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d ~ 192'960'000 km: day 75

We're coming up to the spring equinox, the official start of spring for many people, and it really feels to me like the seasons are changing.The seasons on earth are, of course, the result of the 23 degree tilt in our axis of rotation.

But what might our world be like without that tilt, and without its changing seasons? We wouldn't experience the usual swings between summer and winter, obviously, but would we have a permanent spring or autumn climate, or might the lack of axial tilt have different implications for our environment?

The earth hasn't always rotated with a 23 degree tilt. Pretty much nothing about the Earth's climate stays constant if you wait long enough, and that tilt is no exception. It wobbles up and down by a couple of degrees every 41,000 years or so (at the moment the tilt is slowly decreasing), and the strength of the seasons the earth experiences changes with it. When the tilt is greater, summers are warmer and winters are colder, and when the tilt is smaller there's less of a difference in the seasons. These repeating cycles in the strength of the seasons probably play an important role in forcing the huge climate shifts of the glacial cycles that the earth has experienced over the last million years - and that's all with changes of just 2 or 3 degrees in the tilt.

For fun, I set up a relatively simple model to simulate what the climate on an earth with a 0 degree tilt might be like. There are a few details that make this more of a toy than a serious scientific study, but we can still use it to illustrate some of the things that could happen in a 0 degree world. To start with, of course, the seasons disappear: although the weather is still different from day to day, February is much the same as June and October. However, if you guessed that the earth's climate in a 0 degree tilt world would permanently be stuck halfway between our usual summer and winter, you'd be wrong!

A good way of imagining what it would be like to live on the 0 degree tilt world is to see how the ecosystems that we know from our 23 degree world would fare if we and they moved there*. The top panel shows a very simple way to characterise the climate of our 23 degree world in this kind of scheme. Greens show areas predominantly suitable for types of forest, browns are drier areas and grasslands, with grey for tundra, yellow for deserts and barren areas and ice caps in blue. There's a lot of fertile vegetation in this view of our world, with some desert in the hotter, drier areas and tundra and polar ice right up in the north.

The bottom panel shows what our toy simulation of a 0 degree world looks like. This climate is much less suited to our usual types of vegetation, with much larger barren desert areas, and a huge expansion of polar ice over Asia and North America. The
area suitable for vegetation at in the northern hemisphere shrinks dramatically, and northern Europe swaps its forests for tundra. The average temperature here in Britain sinks to a cool 7 degree C all year round, only varying by a couple of degrees warmer or cooler at most. Not everything would change for us, though - we'd still get about as much rain every year in a 0 degree tilt climate as we do now.

So, the earth's 23 degree tilt doesn't just give us the variations of the seasons and all the wonderful things we'll be seeing from this series - it's really important for setting the basic foundations of the environment we take for granted in our part of the world. As you can see, we'd have a very different planet without those 23 degrees.

NCAS-Climate Dept. of Meteorology, University of Reading

* if the earth really had a non-seasonal climate, totally different types of vegetation would certainly evolve, so this is just a simple way of visualising what the different climates would be like.

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d ~ 102'912'000 km: day 40 of Earth's orbit

Your average cute fluffy cumulus cloud consists of about 400 metric tonnes of liquid water. And it's up there, a couple of kilometers above your head, looking innocent while apparently defying the laws of gravity. Surely, clouds should fall down. Why don't they?

The first thing to know about these clouds is that they're not static. They're like a fountain inside, as warm air from below pushes up through the middle of the cloud. When each individual updraft reaches the top, it pushes out a bump of cloud that we can see. That's why cumulus clouds are lumpy on top - each bump is the result of one of the warm air plumes that helped build the cloud. Here's a great time lapse video of this internal fountain process.

You can see the continual building of clouds from successive thermals. The important point to take from this clip is that a lot of the air inside a cloud is moving upwards.

How about the water?

