Orbit: Earth's Extraordinary Journey - Learning Zone


Orbit: Earth's Extraordinary Journey - Learning Zone

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We're all on an amazing journey.

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A 940 million kilometre voyage through space.

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Even though we can't feel it,

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we're travelling at over 100,000 kilometres an hour,

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circling a star we call the Sun.

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Every year, our planet, the Earth,

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travels around the Sun - and we go with it.

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We're looking at the huge impact this journey has on our world.

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Wow, look at that!

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We'll see how the Earth's tilt gives us our seasons

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and leads to monsoon rains.

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How the planet's spin stirs the atmosphere

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into giant, rotating hurricanes.

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And how small changes in the Earth's movement

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can cover the planet with ice.

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Our yearly orbit around the Sun has created

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and shaped the world we live in.

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We start our circumnavigation of the Sun

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at a very special place,

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and on a very special day.

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It's March the 20th, the spring equinox.

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This is the great pyramid in Chichen Itza -

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an ancient Mayan city in Mexico.

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Built 1,500 years ago,

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the city is one of the world's great archaeological sites.

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And it contains a remarkable insight

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into our journey through space.

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The ancient Maya had developed a deep understanding

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of the Earth's movement around the sun,

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and they built it into the very fabric of this city.

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But it's something that can only be seen at two very precise

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and magical times of the year.

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One of those is today, March the 20th.

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As afternoon approaches, the city fills

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with followers of Mayan beliefs...

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..and those curious to see an ancient wonder.

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There is a unique and particular feature of our planet

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as it orbits the sun,

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and it's encoded in the way that light and stone

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interact at the great pyramid.

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CHEERING

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This is the moment that all these thousands of people

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have been waiting for, they've all stood up

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and there are hands raised to welcome in the sun,

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and it's now aligned perfectly on the edge of the steps here,

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creating this very specific pattern of light and shade which resembles

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the body of a snake. And that's no coincidence

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because it joins up with the carved snake's head

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at the bottom of the pyramid.

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The Maya believed the snake, known as Kukulcan,

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was a messenger between gods and man.

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This is a remarkable display of Mayan architectural design.

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The appearance of this snake isn't an accident,

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they absolutely planned it

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and it happens on the same day every year.

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This is the spring equinox.

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DRUMS BEAT

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So, more than 1000 years ago,

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the Maya recognised the equinox as a pivotal moment in the year.

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Here on Earth, there are a few moments that we all share,

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because we're all on the same journey around the Sun.

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And one of those moments is the equinox,

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when day and night are equal.

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'It's a time of balance we can all experience,

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'wherever we are on the planet.'

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So whether you are here in Britain,

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amongst the fitful showers and overcast skies,

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'or in the bright spring sunshine of Mexico,'

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on the March equinox

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you'll get 12 hours of daylight and 12 hours of night time.

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That's if the sun ever comes through the clouds!

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But it's more than just a time of balance.

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It's also a turning point in our year.

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From the March equinox onwards,

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the days get longer in the northern hemisphere,

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'while in the southern hemisphere, the opposite occurs.

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'This is because of a special feature of our planet

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'as it journeys through space.'

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Let's say this rock is the sun.

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This is going to be our Earth,

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and as the Earth travels around its orbit

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spinning like this, it travels around on a flat plane.

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So you would think that its axis would point upwards

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but it isn't, it's tilted over at 23.4 degrees.

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'This means that the North Pole, the stem of the apple,

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'isn't vertical, it's at an angle.'

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And that tilt stays pointing in the same direction

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as the Earth travels around on its orbit.

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Because of this tilt for part of our orbit,

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the hemisphere north of the equator leans towards the sun.

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This brings with it extra solar energy,

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which fuels spring and then summer.

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Six months later, the situation is reversed.

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The southern hemisphere now leans towards the sun,

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while the northern hemisphere experiences declining energy,

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ushering in winter.

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Tilt creates the Earth's seasons.

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But there's a moment, twice a year as we orbit,

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when the sun favours neither hemisphere.

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At this point, both experience 12 hours of daylight and night time.

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This is the equinox.

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If the Earth wasn't tilted,

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every day would be like the equinox,

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with the 24 hours equally split between day and night.

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And that would mean no seasons.

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But the Earth's tilt means as we travel from the equinox,

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seasonal changes do occur.

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On the equinox,

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the Sun's energy is felt most strongly directly on the Equator.

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As we pass through spring,

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this focused energy moves slowly northward.

