BBC | 23 Degrees

Day 363: Two things have stood out this year

Thu, 29 Dec 2011 09:00:00 +0000

Distance travelled ~ 932'318'400 km

It’s very liberating to be completely and utterly soaked by a rainstorm, especially when what’s falling out of the sky are raindrops the size of large peas.

I tried to explain this to the director and crew who were huddled beneath enormous umbrellas, missing out on all the fun. They were not convinced. They had not come to India during the monsoon to get wet. That was my job.

The thing is, weather is fun. We are brought up to hide from it a bit, to carry on (usually with a British stiff upper lip) in spite of it. But it’s not going away, so I think that we might as well appreciate it. As long as it’s not giving you hypothermia or sunburn, why not just play with whatever the atmosphere is doing today?

For 23 degrees, we’ve been lucky enough to travel to some fantastic places in our global weather patterns. Different parts of the planet receive different amounts of energy from the Sun, and this is just the start. That energy is carried around the planet by the ocean and atmosphere, and the result is a giant pattern of hot and cold air, dry and moist air and huge swirling wind systems. The pattern is never exactly the same from one year to the next, but there are features that are present all the time (tropical thunderstorms and the jet stream), or that return every year (spring showers and hurricanes).

Two things have stood out for me. The first is how little we appreciate the depth of the atmosphere. I realized this properly while looking at the tornado we found in June.

It’s almost certainly the biggest thing I’ve seen whose scale I’ve been able to understand. We know that the clouds are high up, but until you see a single thing joining the clouds to the ground, you have no idea what “high up” means. The atmosphere is big, vertically as well as horizontally.

The second thing is how little we actually look at the sky, especially in Britain. We’re too busy getting on with things on the ground, and anyway there are lots of buildings and trees in the way. Above us thousands of tonnes of nitrogen and oxygen are flowing around, carrying water and energy, and all we do is complain about it when it gets uncomfortable down here. But if you look up, you can usually see some of the structure of the atmosphere, and that gives you a hint about the larger scale patterns that cover our continent and our planet. Next time you look up at the sky, imagine how all this is connected to the weather over Iceland and Morocco and Costa Rica.

The last day of filming for this series was on the south coast of England, near Beachy Head. We haven’t done that many days filming in the UK, and it was as though the weather was determined to prove that it shouldn’t have been neglected.

As the day went on, we had incredibly hard rain followed by hail, very strong winds and occasional spells of sunshine. My boots filled up with water and at the end of the day I felt as though I’d been in a giant washing machine for a few hours. It was impossible not to be impressed by what the atmosphere was up to, even on our own doorstep.

Behind the scenes: Physicist in freefall

Wed, 23 Nov 2011 17:00:00 +0000

Distance travelled ~ 840'555'200 km

Being surrounded by sky is not a natural place for a human being. We have evolved to scoot about on the bottom of the atmosphere, stuck to the ground, and we don't often look up. Even when we do, we tend to see the sky as flat - clouds, the moon and aeroplanes move sideways across the sky. It's easy to forget that the sky has depth too, and that air in the atmosphere moves up and down as well as sideways.

Your perspective changes quickly when you're in freefall, three thousand metres above the Earth's surface and travelling downwards at 120 mph.

Skydivers relish the sense of freedom that falling through the sky brings. There is nothing to get in the way, nothing touching you and a whole extra dimension to play in. For the air in our atmosphere, three-dimensional movement is normal. At the place where I jumped out of the plane, in Arizona, air that starts about 10 miles up is gradually sinking towards the ground. The air doesn't make the squeaking noises that I did, but then it isn't falling nearly as fast - it's a few millimeters per second on average. I was falling through a giant atmospheric waterfall, but a very slow one.

It's not just in Arizona that this happens. Although weather maps tend to show sideways winds, the air making up those winds is all rising and falling as it travels around the Earth. The paths of air parcels weave in and out of each other, making the Tokyo subway map look simplistic by comparison.

Image courtesy of Tokyo Metro

All this is very interesting, but not much comfort to a plummeting physicist. I don't think that I really breathed during the 40 seconds of freefall. Then the parachute opened, everything slowed down, and my brain stopped panicking and started appreciating what was going on around it.

Seeing the layers of the sky is fascinating. Floating down past a cloud is amazing - a fluffy cumulous cloud is telling you that there's been a little puff of air upwards in that location. We can't really see the structure of the atmosphere, but seeing a cloud from the side makes it easy to imagine the turbulent swirls that are mixing all that air up.

After five minutes of sharing the three dimensions of the sky with the clouds, we arrived at the landing zone and my feet touched the ground again. I was very happy to feel something solid under my feet, but there was also a small sense of loss. I was back to crawling around on the bottom of our fabulous three-dimensional atmosphere, and my understanding of the depth of the atmosphere was again limited to hints given away by the clouds. But I remember what it felt like, and my view of the sky will never be quite the same again.

The Santa Ana winds and your bicycle pump

Mon, 14 Nov 2011 18:00:00 +0000

Distance travelled ~ 817'507'200

Air is funny stuff. The oxygen that we take from it is our most fundamental necessity, but air is invisible, odourless, colourless and easily ignored. Day to day, we have better things to think about. But air is doing some interesting things while we're busy ignoring it, and one of those things has the potential to cause huge damage in Southern California this month.

