Episode 1 Orbit: Earth's Extraordinary Journey

All of us, every day of our lives, are on the move.

And we don't mean the morning commute

or taking the kids to school.

But a journey of epic proportions.

Even now, as you are watching this,

you're hurtling through space at 100,000 kilometres an hour.

Every year, our planet, the Earth, travels around the sun

and we go with it.

'I'm Kate Humble.'

This is it. The sun is directly overhead.

My shadow is directly below me.

In this series, we are going to follow

the Earth's voyage through space for one whole year

to witness the astonishing consequences

this journey has for us all.

'I'm Dr Helen Czerski and I study the physics of the natural world.'

Wow, look at that!

I'll be investigating how our orbit powers the most spectacular weather

and how it's also shaped and reshaped our planet.

We'll experience first hand the planet's most powerful forces.

This is the moment we've been waiting for all day.

And it's really raining hard now!

We're going to dive to the deepest depths.

And we'll reach for the greatest heights...

..all to bring you the story of our planet's voyage around the sun.

The Earth takes just over 365 days

to make one complete orbit around the sun.

In that time, it travels 940 million kilometres.

For one year, we've been following that epic journey,

every step of the way.

We're going to begin on the island of Andoya,

just off the northwest coast of Norway.

It's July the 24th,

and I'm here to enjoy a landmark in our journey around the sun.

Back in late May, the sun rose here and since then it's never set.

This is known as the midnight sun.

It's risen and fallen in an arc above the horizon

for the last nine weeks, but it's never dipped below it.

Today, all that is about to change.

The sun is going to set below the horizon completely,

for the first time in 64 days.

The midnight sun exists

because of a special feature of Earth's orbit.

As Earth travels around the sun, it doesn't spin upright.

It spins round an axis that's tilted by just over 23 degrees.

That means that in June,

the northern hemisphere is facing the sun to its fullest extent.

So, despite the Earth's rotation,

all the land north of the Arctic Circle

is bathed in sunlight all day and all night.

But, as the year progresses,

the northern hemisphere begins to point away from the sun

and periods of darkness gradually return to the Arctic.

The hour of sunset has come. It is now night-time, I think,

although it's slightly odd, actually. It hasn't gone dark,

but all the light has sort of leached out of the sky

and this is the very first time

that the sun has actually dipped below the horizon,

in this part of Norway, in over two months.

This first night is very short,

but from now on, they will get longer and longer.

By the time of the end of December,

Andoya will be dark around the clock.

Ten minutes ago, the sun set, and although we can't see it

because of the bank of cloud right on the horizon,

it's obviously beginning to rise again,

because the light is coming back into the sky.

This is because the northern hemisphere

is no longer pointing so directly at the sun,

The area experiencing 24 hours of sunlight has shrunk.

Andoya is now just on the wrong side of the line.

The coming of night to the Arctic

is an evocative symbol of the seasonal change

that we'll follow for the next five months, from July to December.

The cycle of sunset and sunrise is also a reminder

that the Earth isn't just moving around the sun.

It's also spinning on its axis.

Every 24 hours, the Earth makes one complete rotation.

As it does, day gives way to night...

...and back again...

...365 a year.

But the Earth's spin controls far more

than the cycle of day and night.

As we'll see over the next five months,

it plays a central role in creating

some of the most dramatic natural phenomena on Earth.

To understand how the Earth's spin can have so much influence,

we need to explore the place where it has its greatest impact...

...the atmosphere.

The spin of the Earth has a crucial influence on our atmosphere.

To find out why spin is so important,

we're going on a 25,000-metre journey up into the sky.

'To get there, I'm going to need the help of a team of people

'who know how to get to the edge of our atmosphere.'

OK, it's on. It's in mode five.

'This isn't exactly NASA, but, even so, we are about to visit a place

'that normally only astronauts can go to.'

Our vehicle is a balloon.

we're doing now is putting helium gas into it

and helium gas is lighter than the air around us here.

So, once it's full, this balloon

will just float upwards all the way through the atmosphere.