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All over the northern hemisphere, this solar shift means the Sun

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arcs higher in the sky, and the hours of daylight increase.

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The Earth orbits the Sun not standing up straight,

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but tilted over at an angle of 23.4 degrees.

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Our 23 degree tilt is just right.

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It's enough to provide a relatively benign seasonal shift.

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It makes our planet habitable.

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However, it may have been the result of a cosmic accident,

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and here in America, we can gain an insight into what happened.

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This is the Barringer Crater in Arizona.

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50,000 years ago, a meteorite struck this site

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and just look what it left behind - this enormous hole in the ground.

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'This impact would have thrown debris out

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'over tens of thousands of square kilometres.'

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And all the rock around here, like this,

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is what's left after that explosive event.

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This enormous crater is like a lesson in how size isn't everything,

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because the crater itself is a kilometre across,

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but the thing that caused it was only about 50 metres in diameter,

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which is really quite small.

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And the reason that such a small thing could cause such a big hole

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is because it was travelling so fast.

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'Impacts like these are extremely rare,

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'but in the Earth's past, they were far more common

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'and a lot bigger.'

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Around four and a half billion years ago,

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the solar system was still in the process of formation.

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The Earth was just one of many of protoplanets that orbited the sun.

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Amongst these protoplanets

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was a small Mars-sized planet that's been named Theia.

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Its orbit put it on a collision course with the Earth.

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Theia smashed into the larger Earth and was obliterated.

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The impact very nearly destroyed our planet too.

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The collision knocked the planet over,

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tilting the Earth's axis of rotation.

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This tilted Earth might still be oscillating madly,

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were it not for another consequence of Theia's impact.

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A huge amount of debris was blasted into space.

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Gradually, this debris coalesced,

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captured by the Earth's gravity...

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..and it formed the moon.

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Billions of years later, the gravity of the sun and the moon together

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act as a sort of counterweight, stabilising our tilt.

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It's extraordinary to think that the moon is both evidence

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of what caused Earth's 23 degree tilt

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and the celestial object that helps maintain it.

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Without this stabilising effect, the planet would wobble in space.

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There would be no seasons, and the weather would be chaotic.

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Spring triggers a seasonal transformation on land.

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But the rising temperatures also transform our weather,

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in some places with dramatic effect.

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A tornado is the most volatile of these seasonal weather events.

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They occur most frequently in the spring

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and especially in the Midwest of America -

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a region known as Tornado Alley.

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MAN: 'Did you see that? The whole house came apart!

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'Oh, my God!'

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But despite its violence,

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at the core of a tornado is a very simple process.

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This goes on like a backpack.

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'To experience it, I'm taking to the air,

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'over the Midwestern state of Colorado.'

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One, two, three, go. Run!

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Paragliding pilots like Honza Rejmanek,

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love this time of year.

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Spring provides the perfect conditions for soaring...

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..because the increasing temperatures generate thermals.

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So right now we are in a thermal.

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These are basically almost like invisible smokestacks of rising air.

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Right now we've found one, I'm going to take a turn in it

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and circle around and try to gain height.

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'Thermals form when the sun warms the ground,

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'and the ground, in turn, warms the air above it.'

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What I'm experiencing

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is one of the most fundamental principles of atmospheric physics -

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warmer air rises.

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'When air warms, it expands and becomes less dense.

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'So this air has a lower atmospheric pressure

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'than the cooler air that surrounds it.'

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So it floats upwards, forming this rising thermal column.

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The atmosphere tries to even out differences

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in air temperature and pressure,

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attempting to return to equilibrium.

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So the rising thermal will mix with the cooler air above.

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This basic process of moving towards equilibrium

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lies at the heart of every significant weather event

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on the planet.

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'But in the springtime air over Tornado Alley,

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'particularly powerful storms can develop.

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'This is due to the unusual conditions here

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'that intensify this basic atmospheric process.'

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There's a stable layer of dry air that acts as a barrier

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between the warm air down below and the cooler air higher up.

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So the warm air is trapped,

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and what's more, the ground keeps heating it as the day goes on.

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WIND WHISTLES

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THUNDERCLAP

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The thermals get more and more powerful until, by late afternoon,

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they finally punch through the barrier layer at colossal speed.

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These rapid updraughts of less dense, lower pressure air

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are so strong that they generate huge thunderstorms.

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THUNDER RUMBLES

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It's from these thunderstorms that, in certain conditions,

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tornadoes can emerge.