Let's have a think about what air is. What you're taking into your lungs at this very moment is a bustling crowd of billions of molecules, zooming about at speeds of about 1150mph, bouncing off each other and anything else they hit. It's busy down there in the microscopic world that we can't see.

To get to Californian weather, we need to know something about gases. Air temperature is just a way of measuring how fast the gas molecules are all zooming about. If they're travelling on average at 1100 mph, it's about zero degrees Celsius. If they're moving at 1300 mph, it's 100 degrees Celsius, and so on. So temperature represents the amount of energy that's carried by those air molecules.

Now here's the really interesting bit. It happens every time you open a pressurized fizzy drink. As you unscrew the bottle, high pressure air rushes out and when it meets the lower pressure outside, it expands. But for the gas to expand, those molecules have to move further apart from each other and that takes energy. So they use some of their movement energy. As they move apart, the molecules slow down, and that means that the temperature goes down. Put your hand over the bottle opening, immediately after you open it, and you'll feel the cold. So when air expands, it cools, and when it's compressed, it heats up - that's why your bicycle pump gets hot as you pump air into a tyre. Physicists call this adiabatic heating.

Why should all this matter for California? It's because an atmospheric version of the bicycle pump happens there on a huge scale at this time of year. To the east of California there are vast deserts at an altitude of 2 km. Weather systems over those deserts push air westwards, so it flows down the slope towards the ocean. Air pressure gets higher as you go downwards, because there's a greater weight of air above you. So the air flowing down the slope is compressed as it goes and it heats up. Then there isn't just a wind, there's a really hot wind, about 20 degrees Celsius higher than temperature in the desert. These winds are called the Santa Ana winds, and as the air flows down towards the ocean they get funnelled down canyons, increasing the wind speed even more.

Image credit: NASA/GSFC/LaRC/JPL, MISR Team

The result is that after the hot dry summer, San Diego, LA and everything in between get dried out even more by a massive atmospheric hair dryer blowing down from the high deserts. And if a spark starts a fire, there isn't much to stop it. This is why wildfires are such a hazard at this time of year, and why California's fire service is now on high alert. There have been huge destructive fires in the past few years, and Californians just have to prepare for them and do everything they can to prevent a blaze starting. It all happens because of a fundamental rule of physics - that air gets hot when you squash it. At this time of year, it's a rule that many Californians probably think they could do without.

Why are clear nights so cold?

Wed, 09 Nov 2011 15:00:00 +0000

Distance travelled ~ 804'321'600 km

It's the season for baked potatoes, parkin, treacle toffee and bundling up to stay warm. I love the sharp, cold starry evenings and being able to see my breath - it's not every day that you get to make your own cloud! But in the past few days I've remembered that there's a price to pay for being outside on those fabulous clear evenings. It's cold. Frigid, frosty, freezing. Your knuckles go red and the inside of your nose feels like it's full of ice. Why can't we admire the autumn stars in comfort?

The answer is to do with how energy gets from place to place, and how much clouds get in the way. We may associate clouds with bad weather, but when it comes to nighttime, clouds are our friends.

Image credit NASA

Our planet's energy comes from the Sun, mostly as visible light. We know that - it lights up our world. Air is invisible, and by definition visible light travels straight through it. So on the way in, the Sun's energy is carried by all the colours of the rainbow, straight through the atmosphere and all the way to the ground. The ground absorbs that energy and warms up. Black tarmac absorbs more heat than white sand, but they all capture some.

Next time you make some toast, watch the element in your toaster. As it gets hotter, it glows, first dull red, then bright red and then orange and yellow. Hot things give away their energy by glowing - it's a fundamental rule of physics - and the colour tells you their temperature. The ground under our feet, along with you and everything else around you also glows. But because those things aren't as hot as your toaster, they glow in the infrared, which we can't see directly.

So the ground glows in the infrared all day and all night, constantly emitting invisible energy back upwards. Some of this energy heats the air near the ground, but some keeps going upwards. And here's where clouds matter at night. Clouds are really good at capturing that infrared radiation and sending it back down the Earth. They act like a blanket, trapping heat between the ground and the clouds. If there are no clouds, the energy from the ground just goes up, up, and away...

Whenever it's a clear night and you can see lots of stars, there is nothing to trap all that infrared energy, so it's lost to space and we feel cold. If it's cloudy, there are no stars to see, but we have a nice warm blanket above us, keeping the heat in. The fact that Earth gains energy as visible light and loses it as infrared light is really important for the heat budget of our planet, not just for freezing astronomers.

Sadly, this means that stargazing will always require extra layers. Happily, that means extra excuses in life for hot chocolate. In fact, just writing this has made me feel chilly. It might be hot chocolate time right now!

Keeping track of time

Fri, 28 Oct 2011 18:00:00 +0000

Distance travelled ~ 773'769'600 km

Noon is personal. It's the time of day when the sun is highest in the sky and your shadow is shortest. It's the reference point for a clock that is always with you, because you can be your own sundial. The spinning Earth provides us with a built-in natural chronometer. Brilliant!