'The balloon's journey will show us why the Earth's spin

'has such a strong influence on the atmosphere.

'We've attached a GPS transmitter to track its journey

'and four cameras will record everything the balloon sees.'

We've finished setting up now

and hopefully this is the last this camera

will see of ground for about three hours. See you when we get back.

So it's gone.

I can still see it, just that tiny speck in the sky now.

To begin with, the balloon goes pretty much straight up.

This is what you'd expect,

because the atmosphere spins with the planet.

But then the balloon starts to move sideways.

We're going to follow it.

The balloon is being carried away from us.

This reveals a crucial fact about the atmosphere.

Although it spins with the Earth,

the atmosphere isn't completely locked to the surface.

It's actually a fluid, so it can move in different directions,

at different speeds.

Today, the balloon is being pushed east.

'We're driving at 50 miles an hour,

'yet it's still racing ahead of us, as it continues to climb.'

It's like a children's party game on an enormous scale, isn't it?

Yeah!

The balloon is now at 20,000 metres,

twice the height at which airliners fly.

It's nearly midday, but the sky is black.

We think the balloon right now is up near the top of its trajectory,

and up where it is, there's very little,

very, very low air pressure,

so about 95% of the atmosphere is below where this balloon is.

And because the pressure's so low, the balloon will have expanded

to about three times its initial diameter,

but that is about as much as it can take.

EXPLOSION

As it falls back through the atmosphere,

the burst balloon reaches speeds of over 100 mph.

That's just shredded balloon! That's amazing! Look at that.

That's just a fantastic picture.

We can see the Earth and we can see

black outer space outside it,

and in between the two, there's this fuzzy, blue line,

which is the atmosphere.

It's fantastic.

The atmosphere is a thin layer of air that spins with the Earth.

But it's also full of moving currents that help create the weather.

It's tempting to think of these currents as random and chaotic,

but they're not. They're organised into distinct patterns.

And the way these patterns are organised is controlled

by the spin of the Earth.

To see how spin can play such a powerful role,

I've travelled to Ecuador in South America for a very special drive.

Today, I'm going to get into a car

and drive faster than I've ever driven before.

Here's the car in question. Very ordinary.

Absolutely nothing special about it.

It doesn't have a huge engine. It doesn't run on rocket fuel.

What is special, though, is not the car,

but the road I'm going to be driving on.

It may look like a perfectly normal road,

but it's got two particular features.

Its location and the direction it's heading.

This road is right on the equator, and it's heading due east.

The speedometer of this car

is reading about 96 kilometres an hour,

which is roughly 60 miles an hour, but that's not strictly true

because I'm actually travelling a lot, lot faster than that.

As we travel around the sun,

the Earth's surface is spinning through space.

And the place where it moves fastest is the equator.

This road is spinning at over 1,000 miles an hour.

And because I'm heading due east,

in exactly the same direction as the Earth's rotation,

I'm not travelling at a mere 60 miles an hour, oh no.

I am also travelling at well over 1,000 miles an hour.

And given that I am the only car

on this piece of road, at this precise moment...

..I could be the fastest driver in the world!

The Earth moves fastest at the equator

because this is where its circumference is greatest,

so it has the longest distance to travel in a single day.

But this also means that the further away from the equator you go,

the slower you turn,

until, if you stood at the poles, you'd barely be moving at all,

just rotating gently on the spot in a 24-hour pirouette.

These different speeds create an atmospheric force

that has global significance.

You can see it in action at one very particular time of the year,

when it helps create the most destructive weather event

on the planet.

It's now early September.

Although the summer is almost at an end,

in the northern hemisphere, it has a sting in its tail.

Because this is hurricane season.

The development of a hurricane is a wonderful example

of how the Earth's spin controls the weather.

I'm hoping to see one in action.

So, we've been following these storms for the past few weeks

and all of them have their own stories.

So, here, I can see that Tropical Storm Maria

came all the way across the Atlantic and then fizzled out.

Katia came around and swerved around the coast of the US,

but didn't hit land at all, so we can't visit that one.