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'I'm going to investigate how this happens...'

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Not as bad as north of us.

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...with the help of atmospheric scientist, Josh Wurman.

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I don't know what to make of these stringy little features.

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The first step in our quest for a tornado

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is locating a promising storm.

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After a couple of days on the road, we manage to intercept

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one moving north through Colorado.

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So what's happening behind me is the storm is building

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and in the middle of that storm over there,

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there's an updraught with low pressure at the centre of it.

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And all the air around the outside has higher pressure,

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and that high pressure is pushing air into the centre

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and up into the storm, and that's what building the storm.

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The atmosphere tries to even out

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the extreme differences in temperature

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that have been generated.

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So the air movements at the core of the storm

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become exceptionally powerful.

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'Hail is one characteristic product of this atmospheric violence.'

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'The hail formed when an updraught cooled rapidly,

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'so that water condensed out of the air, and turned immediately to ice.'

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SHOUTING: This is what was carried from the south,

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and it was pushed up into the storm

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and it gave the storm its energy.

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And now it's falling back down on me!

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GIGGLES

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Wow!

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CAMERAMAN: That's it. Let's get inside. This is too hard now.

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And even though this is chaotic and messy,

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what this is, is a demonstration that the atmosphere is an unstable place,

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and there are all these differences in temperatures and pressures.

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And this is what happens when the atmosphere moves around

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to even everything out, and make it all the same.

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When tornadoes do form, they are often preceded by hail.

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But this time, there's no twister.

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So we're back on the road,

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still trying to see a storm spawn a tornado.

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Josh's specialist radar detects one

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which shows a revealing swirl of clouds.

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JOSH: Going out ahead, this big dark area's the core.

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So we're basically going to penetrate through the core

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and see what's interesting.

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Tornadoes form when powerful rotating cylinders of air

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within the storm

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get caught by an updraught and are knocked on their side

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by a powerful atmospheric wind called the jet stream.

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Right now, we're kind of in the centre of the coiled part.

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When that column of rotating air touches the ground,

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a tornado is born.

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At the tornado's core is an area of intense low pressure,

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which draws high pressure air towards it.

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The dust and debris picked up by the tornado

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reveal the swirling pattern of winds.

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Just 15 minutes after it first touched down, the tornado dissipates.

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There's still so much that we don't understand about storms.

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We don't understand when they're going to produce hail,

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when they're going to produce rain,

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when they are going to produce tornadoes.

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But what we do understand is that a storm like this

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is a manifestation of something happening round us all the time.

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Our planet's atmosphere is a mosaic of warmer and cooler air masses,

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constantly in motion.

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The air is rising, falling and swirling around

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as it seeks to balance differences in temperature and pressure.

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During April and May,

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the effect of the Earth's tilt is to enhance those differences.

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So all over the northern hemisphere,

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spring is the season for volatile storms.

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Tornadoes are only one consequence.

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The heavy and sudden downpours from storms can result in flash floods,

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like the one that hit the town of Barranquilla in Colombia

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in May 2011.

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These occur when the rain inundates densely saturated ground.

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The water isn't fully absorbed, but instead flows rapidly downhill

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in a near-instantaneous torrent.

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Thunderstorms can also give birth to an unexpected phenomenon...

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..massive dust storms called haboobs.

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This one blew into Phoenix, Arizona in 2011.

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Haboobs are produced in normally arid regions,

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when the leading edge of a storm collapses,

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generating a super-fast downdraught

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that kicks up a wall of dust and sand in front of it.

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As May turns to June, more solar energy

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is reaching the northern hemisphere,

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and it drives the biggest single weather event on the planet.

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An event centred on the Indian subcontinent.

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TRAFFIC HUMS

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CAR HORNS TOOT

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This is the city of Udaipur in Rajasthan.

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It's in the northwestern corner of India.

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Since March, temperatures here

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have been steadily rising as the Earth's tilt

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has warmed the northern hemisphere.

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But by June, everything is on the brink of an exhilarating change.

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I'm here at the time of an epic weather event of huge importance

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not just to Rajasthan but to the whole subcontinent

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and the over billion people who live here.

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'There's a wonderful place to appreciate the event's significance,

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'on one of the hills that overlook the city,

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'here, at this cliff-top palace.'

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It was built at the end of the 19th century

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by the 72nd Maharana of Udaipur

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and it's known as Sajjan Garh.