That clock has served nature well for millennia - birds sing at dawn, foxes come out at dusk, humans go to sleep when it gets dark, and we all live day to day. One daily cycle follows another. But the growth of human civilizations and the need for greater co-operation than ever before meant that humans had to control time instead of being controlled by it. Clocks were standardized. The day was split up into hours, and humans had to agree to start work, meet or provide services at specific times. It was the only way of co-ordinating a civilization. But the Sun was still the reference point.

Faster travel, and inventions like the radio and telephones, meant that time zones had to be invented. Local noon where I am, in Southampton, happens four minutes later than local noon in London, so society agreed that all of the UK would be in a single time zone, for convenience. 12 o'clock in Southampton now happens at the same time as 12 o'clock in London. The Sun is no longer the reference point. The shortest shadows still happen around lunchtime, but you can't set your watch by that any more. And as clocks got more and more accurate, we discovered that the shape of Earth's orbit means that the length of a day varies by about a minute over the course of each year. Solar time seemed to be almost unhelpful in our standardized world.

So humans weren't living and working according to the Sun any more, but sunlight hadn't gone away.

The standardization of time meant that some people were sleeping when it was light and working when it was dark. And so Daylight Savings Time was invented, to try and compensate for the limitations imposed by standard time. The clocks go back in the UK Sunday 30 October 02:00am in a return to GMT, after a summer of allowing us an extra hour of daylight in the evenings (upcoming DST times). Of course, we don't actually get an extra hour of daylight - we just move our time reference to take that hour off the start of the day and tack it on the end. We couldn't have built the modern world without standardized time, and now we're trying to patch up some of its inconveniences.

Image courtesy of C.G.P Grey. For more on daylight saving watch his video.

Should we bother? Every clock change causes sleep deprivation, a demonstrated drop in productivity and a day where the whole country risks turning up at the wrong time. It's a nice ritual to mark the changing of the seasons, but is it worth it?

I think that the crux of the argument might be in how society is changing. Fifty years ago, a giant siren marked the time when work began and ended in factories. The development of our society relied on us all working together, at the same time. It was an enormous example of human co-operation. But now, we live less constrained lives. We work flexibly, and internationally. The standardization of the working day is disappearing - some businesses start work at 8am, some at 10am. I adapt my daily routine so that I can go running when it's light, whether that's in the morning or the evening.

As long as I get my work done, maybe it doesn't matter when I do it. So I can choose for myself what I do with my daylight hours, irrespective of the official time that they start and end.

Do we even need time zones any more? Maybe the logical end to this argument is that we could have just one Earth time, so that everyone has lunch at a different official time, but it's still when the sun is more or less overhead. I'm not necessarily advocating for that, but it's not as close to science fiction as you might think. Scientists in every country frequently record data using "Universal Time" or Zulu time, which is GMT. That way there's no confusion at all over when it was recorded, wherever you were on the planet.

So, is the era of British Summer Time/Daylight saving time over? What do you think?

Behind the scenes: sharks and stalactites

Mon, 03 Oct 2011 14:00:00 +0000

Distance travelled ~ 709'020'800 km

Getting geared up for the dive

Sharks and stalactites may be close to each other in the dictionary, but you would think that reality keeps them a safe distance apart. For a start, sharks aren't known for inhabiting caves, and every stalactite I've ever seen has been in a cave. Secondly, stalactites can't grow underwater and sharks can't breathe if they're taken out of water. That sounds like a clinching argument if ever I heard one, but the thing I love about science is that our world is more complicated and interesting than that. Not only did I see lots of sharks swim past lots of stalactites this week, but this weird combination tells us something fundamental about our planet. And it's not that a flock of flying sharks has started spelunking because they suddenly fancied bats for dinner.

Belize is just next to Guatemala and south of Mexico, tucked into the back of the Caribbean sea. Its coastline is littered with islands and coral reefs, but what brought Jacques Cousteau here in 1970 is circular deep blue hole in the reef. We arrived in Belize last Monday laden with SCUBA gear, all ready to explore that hole.

Going into the hole was pretty eerie. There is sand and coral right up to the edge, and then the vertical wall just drops away into the darkness. We left all the brightness and light and colourful fish behind, and sank slowly. After going down a little way, all I could see was the rock wall stretching into the gloom. I found looking away from the wall a bit disconcerting because it felt as though anything could swim out of the black, even though I knew perfectly well how unlikely that was. We kept going down further and further, and I stared at the wall, straining to see what on earth brings people here. A reef shark swam past just two metres underneath me. And then the gloom readjusted itself just in front of me and I was looking at a stalactite that was nearly a metre wide at the top where I was, and was probably 5 metres long, pointing downwards into the depths. It was monstrous. There was an overhang, like an upside-down shelf a few metres deep, and looking along it I could see other stalactites hanging down, all of a similar size. We swam along the overhang, and the sharks cruised past us a few metres further out from the wall.

Dives that deep have to be short, and we had work to do, so it was only that night that the scale and the incongruity of what I'd seen sank in.