Tropical Storm Nate. Now, that one looks like it's got potential.

It's trapped in the Gulf, due to grow into a hurricane by tomorrow

and it looks as though it's almost certain to get to the Mexican coast.

24 hours later, I'm in eastern Mexico,

heading towards the Gulf of Mexico and the oncoming storm.

We're driving northwards up the coast.

Our storm is about 100 miles that way, coming across here.

So we're driving north so that tomorrow

we can be at the place where it crosses the coastline.

Next morning, and the first signs

of Hurricane Nate are reaching the coast.

The winds are building up

and the normal sunny skies

are replaced with cloud and rain.

At this time of year,

the Gulf of Mexico has the perfect ingredients to make a hurricane.

The sea is relatively shallow and close to the equator,

so the water gets particularly hot.

This water is warm, really warm

and the reason for that is that the ocean out there

has been absorbing the sun's energy, storing it up.

And now, it's that energy which can build tropical storms.

The way the storm is built is that the warm ocean

heats the air above it.

And once the air is warm, it expands and rises.

THUNDER RUMBLING

As the warm air rises, the pressure drops,

sucking in even more moist air, creating powerful winds.

But there's one final ingredient needed to create a hurricane.

It needs to start turning.

And that rotation comes from the spin of the Earth.

Out at sea, Nate has the characteristic rotating,

swirling clouds of a hurricane.

But, frustratingly, Nate begins to lose power.

Before it can make landfall, the winds die away.

Instead, the 2011 hurricane season

became famous for a different storm.

Hurricane Irene.

Unusually for a hurricane,

it travelled far enough up the east coast of the USA

to flood parts of New York City.

The powerful circulation of winds within a hurricane

is generated by the Earth's spin,

through a phenomenon known as the Coriolis effect.

Now, the Coriolis effect can be a little bit counterintuitive,

but there's a great way of seeing how it works

with a ball and a children's roundabout.

Now, let's say this is our planet, the northern hemisphere

and that's the North Pole.

Now, this planet isn't spinning,

so when I throw a ball in a straight line...it travels in a straight line.

But we live on a rotating world.

So, let's take our planet and make it spin,

round anticlockwise, like in the northern hemisphere.

So, now I'm on a spinning planet, things look quite different.

When I try and throw a ball in a straight line,

it bends around to the right.

From my point of view, this ball is always curving to the right,

even though I'm trying really hard to throw it in a straight line.

Now, the reason that this matters

is that this ball represents winds on Earth

and when the wind blows in the northern hemisphere,

the wind is also moved to the right.

In the southern hemisphere, the effect is reversed

and the winds bend to the left.

And that is all the Coriolis effect is.

A hurricane shows the Coriolis effect in action.

Winds are drawn inwards towards the low pressure

at the centre of the hurricane.

But as they head towards the centre,

the Coriolis effect makes them turn to the right.

This creates the hurricane's characteristic

circular swirl of wind.

It also means that the wind never reaches the centre of the storm.

So the eye of the hurricane remains calm.

The Coriolis effect is a direct consequence of our planet's rotation.

But it does more than just make hurricanes spin.

It's responsible for our climate patterns on a global scale.

When you look at the Earth,

you can see some fairly obvious bands.

White snow at the poles,

yellow deserts and then green vegetation in the tropics.

Each band reflects a dramatically different climate zone,

with its own distinctive weather.

These major climate zones are caused by the spin of the Earth

and the Coriolis effect.

To see how the Coriolis effect creates these global climate bands,

I've stayed on the equator

and travelled into the highlands of Ecuador.

It's late September and I'm here on a particularly significant day.

For thousands of years,

human beings have tracked and celebrated the progress of the sun.

This spectacular plateau was once the sacred place

of an ancient culture called the Quitos, who came from this region.

And you can see why they chose it.

It has uninterrupted, 360 degree views of the sky,

so they would have been able to watch the sun rise and the sun set.

Today is especially significant in understanding

why the equator plays such an important role

in the Earth's climate system.