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He built this palace to get a pure, unadulterated view of the sky

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and the clouds that start to build at this time of year.

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Sajjan Garh is the monsoon palace.

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When the rains do eventually arrive,

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they'll be an essential relief from the heat of the Indian summer.

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But what's intriguing is that the monsoon is actually a consequence

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of the rising seasonal temperatures that precede it.

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To reveal why this is, we need to travel 2,000 kilometres...

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..south.

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I'm in the coastal state where the monsoon first arrives in India -

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Kerala.

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The key to understanding the monsoon is here, on the beach.

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The monsoon is powered by a simple,

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but incredibly significant difference -

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the difference between land and sea.

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And in particular, the differing ways in which they respond to the sun.

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Take this sand as an example.

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The sun's energy is heating all of this surface,

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but if I dig down just a little way...

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..the sand underneath is quite cool, and that's quite familiar,

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we see that on sunny beaches all the time.

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And here, where it gets really hot,

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the surface can reach 40 degrees Celsius.

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Just 15 centimetres down into the sand,

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it can be only 7 degrees Celsius.

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So, all the sun's energy is going into a really thin surface layer,

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and that layer heats up really, really, quickly.

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The sun is also beating down on the ocean,

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and that responds very, very differently.

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This water is much warmer than the sea at home

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but it's much cooler than the beach,

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and the reason for that

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is that the ocean takes much more of the sun's energy to heat it up.

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So a kilogram of water will take three times as much energy

0:27:180:27:22

as a kilogram of sand to heat by one degree.

0:27:220:27:24

The ocean is also relatively cool because to heat the surface

0:27:260:27:29

you have to heat much more than just a thin layer.

0:27:290:27:32

What happens is that winds that blow across the surface of the ocean

0:27:320:27:36

generate turbulence which mixes that top layer.

0:27:360:27:38

So as soon as some water's been heated at the top,

0:27:380:27:41

it gets mixed down below.

0:27:410:27:43

'This means that, unlike the land, the ocean warms up only very slowly,

0:27:460:27:50

'as the sun's energy is absorbed.

0:27:500:27:53

'So as we enter summer, the land heats up quickly,

0:27:550:27:58

'while the ocean lags further and further behind.'

0:27:580:28:01

This increasing temperature difference is critical,

0:28:030:28:06

because both land and sea heat the air above them.

0:28:060:28:10

As the sun has baked the Indian subcontinent,

0:28:130:28:16

the land has warmed the air above it.

0:28:160:28:19

The warmer air is less dense, so it rises.

0:28:190:28:23

This draws in the cooler air from the ocean.

0:28:230:28:26

Because of India's particular geography,

0:28:260:28:30

this process is magnified.

0:28:300:28:33

It's a triangular peninsula, with wide, hot plains

0:28:330:28:36

and, crucially, a very long coastline.

0:28:360:28:40

This combination sets up a powerful

0:28:400:28:42

and sustained movement of cooler ocean air -

0:28:420:28:46

the monsoon wind.

0:28:460:28:48

Of course when most of us think of a monsoon

0:28:510:28:53

we think not of seasonal winds, but of rain.

0:28:530:28:55

'By setting up a time-lapse camera,

0:28:580:28:59

'I'm hoping to watch the rain clouds forming.'

0:28:590:29:02

THUNDERCLAP

0:29:080:29:11

Wow!

0:29:130:29:16

There is an enormous process on the go here.

0:29:250:29:27

When the sun shines down on the ocean surface,

0:29:270:29:29

some of the water at the surface will evaporate,

0:29:290:29:32

so water and energy are carried up into the atmosphere.

0:29:320:29:35

And as the monsoon winds come inland

0:29:350:29:37

and they carry that water vapour with them,

0:29:370:29:39

the heated land makes that moist air rise,

0:29:390:29:43

goes up into the clouds and there droplets condense -

0:29:430:29:46

the water condenses out, becomes visible, we see clouds.

0:29:460:29:49

When those droplets join together to form droplets which are large enough,

0:29:490:29:53

we get rain like this.

0:29:530:29:55

And it's really raining hard now!

0:29:550:29:58

None of this would be happening if it wasn't for the Earth's tilt.

0:30:010:30:04

It's the seasonal heating is what widens the gap in temperature

0:30:060:30:09

between the land and the sea, and this drives everything.

0:30:090:30:12

And this massive system of rain and wind rushes inland

0:30:120:30:16

and that's the monsoon.