The size of the stalactites helps you understand the size of the story they're telling. Both are gigantic, almost too big to fit into a human brain. The reason that the stalactites are down there at all is that during ice ages, sea level gets much much lower. 15,000 years ago, the last time those stalactites were growing, they were on a cliff in dry air because sea level was 120 metres lower than it is today. That's the sort of fact that you can read and understand logically, and it's something that I had known for years, but it's hard to digest properly. Read it again: 120 metres lower. That is an awful lot of ocean that wasn't there. Floating in the darkness with 40 metres of water above me, next to a rock wall that kept going downwards as far as I could see, I came closer than I ever have to really understanding the enormity of the changes that ice ages bring to Earth. Oh yeah, and there were sharks too.

Behind the scenes: Nate got "disorganised" while remnants of Katia came to UK with a force

Wed, 14 Sep 2011 12:30:00 +0000

Distance travelled ~ 659'976'800 km

Hurricane hunting was not supposed to be like this. The Sun was shining, there were butterflies everywhere, and there wasn't enough wind to blow out a candle on a birthday cake. On the plus side, we had to give ourselves top marks for trying and I didn't have to get drenched again.

Twenty four hours earlier, things had looked very different. We had been tracking several Atlantic storms, and finally Tropical Storm Nate was forecast to make landfall in the Gulf of Mexico as a category 2 hurricane. I've never paid that much attention to tropical storms in the past, but it turns out that storm-monitoring is surprisingly addictive. Tropical disturbances in the Atlantic often start out near the coast of Africa, and then they crawl across the ocean to the west, growing or petering out as they go. The storms move at about 15 mph, so they'd lose a race to any half-decent cyclist. That gives the nascent addict many happy days of monitoring storm strength and direction. There are also exciting milestones such as the day the storm is given its name, and most important of all, the day the maximum sustained winds first reach 74 mph and the storm is declared to have graduated to hurricane status. Most storms don't make it that far. If they drift too far north, they get broken up or run out of fuel, and they can be decapitated by high-level winds, never giving them a chance to grow.

Satellite image captured 09-sep-2011

Tropical Storm Nate was interesting because it had skipped the slog across the Atlantic ocean, and had instead formed entirely inside the Gulf of Mexico, stuck in the gap between the Yucatan and the rest of Mexico. It was pootling westwards at only 3 or 4 miles an hour, feeding off the nice warm bath it was trapped in, and forecast to hit the Mexican coastline near Veracruz as a category 2 hurricane. We thought that we finally had a winner, and off we went.

The five of us arrived in Veracruz in the dark, only 18 hours before the centre of the storm was due to hit the coastline. It was horribly hot and sticky, and the evening gloom made everything feel very ominous. The wind was picking up and we were excited and a bit nervous about what would happen in the morning.

What happened was that we learned that Tropical Storm Nate had apparently become "disorganized" overnight. I've got friends like that, but I wasn't expecting it from a giant atmospheric whirlpool. Josh Wurman (our hurricane expert) inspected the satellite images on his computer screen and made "meh" noises whenever the director asked him where exactly the storm had gone.

This visible image from the GOES-13 satellite on Sept. 12 at 10:45 a.m. EDT shows Nate's remnant clouds southwestern Mexico and moving into the eastern Pacific Ocean. Credit: NASA/NOAA GOES Project

The tight spiral that we had seen the previous day had widened, split and was indeed looking pretty disorganized. It rained hard for a couple of hours that morning, so we did film some nasty weather, but soon the sun and the butterflies came out again. We stared at the flat calm ocean and wondered whether to blame the butterflies for flapping their wings.

In our absence, of course, the remnants of Hurricane Katia were passing over Scotland. The winds in Scotland this weekend reached twice the speeds we saw in Mexico. We are not bitter about this. Honest. We had all thought that filming a hurricane would be much easier than filming a tornado, just because hurricanes last for weeks and their tracks can now be predicted very accurately. But we learnt the hard way that the complications of our atmosphere are still not perfectly understood, and that even a large storm can vanish almost overnight if the conditions are right. But still, it's all part of experiencing the weather, and I'm actually quite glad that the town where we were was able to have a normal Monday morning, rather than dealing with the damage and flooding that a hurricane would have left behind.

Coriolis force.....what force?

Thu, 01 Sep 2011 09:00:00 +0000

Distance travelled ~ 626'155'200 km

Last week we all watched Hurricane Irene march up the east coast of the US, a gigantic atmospheric whirlpool that caused a huge amount of damage. Out in the mid-Atlantic, tropical storm Katia has just graduated to being Hurricane Katia on her way west, and there are likely to be more as the season goes on. These storms start as isolated disorganized thunderstorms, but as they grow, they also start to rotate.

(This GOES-13 satellite image is of Hurricane Irene just 28 minutes before the storm made landfall in New York City. The image shows Irene's huge cloud cover blanketing New England, New York and over Toronto, Canada. Shadows in Irene's clouds indicate the bands of thunderstorms that surrounded the storm. Credit NASA/NOAA GOES project.