It's noon on September the 23rd, the autumn equinox.

This is it. The sun is directly overhead.

My shadow is directly below me.

And for all those people living in the southern hemisphere,

they can think, "Yay!", cos summer is on the way for them.

But for us, who live in the northern hemisphere,

well, sadly, winter is on its way.

The midday sun is overhead on the equator on the equinox

because of the Earth's tilt.

After this September equinox,

the path of the midday Sun travels south until December,

before tracking back to the equator for the next equinox

and then into the northern hemisphere to bring us summer.

Which means that throughout the year, the equatorial regions

receive more of the sun's heat than anywhere else.

This has a profound effect on the Earth's climate.

The heat of the equatorial region

is the engine room for the world's climate.

A system of wind starts here

that dictates the climate across the whole of our planet

and that is all controlled by the spin of the Earth.

To find out how it works, I need to leave these beautiful highlands

and head down into the rainforest.

I'm travelling into the Amazon basin,

5.5 million square kilometres of rainforest

stretching across Ecuador and into Brazil.

This is the heart of the first climate zone,

the green band that's centred on the equator.

With all that heat concentrated at the equator,

you wouldn't necessarily expect it to be so wet and lush,

yet the majority of the world's rainforests

are found in the equatorial region.

There's about 2,000 species of tree here

and it's dark, in amongst the trees,

and that's because the very intense equatorial sunlight

doesn't really penetrate through that thick canopy of leaves.

The reason the heat of the equator creates this dense forest

also explains why this region

is the engine of the global climate system.

To see how it works, I need to get above the canopy.

The morning mists are a breathtaking sight,

but the movement of air also shows

an important atmospheric process in action.

If you remember from your physics lessons, hot air rises,

as we can see it doing beautifully over the trees here.

And as it rises, it cools and some of it then condenses

and falls back as rain.

But some of the air keeps on rising.

It's the start of an epic journey

that will see it come under the influence of the Coriolis effect

and then move on to create another huge climate band.

The air rises high into the atmosphere to around 15,000 metres.

This warm, rising air mass is then drawn towards the poles

as it tries to equalise the temperature gradient

between the heat of the equator and the cold of the poles.

But the air doesn't reach the poles.

Instead, it's bent to the right by the Coriolis effect.

By the time it reaches about 30 degrees north of the equator,

it's moving almost parallel to the equator.

But it doesn't stay here for ever.

The next leg of its journey

will see it create a second great climate band that circles the planet.

I'm going to join that air and travel with it

as it changes direction once again.

In the northern hemisphere,

the Coriolis effect is bending those winds to the right,

so they're not going northwards any more.

And after a while, they cool down and get more dense

and that wind starts to sink.

And that's happening right here.

This is a great place to join that air

and I'm going to do it by going out of this door.

OK, here we go!

The ground is 3,000 metres below me...

..but getting closer.

As I fall, I'm travelling with the air that left the equator,

because it too is now falling towards the ground.

So we're falling now together along with all that air,

this waterfall in the sky.

I've got a fabulous view of where all that air is going.

I'm glad to be back on the ground.

It's so different from the rainforest that this air came from.

Definitely, I've landed in a desert.

This is the Sonoran Desert in the American state of Arizona.

It's about 30 degrees north of the equator.

Out here in the summer, it can reach 50 degrees centigrade,

just horribly hot,

but, now, the summer's long gone and it's actually quite cool here today.

And what that tells us is that it's not heat that defines a desert.

It's lack of water.

Yearly rainfall can be as low as eight centimetres.

The reason it doesn't rain much

is that the air here has left much of its moisture

behind at the equator.

The Sonoran Desert is only here

because the Coriolis effect deflects the winds that began at the equator.

But it's not just the Sonoran Desert where this happens.

The same effect helps create deserts

at this latitude all around the world.

There's the Thar Desert in India,

the Arabian Desert

and, of course, the Sahara.

They're all created by falling dry air

that originated close to the equator.

Here we can really see the influence of the Earth's spin.

This pattern of wet rainforest at the equator and dry deserts here

is created by a giant system of winds.