The liquid water in a cloud is in very tiny water droplets, and each droplet is around 0.01mm in diameter. That means that if you took a lump of water with the same volume as a sugar cube and you split it into a billion pieces, each fragment would be the same size as a cloud droplet. The droplets are very spaced out, so the amount of liquid water in a litre of cloud is pretty small. That's why you can't drink fog.

If there was no air in the way, gravity would make a cloud droplet and a lump of lead fall at the same speed. But air pushes back on things that are falling through it, and that push becomes more important as the object gets smaller. I like the way that said this, although in a different context: "You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, and a horse splashes." Yuck. The tiny cloud droplets have a very large surface compared with their mass, so the air slows them down even more than it slows the mouse and the final droplet speed is only about 3mm per second.

The end result? Individual cloud droplets are falling down, but very very slowly. And the air that carries them is moving upwards faster than they're falling. Imagine a slinky coming down an escalator moving upwards, and you'll get the idea. So those tiny droplets are stuck, up there in the cloud. And that suits me just fine. I don't have an umbrella that would withstand 400 tonnes of water falling on it all at once!

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d ~ 61'747'200 km: day 24 of Earth's orbit

"Drivetime" seemed a misnomer for Simon Mayo's early evening show on Radio 2 as my car inched out of Ö÷²¥´óÐã TV Centre and into the homeward crawl.

Twenty minutes and 200 yards later, Simon read out a question from a listener whose five year-old daughter had asked "What would be different if the Earth spun the other way?" Easy, I thought - everything would travel the other way across the sky and we would greet the sun from the west every morning.

My smugness lasted until Chiswick, when I realised I'd forgotten the little matter of Coriolis.

The Coriolis effect transfers the spin of the earth into the circular motion of winds around a weather system. Storms spin anti-clockwise in the northern hemisphere and clockwise in the south. Reverse the rotation of the Earth and you put the storms into reverse too. Interesting, but apart from confused weather forecasters I couldn't imagine a huge impact.

Reaching the M4, the pace of traffic is picking up to a gentle jog and this seemingly simple question is quickening my thought processes too. What about the jet stream?

This river of high altitude, fast-moving air steers the mid-latitude depressions across the planet from west to east. Swirling masses of cloud and rain are pushed from Japan to the Pacific coast of America, and from Newfoundland to Cornwall. Reverse the flow and climate changes dramatically. The British Isles loses the moderating effect of weather from the Atlantic. A harsher continental climate becomes more likely, with a predominantly easterly flow bringing bitter Siberian winds in winter and hot, dry weather in summer. Goodbye green and pleasant land.

Finally on to the M4. Now that we're really moving, the constant stream of traffic reminds me of the trade winds, another crucial part of our planet's circulation system.

The sun heats the atmosphere more at the equator than it does at the poles. On a stationary Earth the warm air would rise at the equator, moving to the poles where it would sink and flow back to the equator along the surface. Nice and simple.

Rotation complicates things. The flow breaks up into three separate cells known as the , the Ferrell cell and the Polar cell. Northward and southward-moving surface winds generated by the cells are then deflected to right or left by our old friend the Coriolis effect and we end up with the trade winds.

These constant easterly winds in the tropical regions were the motorways of the seas for sailing ships. A captain heading out of southern Spain could depend on picking up the northeast trades for a free ride to the Caribbean. Again, reverse the Earth's spin and the whole thing switches. Patterns of human discovery, subsequent empire-building and the resulting political geography would all be different.

The trade winds also affect the distribution of rainfall across large parts of the planet, influencing the position of some deserts and rainforests and interacting with periodic events like El Nino. It's reasonable to assume that a reversal would alter the pattern of habitable land.

Conclusion - change something as simple as direction of rotation and you change the planet we know.

And beware the innocent questions of five year-olds!

Peter Gibbs is a Ö÷²¥´óÐã weather forecaster and Met Office meteorologist

We want to hear from you. If you have a weather-related question for the 23 degrees team to investigate, let us know.

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