the temperature is. At -2C it forms small thin hexagon shapes; go down a few more degrees to -5C and it forms long slender needle-shaped crystals. Go colder still to -15C and you get the classic six-arm stellar snowflakes we all know and love: these are often 100 times as wide as they are thick. A bit colder still at -20C we get crystals shaped like pencils. Below -25C it gets even more complicated, with many individual pencil or hexagon crystals growing out from a single point to form messy complex structures.

So the shape of a snow crystal depends on the temperature at which it formed in the atmosphere right? Well this is often kilometres above us in the atmosphere: so by the time they fall to earth they've been falling for a couple of hours. During this time they will have passed through a whole range of different temperatures, modifying the shape of the crystal as it grows. To complicate things further, as the crystals fall through the cloud they inevitably collide with each other, sticking together to form clusters. The closer the temperature is to 0C, the 'stickier' the surface of the ice gets - that's why you get the really huge fluffy flakes when the temperature is hovering close to the freezing: these big flakes are dozens of individual crystals stuck together like the one in the centre of my photo.

So how can researchers possibly observe this complicated evolution of snowflakes as they grow, fall and stick together? Well one way is using radar. Just as aircraft reflect radio waves, so believe it or not, do snowflakes. Our research group at the University of Reading uses radars based at the Chilbolton Observatory to unravel the evolution of snowflakes as they fall through clouds, monitoring their shape, fall speed and number. This may seem a bit academic, after all many of us in the UK only get a few days of snowfall each year. But of course, even if the temperature at the ground is above 0C, the temperature several kilometres above us is well below freezing. So clouds of snow crystals are above us throughout the year - it's just that if they fall to earth, they melt into raindrops first. In fact, the majority of rain in the UK forms this way - by melting snow formed many kilometres up in the atmosphere.

Chris Westbrook is based in the Department of Meteorology at Reading University

Have you got any interesting observations of snow crystals over the winter? Let 23 degrees know here.

BBC | 23 Degrees

Observing ice crystals

What determines the shape of a snowflake?