Even this is not the end of the cycle.

The air that reaches the ground in the desert belt

now heads back to the equator,

drawn by the low pressure zone at the hottest part of the planet.

And as it travels back to the equator,

it's subject to the Coriolis effect,

so, once again, it bends to the right.

This creates very regular and reliable surface winds

that blow from the northeast to the southwest - the trade winds.

And that was used by the early human explorers

and by traders for centuries,

because it's much easier to travel if you're moving with the wind.

From the 15th century onwards,

European sailors increasingly exploited the trade winds.

They played a central role in the colonisation of the Americas

and the establishment of empires around the world.

And so the history of human exploration of our planet

would have been very different if our planet wasn't spinning.

When the trade winds arrive back at the equator, it closes the circle.

The resulting pattern of wind is called a circulation cell.

But this circulation cell at the equator isn't the only one.

The pattern repeats, so that, in total,

there are three circulation cells in each hemisphere, making six.

In each cell, air rises and is then bent by the Coriolis effect

until it cools, sinks and returns to the surface.

So far on our journey,

we've experienced the way the Earth's rotation

organises the atmosphere,

creating spectacular weather

and huge climate systems.

But that's not the end of the way

the Coriolis effect influences our planet.

It also affects the oceans.

I've come to Chile, to the world's biggest ocean, the Pacific,

to see a dramatic example of how the Coriolis effect

has transformed the ocean and the life that depends on it.

This is such a treat for me.

There's this fantastic colony of birds perched up on this rock.

Pelicans, just taking off and flying over here.

There are cormorants but, most excitingly,

just on the back there are Humboldt penguins.

Now, they are a species of penguin that live in temperate climates.

They're endangered, so it's a real treat to see them,

and all these birds are here for just one thing.

Fish.

Millions of them.

This is one of the richest fishing grounds in the world.

Less than 1% of the planet's ocean

provides up to 20% of the total fish catch.

And they're all here as a direct result of the spin of our planet.

There are plenty of fish here

because of a nutrient-rich current of water

that flows right along the west coast of South America.

It shares its name with the penguins.

It's called the Humboldt Current.

Just as the Coriolis effect deflects the winds,

it also deflects surface currents in the ocean.

It turns them to the right in the northern hemisphere,

creating a clockwise spiral, and here, in the southern hemisphere,

it turns the currents to the left, forming a counter-clockwise spiral.

These spirals are called gyres.

Despite the fact that we're so close to the tropics,

the water is freezing. It's about 15 degrees.

Well, that's because the current originates in Antarctica

and travels all the way up here.

That cold Antarctic water is driven up along the coast

by the South Pacific Gyre.

The current pulls nutrients up from the depths,

and these sustain the largest fishery on Earth.

And, in turn, quite a lot of sea birds.

This circular flow of water is a phenomenon

repeated around the world.

There are similar ocean currents in the North and South Atlantic,

and in the Indian Ocean.

The Earth's spin creates large-scale circulation patterns

in both the oceans and the atmosphere.

These patterns define the weather

and control the ocean circulation across the planet.

But the Earth's spin has another influence on the oceans.

And it reaches a peak at this time of year, in early autumn.

The tides.

The tides are what make our shoreline endlessly fascinating.

WATER RUSHING

I love the noise. It's brilliant.

The scene here is always changing as they go in and out twice a day.

And you're probably familiar with the idea

that the main factor driving all this is the moon.

Our cycle of tides happens because

the moon's gravity tugs at the ocean...

..creating a bulge of water

that's pulled away from the Earth towards the moon.

But that's not the whole story.

It's the spin of the Earth

that's responsible for this daily cycle of the tides.

As the Earth rotates underneath the moon,

it also rotates underneath this tidal bulge.

And when that water hits land, it creates our tides.

Here in Britain, we have some pretty big tides.

But to see the largest tides of all, you need to cross the Atlantic.

It's now September the 29th.

We've come to the eastern coast of Canada,

the best place - and today is the best day -

to witness one of nature's great events.