0:30:160:30:18

80% of all India's rains arrive in this seasonal deluge.

0:30:220:30:27

It's not just the volume of the monsoon rains which is impressive.

0:30:290:30:33

It's the distance they travel.

0:30:330:30:35

As summer progresses in India,

0:30:380:30:40

the difference in temperature between land and ocean actually increases.

0:30:400:30:45

This makes the whole monsoon system more powerful,

0:30:450:30:51

drawing this moisture-laden air further and further inland.

0:30:510:30:55

From when the monsoon first arrives on the Kerala coast

0:30:590:31:03

around June the 1st,

0:31:030:31:04

it spreads more than 2,000 kilometres

0:31:040:31:07

until it eventually reaches the far north of the country.

0:31:070:31:12

It's now early September.

0:31:210:31:24

Although the summer is almost at an end,

0:31:240:31:26

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

0:31:260:31:29

Because this is hurricane season.

0:31:300:31:32

The development of a hurricane is a wonderful example

0:31:370:31:40

of how the Earth's spin controls the weather.

0:31:400:31:43

I'm hoping to see one in action.

0:31:440:31:47

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

0:31:470:31:50

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

0:31:500:31:54

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

0:31:540:31:57

24 hours later, I'm in eastern Mexico,

0:32:030:32:07

heading towards the Gulf of Mexico and the oncoming storm.

0:32:070:32:11

The winds are building up and the normal sunny skies

0:32:170:32:20

are replaced with cloud and rain.

0:32:200:32:26

At this time of year, the Gulf of Mexico

0:32:260:32:29

has the perfect ingredients to make a hurricane.

0:32:290:32:33

The sea is relatively shallow and close to the equator,

0:32:330:32:37

so the water gets particularly hot.

0:32:370:32:40

This water is warm, really warm

0:32:400:32:43

and the reason for that is that the ocean out there

0:32:430:32:46

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

0:32:460:32:48

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

0:32:480:32:52

The way the storm is built is that the warm ocean

0:32:580:33:01

heats the air above it.

0:33:010:33:02

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

0:33:020:33:05

THUNDER RUMBLING

0:33:050:33:08

As the warm air rises, the pressure drops,

0:33:080:33:11

sucking in even more moist air, creating powerful winds.

0:33:110:33:15

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

0:33:200:33:24

It needs to start turning.

0:33:250:33:27

And that rotation comes from the spin of the Earth

0:33:270:33:32

through a phenomenon known as the Coriolis effect.

0:33:320:33:37

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

0:33:370:33:41

and that's the North Pole.

0:33:410:33:43

Now, this planet isn't spinning,

0:33:430:33:45

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

0:33:450:33:50

But we live on a rotating world.

0:33:520:33:55

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

0:33:570:34:01

round anticlockwise, like in the northern hemisphere.

0:34:010:34:04

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

0:34:090:34:12

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

0:34:120:34:15

it bends around to the right.

0:34:150:34:17

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

0:34:240:34:28

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

0:34:280:34:31

Now, the reason that this matters

0:34:340:34:37

is that this ball represents winds on Earth

0:34:370:34:40

and when the wind blows in the northern hemisphere,

0:34:400:34:44

the wind is also moved to the right.

0:34:440:34:46

In the southern hemisphere, the effect is reversed

0:34:470:34:50

and the winds bend to the left.

0:34:500:34:52

And that is all the Coriolis effect is.

0:34:530:34:57

A hurricane shows the Coriolis effect in action.

0:35:030:35:07

Winds are drawn inwards towards the low pressure

0:35:080:35:12

at the centre of the hurricane.

0:35:120:35:14

But as they head towards the centre,

0:35:150:35:18

the Coriolis effect makes them turn to the right.

0:35:180:35:21

This creates the hurricane's characteristic

0:35:240:35:28

circular swirl of wind.

0:35:280:35:31

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

0:35:310:35:35

So the eye of the hurricane remains calm.

0:35:360:35:40

Out at sea, Nate has the characteristic rotating,

0:35:470:35:50

swirling clouds of a hurricane.

0:35:500:35:52

but frustratingly, Nate begins to lose power.

0:35:540:35:58

Before it can make landfall, the winds die away.

0:36:000:36:03

Instead, the 2011 hurricane season

0:36:100:36:14

became famous for a different storm.

0:36:140:36:17

Hurricane Irene.

0:36:170:36:20

Unusually for a hurricane,

0:36:200:36:22

it travelled far enough up the east coast of the USA

0:36:220:36:25

to flood parts of New York city.