And while watching the stunning satellite images of these mammoths of the atmosphere lumbering across the ocean, I thought about why that is.

Spinning tops and sycamore seeds, wind turbines and the wheels on the bus... lots of things in our world go round and round. But all of those examples are solid objects with an obvious central axis to rotate around. A hurricane is a pattern of flowing air and there's no visible container to hold that air together. The air on one side of it is not attached to the air on the other side of it. Even an average hurricane can be 1400 km across, covering 13 degrees of latitude. What could possibly be happening to make that much of our atmosphere spin in such a coordinated way?

The short answer is that a hurricane spins because our planet spins, and the link is a funny thing called the Coriolis force. It's funny because it's not actually a force at all, but a consequence of the fact that we're going round and round in circles while we're looking at the weather. Ours is an odd point of view, but since we're stuck with it, science came up with the idea of this extra force to compensate. It's a neat psychological trick - instead of accepting that we're the ones doing odd things, we claim that we're normal and the weather is behaving oddly by adding in the Coriolis force. So what is this mysterious force?

Imagine that you're on a roundabout. You want to play catch with a friend, but they don't like roundabouts so they're standing on the ground a few metres away. The roundabout is going round anticlockwise and you are standing on the edge facing outwards. Next time you pass your friend you throw the ball to them, and as soon as it's in the air, it travels in a straight line. You keep rotating around, so it looks to you as though the ball is mysteriously curving to the right. It isn't, it's just that you're rotating to the left. This magic force that seems to have taken over the ball from your point of view is called the Coriolis force. Try it, next time you're at a children's playground. You'll find that whatever direction you throw the ball in, if you're on a roundabout going anticlockwise, the ball always looks as though it's curving mysteriously to the right. And since in the northern hemisphere, Earth is rotating anticlockwise beneath us, anything that moves always ends up slightly to the right of where you think it should be.

Let's get back to that hurricane. Its centre is a huge area of low pressure, so all the air is trying to rush from the outside where the pressure is high towards the middle. But as it moves inwards, all that air is curving to the right because of the Coriolis force. The last part of the puzzle is something you can see in action on our roads all the time. Next time you're at a roundabout, stop and look.

Every car comes in towards the roundabout and has to turn left. But that means that the flow around the whole roundabout has to be to the right. And this is why hurricanes always spin anticlockwise in the northern hemisphere. All that inward rushing air is turned to the right, so the whole system goes round to the left.

These forces are relatively small, and to see their effects you need a very large weather system. So the spin of a hurricane is just a reminder that we travel 17 thousand miles every single day in the UK, as we spin around the Earth's axis. I feel dizzy just thinking about it...

Cloudbursts in Kullu-Manali India cause havoc...

Wed, 17 Aug 2011 15:00:00 +0000

Distance travelled ~ 588'206'400 km

"Cloudburst" sounds dramatic, doesn't it? As though a cloud was a large balloon filled with water and someone had just arrived with a very large pin. Pop! And then what went up must come down and the one place that you don't want to be is right underneath.

Although the real thing isn't quite like that, the people underneath can be forgiven for not caring about the difference. This week a cloudburst in the town of Kullu produced 176 mm of rain in three hours and drenched three villages. A cloudburst is defined as any rainstorm where the rain rate is greater than 10 cm each hour, and they are usually only a few km wide. Just stop and think about that... 10 cm of rain in one hour. That is enough to turn an entire village into a temporary river bed. And these are large damaging drops, accompanied by strong winds and thunder. Cloudbursts are very hard to predict, but devastating to the places that they hit.

Not long ago the 23 Degrees team were in India to film the monsoon rains

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A cloudburst is a hard thing to study, because it's tough to be in the right place at the right time. So we don't have a good description of exactly how they form or what causes them. They're relatively common during the monsoon in India, especially close to the mountains. Scientists think that these huge amounts of rain come from very tall clouds, reaching up to 15 km into the air, and so the droplets falling from the top of that huge cloud have been able to hoover up lots of smaller drops along the way. That's why the raindrops are so big when they get to the bottom.

The trigger for the storm seems to be that something nearby (for example shape of the local mountains) starts a huge upside down fountain of air that's very strong but only covered a very small area. Warm moist air pours upwards, releasing energy as it rises, and this builds the storm cloud very quickly. Once it gets big enough and if the conditions are right, all the water that had been lifted up comes down. Very quickly. This is similar to the process that generates normal storms, but what makes cloudbursts different is that this whole process happens inside a small area and very powerfully.

The problem for the areas that are vulnerable to these events is that accurate prediction of cloudbursts is hard to do and would require some really detailed weather monitoring. You would need to have a weather station every couple of kilometres all over those areas, and installing and maintaining that system would be very expensive. To understand and predict really local events, you need to have really local measurements.

Hopefully, new technologies and new monitoring systems will solve some of these problems soon. But until then, cloudbursts are going to remain a dramatic reminder of the invisible complexity of the air we breathe, and how little control we have over it.

Next few days of rain in UK great for rainbow appreciation

Thu, 11 Aug 2011 15:30:00 +0000

Distance travelled ~ 572'823'200 km

What is a rainbow?