This is the Bay of Fundy,

a massive stretch of water, at high tide.

But at low tide, all the water has gone,

leaving me just enough time to reach those islands before it returns.

Now, the Bay of Fundy is famous

for having the greatest tidal range anywhere in the world

and today, it's due to be the biggest tide of 2011.

It's a three-mile walk from the high-tide mark.

We've come from right back there on the shoreline.

A certain amount of wading -

low tide doesn't mean it's completely without water -

but, we have now reached

the sea, precisely on low tide.

So, we're looking at pretty much slack water now,

but when the tide turns, a vast volume of water

is going to be dragged back down this channel,

filling this entire bay.

At its peak, the water advances at around 10 metres a minute.

I can't believe how fast the water's coming in now.

You can see it rippling in over all these dips and troughs,

filling them and then moving them on in a constant wave of water.

Look, all that land that was behind me has totally disappeared.

As the tide comes in, 115 billion tonnes of water flow in to the bay.

The Bay of Fundy has the highest tides in the world

because of its shape.

The water is funnelled up a channel that gradually narrows.

It seems extraordinary that just this morning,

we were walking past this island

and the path that we were walking on is now 30 or 40 feet under water.

But the reason why the tides are at their biggest at this time of year

is down to the interaction between the Earth's spin

and a very particular orbital alignment.

The highest tides happen when the gravity of the moon

and the sun work together.

But around the equinoxes, something special happens.

At this time of year, both the sun

and the moon are tracking along the equator.

And as they pass over the centre of our planet,

we spin right through the middle of a mammoth tidal bulge,

giving us the biggest tides of the year.

Geographical peculiarities create different tidal phenomena

in different parts of the world at this time of year.

Where big tides pour up river valleys,

they often result in a tidal bore.

In Britain, the Severn Bore is one example.

Even bigger is the Amazon tidal bore.

But the biggest of all is in southeast China...

..where crowds gather, often at great personal risk,

to see the bore arrive.

Today's tides are pretty impressive,

but they were actually much, much bigger in the past.

The evidence for that is just off the coast of Bermuda.

Understanding how the tides

have changed gives us a surprising insight

into the history of the Earth's spin.

I'm looking for a particular sort of sea creature.

It's lived on Earth for millions of years

and it can tell us extraordinary things about our past.

It's coral.

Encoded in coral reefs all over the world

is a daily record of a very significant feature

of our planet's history.

Because, as it builds an external skeleton - the coral reef -

it lays down a very thin layer of limestone each day,

a bit like the rings on a tree but daily, rather than annually.

Effectively, corals record how many days there are in a year.

On a piece of modern coral, these orange bands are annual rings

and, in between them, virtually invisible,

are 365 daily growth rings.

But, if you look at a much older piece of coral,

it tells a very different story.

This, amazingly, is about 400 million years old.

It is a piece of coral.

I know it looks like an insignificant bit of rock,

but, to the expert eye, this is as good as a history book.

And it gives a really faithful record

of what life on Earth was like, way back then.

There are rings, just like the modern coral.

What's really surprising, though,

is that if you count those daily growth rings,

you'll get a total of 410.

400 million years ago, a year lasted not 365 days but 410.

So, the explanation for ancient corals like this piece

to have 410 daily growth rings

is that when this was alive, a day wouldn't have lasted 24 hours but only 21.

And for that to happen, the Earth had to be spinning faster.

To find out why,

you have to go back to the earliest days of the Earth's history.

4.5 billion years ago, the Earth was hit by another planet.

EXPLOSION

Debris from the impact created the moon,

which would have been much closer to the Earth than it is today.

It also set the Earth spinning much faster.

When the first oceans formed,

that combination would have meant

the tides would have been simply huge.

Just imagine, instead of these small tides

of a few metres that we have today,

there would be enormous tides, hundreds of metres high,

crashing into the coastlines all around the world.

But as the moon's gravity pulled on these huge tides,

it acted as a kind of brake.

Gradually, this slowed the Earth down.

And the same process meant that the moon drifted gradually further away.