0:36:250:36:27

It caused billions of dollars worth of damage.

0:36:290:36:34

And all this because our planet spins.

0:36:340:36:37

In January, the northern hemisphere is locked in winter.

0:36:430:36:47

And yet there is a paradox about our winter,

0:36:470:36:52

because in January, winter is still getting colder,

0:36:520:36:56

even though the northern hemisphere is receiving more energy from the sun.

0:36:560:37:00

I've come to Northern Canada,

0:37:100:37:12

to the best - or perhaps the worst - place to explore this paradox.

0:37:120:37:17

Whoo!

0:37:200:37:23

Cor! This...

0:37:230:37:27

..is Yellowknife.

0:37:270:37:30

It has the dubious distinction

0:37:300:37:33

of being the coldest city

0:37:330:37:36

in the whole of North America.

0:37:360:37:39

Today is January the 19th.

0:37:410:37:44

On average, this is the coldest day of the year across the northern hemisphere.

0:37:440:37:49

It's minus 35 degrees Celsius, which certainly qualifies as cold to me.

0:37:490:37:55

When you breathe, it hurts.

0:37:570:38:01

It kind of gets you at the back of the throat.

0:38:010:38:04

Your nose feels like it's permanently frozen solid.

0:38:040:38:07

And despite the fact that I've got the feathers of about 25 geese

0:38:070:38:11

stuffed into this jacket, and more thermal underwear

0:38:110:38:14

than I thought possible to wear at exactly the same time,

0:38:140:38:17

I still feel cold.

0:38:170:38:20

In these conditions, even familiar things behave in unfamiliar ways.

0:38:200:38:26

You can take a lovely, hot, steaming cup of coffee,

0:38:260:38:29

throw it in the air, and the steam from that coffee will freeze instantly.

0:38:290:38:35

Well, you've got to give it a go, haven't you?

0:38:350:38:37

Right...

0:38:390:38:41

Here goes.

0:38:410:38:42

Wow!

0:38:460:38:47

That is amazing!

0:38:480:38:51

There's something curious about the way winter peaks towards the end of January.

0:38:580:39:04

The winter solstice falls on December the 21st

0:39:040:39:07

and this marks the day when the northern hemisphere

0:39:070:39:10

receives the least amount of solar energy from the sun.

0:39:100:39:14

So you might expect the December solstice to be the coldest day of the year.

0:39:140:39:20

But it's not.

0:39:200:39:21

On average, temperatures on the 19th of January are colder

0:39:220:39:26

than they are in mid-December.

0:39:260:39:29

But, you say, the days are getting longer.

0:39:290:39:32

The northern hemisphere is getting more sun.

0:39:320:39:35

It should be warming up.

0:39:350:39:38

In Yellowknife, there are people

0:39:410:39:44

whose livelihoods depend on the way winter's peak is delayed.

0:39:440:39:48

In the driving seat is Blair Weatherby.

0:39:490:39:52

His family have been driving through the bitter cold of this region

0:39:520:39:55

for three generations.

0:39:550:39:57

He's not an ordinary trucker. He's an ice road trucker.

0:39:570:40:03

And this is his highway.

0:40:030:40:05

In the summer, what happens here?

0:40:240:40:27

We'd be in a boat!

0:40:270:40:28

That's because we're not driving on land, but on a frozen lake.

0:40:300:40:35

So really to appreciate Yellowknife's splendid isolation, you have to look at a map.

0:40:400:40:44

And here it is, right on Great Slave Lake.

0:40:440:40:49

At this time of year, of course, it freezes.

0:40:490:40:52

So what time of year can you start driving on the lake,

0:40:520:40:57

as opposed to boating on the lake?

0:40:570:40:59

The season starts towards the end of January.

0:40:590:41:03

It's about 30 inches thick at this point. It just keeps getting thicker and thicker.

0:41:030:41:09

So whilst the northern hemisphere's coldest day is the 19th of January,

0:41:120:41:16

here in Yellowknife, it's still bitterly cold for many weeks to come.

0:41:160:41:20

For the truckers, this delayed winter means their work season

0:41:200:41:25

runs from late January well into March.

0:41:250:41:28

So why is the worst of winter delayed so long

0:41:340:41:38

after the solstice on December the 21st?

0:41:380:41:41

It's all about the balance

0:41:420:41:44

between the heat coming in and the heat going out.