We all love rainbows. Even though we take it for granted that the sky changes colour all the time, there's something about these enormous semicircles that makes us want to smile, point and take photographs. And they're worth looking at, because you're seeing nature's paintbox directly. The entire spectrum of visible light is in there, and every single thing that we can see comes to us as a combination of those colours.

The next few days could be great for rainbow appreciation, especially on Saturday and Sunday. A mixture of sunshine and rain is forecast for all of the UK, as several weather fronts pass over us.

Image courtesy of the Met Office

So keep your eyes on the sky, and if you see bright sunshine appearing suddenly after a shower, turn away from it so that you are facing your shadow. If you're lucky, you'll then be looking at a giant arc of colour.

A rainbow is a beautiful example of physics in action. The only ingredients are direct sun and water droplets (rain or mist) some distance away. When sunlight travels into these water droplets, it changes direction (this is called refraction) and the important part is that the path of blue light is bent a bit more than red light. We see sunlight as white, because it's a mixture of all the visible light colours. When sunlight goes into water droplets, bounces off the inside once and then comes out, the blue light comes out at a slightly different angle to the red light. You can only see a rainbow when the sun is behind you, and that's because the light that goes into those droplets and bounces once comes out at about 40 degrees to the direction it went in. The different angles of red and blue light mean that you have to look a bit higher up to see droplets which are sending red light in your direction - that's why red is on the outside of a rainbow.

A rainbow is a very personal thing. Imagine a line that comes from the sun, goes through your head and then keeps going forwards in front of you. You will see a rainbow 40-42 degrees away from that line, and it would be a full circle if the ground didn't get in the way. But the person standing next to you is seeing a different rainbow, because the line joining their head and the sun is a different line. They see the colours from different droplets. You are the only person who can see your rainbow. This is why you can never get to the end of a rainbow - if you move closer to where you think the end is, the rainbow you see is a different one and you can't touch that one either.

If you can't wait for the next shower to see all this, it's easy to make your own rainbow. All you need is a sunny day and a garden hose. I had a go last weekend with my mum wielding the hose (she also saw it as an opportunity to water the garden), and the photo I took is below. Stand with the sun behind you - the lower it is in the sky, the better - and get your helper to spray water a couple of metres in front of you. The shadow of your head will be at the centre of the rainbow circle. It'll be easier to see the rainbow if there's a darker background - in the photo below you can see that the rainbow is easier to see against the darker plants. A rainbow in the sky is exactly the same, but bigger because it's further away.

Even though a rainbow looks ethereal and delicate, bear in mind that the energy from the sun carried by those colours is what powers our world.

If you happen to spot any rainbows over the next coming days, send them to 23degrees@bbc.co.uk

150 years since the first UK weather "forecast"

Mon, 01 Aug 2011 08:00:00 +0000

Distance travelled ~ 546'291'200 km

Victorian England in 1861 must have been a brilliantly exciting place to live. Fabulous inventions and intriguing new scientific ideas were popping out of the population like endless possibility popcorn. London was the largest city in the world, the theory of evolution was being debated in Oxford, Maxwell had just written down the fundamental equations of light, and solar flares and Neanderthal fossils had just been discovered.

Image courtesy of the Met Office

(The forecast itself appears immediately below the table of station data and consists of the following:

General weather probably for the next two days in the-
North-Moderate westerly wind; fine.
West - Moderate south-westerly; fine.
South - Fresh westerly; fine. )

But no-one really knew much about large scale weather patterns. Weather just happened. Every year an enormous number of lives were lost at sea, as ships sailed into storms and never sailed out again. In the late 1850s there was the first hint of a possibility of changing this situation because communications technology had just taken an enormous leap. The electric telegraph now made it possible to collect weather observations from all over England, assemble them and send them out again to whoever needed to know, all in less than a day.

Telling people what had happened was useful, but not as useful as telling them what was going to happen. The man who made the first attempt to bridge that gap, and who coined the phrase "weather forecast", was Admiral Robert Fitzroy. Thirty years before, he had been captain of the Beagle voyage that carried Charles Darwin, so he was no stranger to the perils of weather when at sea. In 1854, Fitzroy was given the job of collecting data on weather at sea, leading what later became the Met Office. After a few years, he saw that weather systems could be tracked. He became convinced that predictions were possible and that storm warnings would save many lives. His bosses did not agree that predictions were realistic, but in spite of them Fitzroy started to issue storm warnings in 1861. Then, on the 1st of August 1861 (exactly 150 years ago), Fitzroy issued the first ever weather forecast for the general public, published in The Times. This earned him a slap on the wrist and a huge amount of criticism, because it was considered that the forecasts could not possibly be accurate. After a lot of debate, the public forecasts ended in 1866.

I think that you have to admire Fitzroy for trying. He saw the big picture, and walked the first few steps along a very hard path. Even now, when we have a sky full of satellites and enormous computational weather models, we don't always get it right.