SHE LAUGHS

As the moon got further away,

its gravitational pull on the tides decreased,

so the tides got lower.

But, even today, the rotation of the Earth is slowing down

and our days are still getting longer

but only by about 2.3 milliseconds every century.

We're now heading into winter.

Because of the Earth's angle of tilt,

the northern hemisphere is leaning

further and further away from the sun

So temperatures are falling and the days are getting shorter.

The natural world responds by starting to shut down.

Trees detect the shortening day length

and stop producing green chlorophyll in their leaves.

The golden colours of autumn take over.

The snow begins to fall.

In Yellowstone Park, black bears

head for their dens and winter hibernation,

100 days without eating.

Birds like snow geese,

that spent the summer in the Arctic, now head south,

often in huge numbers, to spend the winter in more temperate climes.

By the end of November,

most of the Arctic is experiencing 24 hours of darkness.

The polar night.

It's now early December

and we are coming towards the end of our journey.

Our final destination is a place where the Earth's rotation

has a particularly powerful impact on the weather.

Britain.

You can't imagine a day that's more typical

of what we think of British weather.

We arrived this morning and it was raining

and then the sun came out and then there was a squall,

and now the sun's come out again

and we've got this really strong wind.

The British winter is notoriously unpredictable.

Sometimes cold and dry, sometimes mild and wet.

This unpredictability is a consequence of the Earth's rotation.

The key factor is Britain's location.

We sit underneath the boundary

between two of the Earth's climate cells.

This means that, above our heads,

there's a battle going on between two different types of air.

I'm going to draw a map to show you.

This is the south coast

and Scotland's up here.

And we're down here, in Cornwall.

And Ireland is out here.

Up here, to the north of us, there's cold, polar air,

and down on this side, to the south,

is warm air that's come from the tropics.

And the boundary between the two

can lie right over the British Isles.

And what's going on above our heads

is a clash of the cold air and the warm air

and it's where they're pushing against each other and mixing it up

that we get this changeable, messy weather

that we love to complain about in this country.

In December 2011, we saw this battle in action.

A succession of storms battered the country

as warm and cold air struggled for supremacy above our heads.

But there's a further factor

that influences the outcome of this battle between warm and cold air.

The boundary between the cells can move.

This movement can be affected by a phenomenon that's generated

right at the boundary between the cells.

And it's a product of the Earth's spin.

Right at the boundary, high up in the sky,

a wind blows about 10 kilometres up.

It's really, really fast.

It can travel at speeds of up to 450 kilometres per hour.

It coils all the way around the planet, at about our latitude,

and we call it the jet stream.

The jet stream is crucial

because it influences the boundary between the cells,

and therefore between cold air to the north

and warm air to the south.

You can see the significance of this

by looking at the weather 12 months earlier, in December 2010.

The whole country shivered under a blanket of snow and ice.

It was one of the coldest winters since records began.

The reason was that the jet stream had developed a kink.

Over the Atlantic, it sat much further north, near the Arctic.

Then it swung down, over Britain.

This temporarily shifted the boundary between the cells

and brought cold, polar air across the whole country.

Unfortunately for our weather forecasters,

it's particularly difficult to predict

the meanderings of the jet stream.

The spin of the Earth makes the weather

here in the UK unusually changeable,

and particularly hard to predict.

The fact that you wake up every morning

and the atmosphere surprises you

and it just adds to the spice of life.

We've travelled five months

and it's now December the 22nd, the winter solstice.

This is the day when the northern hemisphere

receives the least amount of sunshine in the year.

And it marks the end of our journey, for now.

Over the last five months,

we've seen how the Earth's spin plays a critical role

in defining the weather across the planet.

Spin moves oceans...

SHE SQUEALS

..and it gives us a global pattern

of climate zones that can be seen from space.

Next time, we journey from the winter solstice to the spring equinox.

It's the most astonishing landscape that I've ever seen.

It's a time of paradoxes and extremes.

And I just drop into the abyss.

Subtitles by Red Bee Media Ltd