0:41:440:41:48

Throughout early winter, the northern hemisphere

0:41:500:41:53

receives declining amounts of the sun's energy,

0:41:530:41:57

so it starts to cool down.

0:41:570:42:00

But there's a lag in this cooling,

0:42:000:42:02

because the Earth's surface loses heat relatively slowly.

0:42:020:42:06

So well into January, the Earth's surface is still losing heat,

0:42:060:42:11

even though solar energy is slowly increasing.

0:42:110:42:16

It isn't until around the 19th of January that a tipping point is reached.

0:42:160:42:22

From this day onwards, the increase in solar radiation

0:42:220:42:26

will overwhelm the effects of the heat loss

0:42:260:42:29

and the northern hemisphere will begin to warm up.

0:42:290:42:32

But it'll still be a few more weeks yet

0:42:320:42:35

before the ice here is too thin to support the weight of the trucks.

0:42:350:42:41

We've seen how the Earth's journey through space is critical for life

0:42:460:42:51

and how the Earth's angle of tilt defines our seasons.

0:42:510:42:56

But you only really understand just how important our orbit is for our planet

0:42:560:43:01

when you look into the Earth's past.

0:43:010:43:04

There's evidence in the most unexpected places.

0:43:070:43:11

A few miles out there is one of the most spectacular wonders of the world,

0:43:130:43:17

but I can't see it from here because it's underwater.

0:43:170:43:20

I'm in Belize in Central America

0:43:200:43:22

and what I'm going to see is known as the Blue Hole.

0:43:220:43:26

It's not often that nature produces something

0:43:340:43:37

as beautifully symmetrical as this.

0:43:370:43:39

It's almost a perfect circle.

0:43:390:43:41

But it's more than just a stunning piece of natural architecture,

0:43:450:43:49

because deep down there are clues

0:43:490:43:51

to some of the most dramatic events in Earth's history.

0:43:510:43:54

The bottom here is 120 metres down,

0:44:060:44:09

and I'm just dropping into the abyss.

0:44:090:44:13

Finally, I've reached my goal.

0:44:200:44:23

So down here at 40 metres...

0:44:280:44:31

..it's really eerie.

0:44:320:44:34

Gloomy.

0:44:340:44:35

And this is what I've come to see.

0:44:370:44:40

And they're stalactites.

0:44:410:44:42

But there's only one way I know of for stalactites to form.

0:44:480:44:53

And it isn't down here, in 40 metres of water,

0:44:530:44:57

with sharks swimming about nearby.

0:44:570:44:59

Stalactites are created when mineral-rich water drips from the roof of a cave,

0:45:040:45:10

over hundreds or even thousands of years,

0:45:100:45:13

leaving behind mineral deposits.

0:45:130:45:16

In other words, they didn't form in the ocean.

0:45:160:45:20

That means that when these grew,

0:45:240:45:28

the sea level was much, much lower than it is today.

0:45:280:45:32

Scientists have precisely dated stalactites from the Blue Hole

0:45:350:45:40

and, by comparing these and other sea level indicators from around the world,

0:45:400:45:45

they've built up a picture of changing sea levels

0:45:450:45:49

dating back hundreds of thousands of years.

0:45:490:45:52

It reveals a striking pattern.

0:45:530:45:56

Sea levels across the world have risen and fallen over time.

0:45:560:46:01

20,00 years ago, the entire surface of the world's oceans

0:46:080:46:12

was 120 metres below where it is today.

0:46:120:46:16

And that means if I was standing here 20,000 years ago,

0:46:160:46:21

all of this, including the Blue Hole cave system, would be dry land.

0:46:210:46:25

So where did the ocean go?

0:46:280:46:30

The answer is that it was on land.

0:46:320:46:35

But it wasn't liquid water, it was ice,

0:46:350:46:38

because 20,000 years ago, our planet was in the middle of an ice age.

0:46:380:46:42

The Earth has experienced regular ice ages

0:46:450:46:48

in a cycle going back several million years.

0:46:480:46:52

These dramatic changes to the state of our planet

0:46:540:46:58

are triggered by small changes in the Earth's orbit.

0:46:580:47:02

I've travelled back to Britain

0:47:100:47:12

to uncover the relationship between the Earth's orbit and an ice age.

0:47:120:47:16

Snowdonia's peaks and valleys were carved out in the last ice age.

0:47:190:47:24

It's in mountainous locations like this that an ice age would have begun

0:47:260:47:31

as snow gradually built up.