In the end though, Fitzroy was given the most appropriate recognition possible. When the region that used to be known as Finisterre on the shipping forecast was renamed in 2002, they decided that it would be called Fitzroy. So now, four times a day on longwave radio, Fitzroy's name is broadcast to all shipping in the vicinity of the United Kingdom. Next time you hear "Trafalgar, Fitzroy, Sole, Lundy..." as you fall asleep, think of the man who started the journey towards the weather forecasts we all take for granted today.

(With thanks to Malcolm Walker of the Royal Meteorological Society's History Group for his help with information about the history of Fitzroy's contribution)

Cromarty, Forth, Tyne, Dogger... what does the shipping forecast tell us about our planet?

Wed, 13 Jul 2011 19:40:00 +0000

Distance travelled ~ 499'123'200 km

The absolute magic of the shipping forecast is hard to explain to people outside of the UK. Four times a day, a slow soothing voice reads out 350 words which almost no-one understands and almost everyone loves. But hidden in this swirl of familiar words is the story of giant swirls in the atmosphere, mixing air up and moving energy north. There is always another shipping forecast tomorrow, because there is always more energy from the Sun to take that journey.

Let's imagine the shipping forecast as if we were sailors out at sea. First of all, the General Synopsis gives us the position of a region of low pressure, for example in "Irish Sea". Then area forecasts work clockwise around the British Isles, giving the conditions in every section of the sea, and we can put it together like a jigsaw puzzle. We can get the map out and as we listen, we can draw the wind direction and speed on the map for every section. As we draw on the wind arrows, we see that they point anticlockwise around the pressure low. The words have painted a picture of the weather, and it's a swirl. But what's going round what and why?

The UK is in an interesting place from a meteorological point of view, because we're right in the path of a huge boundary in the atmosphere, the boundary between warm air in the south and cold polar air in the north. The Sun heats Earth most at the equator, and all our weather is just the atmosphere's way of moving this energy towards the poles. The poles lose energy to space faster than the equator does, so it's a bit like a conveyer belt for heat. But it's not a smooth process.

Next time you add some milk to your tea, mix it just a little bit and then watch. Rather than the milk slowly diffusing into the tea, the mixing all happens in the swirls. And the same is true in the atmosphere, but the swirls can be a thousand miles across. These are the cyclones and anticyclones, always moving towards the east, which make up the bulk of UK weather patterns. Where warm and cold air meet, at around our latitude, they get mixed up in swirls. At the centre of each swirl is either a low pressure or high pressure region, and weather fronts (boundaries between warm and cold air) are moving around that region to mix everything up. Warm air moves north, cold air moves south, and overall, energy travels towards the north pole.

So the soothing tones of the shipping forecast are telling the story of the movement of energy on a planetary scale. Next time you're listening to the shipping forecast at bedtime, imagine the swirls in your cup of tea as you drift off to sleep and listen: "Northerly or northwesterly 5 to 7. Moderate or rough. Mainly fair. Good. "

A date with History: Fram polar expedition leaves Norway June 24 1893

Fri, 24 Jun 2011 16:30:00 +0000

Distance travelled ~ 450'240'000 km: day 175

Ice is an important contributor to our weather, but finding out about the Earth's ice was very hard work. It's still less than a hundred years since Amundsen first reached the South Pole (he arrived on the 14th of December 1911, beating Scott by 35 days). My favourite polar expedition is one that is rarely mentioned because it wasn't an expedition in the normal sense. It was made by a ship called the Fram, and there were no sleds or dogs, and there wasn't even any navigation. The Fram was designed to become a piece of pack ice, and it spent three whole years doing just that (from 1893-‐1896). Its voyage was not only a fantastic oceanographic experiment but also an amazing illustration of the dynamic nature of the Arctic.

We tend to think of polar ice as being a bit like giant dollops of whipped cream on top of a dessert, as smooth white splodges that from space almost look as though they're about the run down the sides of the Earth.

Ice at the South Pole is fairly static, but things are very different at the North Pole because the Arctic is an ocean, not a continent. In the Arctic ocean the ice is dense pack ice, always moving, breaking apart and reforming. Pushed by ocean currents below and by wind above, Arctic ice never gets to sit still. The ocean at the North Pole is about 4.2 km deep, and there's no flag there and no base because no piece of ice is ever over the North Pole for any length of time.

Fridtjof Nansen designed the Fram to test the theory that there was an ocean current that crossed the entire Arctic basin. He reasoned that if the jostling ice chunks were pushed along from one side to the other, a ship could be pushed along too. It would be a bit like a conveyer belt to the North Pole, and all he would need was patience and the right ship. The danger was that the ship would be crushed by the huge slow-‐moving pieces of ice, so the Fram was designed with a very round keel so that it would be lifted out of the water and carried rather than crushed. This plan worked beautifully, except that the Fram never quite got to the pole. The ship and her crew were pushed about all over the place [link to map], eventually drifting to the edge of the Arctic ocean and freedom after three years. The furthest north she reached was nearly 86 degrees, 280 miles from the pole.

Even though she didn't get to the North Pole, the Fram clearly demonstrated the dynamic nature of ice in the Arctic, and we now know that individual pieces of ice can indeed travel from Siberia to Greenland in 3-‐4 years, or can go round a sort of giant ice-‐roundabout (called the Beaufort Gyre) in 7-‐10 years. Most polar expedition stories are about humans struggling against natural forces to achieve their goal, but I like the Fram's exploits because they're about humans working with the enormous forces of this planet instead of against them. What are your favourite examples of that?

Update: After weeks in the Midwest the 23 Degrees team finally catch a tornado!

Tue, 21 Jun 2011 16:30:00 +0000

Distance travelled ~ 442'521'600 km: day 172

The team were pleased late yesterday (June 20 2011) afternoon to finally catch sight of a tornado

Right at the end of our last day here, the wait finally paid off. We were driving in the convoy of radar trucks towards a storm system, which looked to us exactly the same as the others we'd seen, and a calm voice on the radio said "tornado at 1 o'clock". We drove up and over a slight hill and suddenly the grey sky in front of us resolved itself into an enormous dark tower, moving sideways across the road in front of us. It was probably a mile or so away, and we were all overwhelmed by just how massive it was. The photos don't do justice to the scale of it. We hopped out of the car and started filming, but after only 3 or 4 minutes, the tornado started to shrink, and had soon vanished. It had looked so solid and substantial, but what we were looking at was just the low-pressure core of the rotating storm. When the pressure drops enough, the moisture in the air will condense and so a tornado is a part of the cloud reaching down to the ground. But if the pressure rises just slightly, all that water will evaporate again, and that huge structure will vanish. And so our tornado evaporated, and the core became invisible again.

We drove down the road, to the place where the tornado had crossed it. There were ploughed fields on either side, and you could see the churned up soil where the tornado had passed across one field, across the road, through the field on the other side and over to a cluster of trees about half a mile away. We looked at a couple of trees next to the road, a short distance from the track. 15 minutes before, these had been healthy trees, but now they were ripped apart, and there was a really strong smell of sap to remind us how recently they'd been shredded.

There was no doubt in any of our minds that it had been worth the wait, and we felt very lucky to have been there for the short time the tornado had touched down. Seeing the destruction was sobering, and it brought home how much energy is swirling around in the atmosphere above us. A tornado is a reminder of how our atmosphere can be simmering away and still be almost completely invisible, until a threshold is crossed and a really destructive storm results.

Midwest USA update: what is it like to chase tornadoes?

Tue, 21 Jun 2011 15:00:00 +0000

Distance travelled ~ 442'521'600 km: day 172

(Helen Czerski and some of the 23 Degrees team stayed back in Midwest USA to continue tracking tornadoes whilst the others moved on to the next stop in Egypt to film the Summer solstice. Here is Helen's update from yesterdays chase).

20 June 2011, DOW stopped to scan the sky

If you stand outside in the Midwest, half of everything that you can see is sky. It's easy to see why everyone here is interested in storms - the land is completely flat and open, and you can watch these huge structures in the sky changing and growing from miles away. The rain, lightning, hail, the sudden darkness and the occasional unexpected rainbows are awe-inspiring. By comparison, a human being is tiny, slow and vulnerable.

The process of finding the eye of the storm is less awe-inspiring. The reality of storm-chasing involves patience more often than it requires adrenaline. As I write this, we're waiting with the storm scientists while they decide where we're going today. There are ten scientists and students with laptops, all looking at the current radar data and weather forecasts, discussing whether different wind conditions are more or less likely to combine to produce supercells, and also just waiting to see how the weather changes as the day goes on. Storms tend to develop later in the day, because that's when the ground and air have heated up enough to start building storm cells. So, we have to wait for the complexities of the atmosphere to call the shots. This is why these scientists are out here - the more they understand about tornado formation, the better the forecasts will be. And it won't just be scientists and tv crews that will benefit, but the people whose homes and businesses might get hit.

20 June 2011, storm that came closest to dropping a tornado

When the storms do start to develop, the pace changes dramatically. The hunt is on. The radar trucks stop every few minutes to scan the sky in detail. You're right underneath a huge black cloud, and it's moving and changing as you watch it. You drive through heavy rain, hail and really strong winds to get ahead of the storm , and all the time it's above you, metamorphosing, with huge blocks of cloud ploughing through the sky below the main storm cloud. You can see the lower clouds rotating around you. And this was the point yesterday when I finally really understood why people spend so much time trying to see tornadoes. I stepped out of our car, and the speed with which the low clouds were spinning around me was stunning. I could see them rotating around a half-mile wide circle almost as if they were gathering themselves together, and the whole thing was huge and incredibly energetic. It was hard not to think of it as alive, because it was so dynamic. Being right underneath such a piece of atmospheric architecture was genuinely awe-inspiring, and definitely worth the wait. And this one wasn't even a tornado, just a mesocyclone, which is the stage before a tornado.

All this is generated by simple physical processes, operating on a huge scale. Most of the time they're invisible, but when they're all concentrated in once place like this so you can see them, it brings home the scale and power of what's going on around us all the time.

The scientists are getting up, and a decision about where to go has been made. Today is our last chance to see a proper tornado, and it all depends on whether the huge forces in the atmosphere cooperate. All we can do is follow behind and admire.