0:47:310:47:34

When we think of ice ages, we think of extreme cold during the winter.

0:47:400:47:44

But, counterintuitively,

0:47:440:47:46

it's summer temperatures which are important in starting ice ages.

0:47:460:47:50

And the reason for that is, now, ice will build up here during the winter,

0:47:520:47:56

but it will all melt away in the summer.

0:47:560:47:58

But if the summer is a little bit cooler, a layer of ice will be left behind.

0:47:580:48:03

And a series of cool summers

0:48:030:48:04

will leave layer after layer, one on top of the other, building up.

0:48:040:48:08

And here, the ice could have been hundreds of metres high.

0:48:080:48:11

Ice ages always start in the northern hemisphere

0:48:130:48:16

because there's so much more land surface on which ice can build up.

0:48:160:48:20

So the question is, what causes cooler summers in the northern hemisphere?

0:48:240:48:29

The answer comes from small changes in the Earth's orbit,

0:48:300:48:34

caused by the gravitational pull of other planets.

0:48:340:48:38

Our orbit isn't exactly the same every time.

0:48:400:48:43

Aspects of it change just slightly, in cycles lasting thousands of years.

0:48:430:48:48

And when all of those cycles reach their most extreme point

0:48:480:48:52

all at the same time,

0:48:520:48:54

that can change our summer temperatures just enough to tip us into an ice age.

0:48:540:48:59

There are three cycles to do with the Earth's orbit

0:49:010:49:04

that must all coincide to trigger an ice age.

0:49:040:49:07

The first of these cyclical changes

0:49:100:49:12

affects the time of year when perihelion occurs.

0:49:120:49:15

This is the day when the Earth is closest to the sun.

0:49:150:49:19

Today, perihelion is in January,

0:49:210:49:25

but over thousands of years, the date of perihelion changes.

0:49:250:49:29

When perihelion occurs in the northern hemisphere summer,

0:49:290:49:33

it makes summers particularly hot.

0:49:330:49:36

But when it occurs in winter, as it does today,

0:49:370:49:40

then northern hemisphere summers are cooler.

0:49:400:49:44

So at the moment, the perihelion cycle is at the right point to generate an ice age.

0:49:450:49:52

But two other cycles are not in an ice age phase.

0:49:530:49:58

The first is the angle of the Earth's tilt.

0:49:580:50:00

The Earth's tilt is currently at an angle to the vertical of 23.4 degrees.

0:50:050:50:10

But that angle changes between 22 and 24.5 degrees.

0:50:100:50:16

It's only when the angle is at its shallowest - 22 degrees -

0:50:160:50:21

that the seasons become less extreme and the summers cooler.

0:50:210:50:24

Today, the angle of tilt is too great for an ice age.

0:50:270:50:31

The final cycle affecting an ice age is the shape of the Earth's orbit.

0:50:350:50:40

The Earth's orbit is an ellipse,

0:50:420:50:44

but over time, it becomes slightly more, and then slightly less, elliptical.

0:50:440:50:49

When the orbit is at its most elliptical, the result is lower summer temperatures.

0:50:490:50:54

At the moment, the Earth is midway through this cycle,

0:50:570:51:01

so again, it's not in an ice age phase.

0:51:010:51:04

It's only when all three of these changes to the Earth's cycle line up together

0:51:090:51:14

that they produce the really cool summers

0:51:140:51:17

in the northern hemisphere that result in ice ages.

0:51:170:51:20

It'll be around 60,000 years before the cycles line up again

0:51:270:51:31

and the next ice age starts.

0:51:310:51:34

In our journey around the sun, it's now the beginning of March

0:51:380:51:42

and the shackles of winter are being loosened in Britain

0:51:420:51:46

as we move into spring.

0:51:460:51:49

The land starts greening as longer days bring more energy from the sun.

0:51:490:51:55

But in some parts of the world,

0:51:550:51:57

the effect of that warming has yet to be felt.

0:51:570:52:00

I've come to Greenland, where there's definitely not much sign of spring yet.

0:52:080:52:13

This is Kulusuk. It's a tiny settlement of just 355 people

0:52:150:52:21

perched on the edge of an island in eastern Greenland.

0:52:210:52:24

To the north of here is the Arctic Circle and the Greenland ice cap.

0:52:240:52:28

Kulusuk is surrounded by the Arctic Ocean.

0:52:320:52:36

At this time of year, it's frozen, covered in a thick layer of sea ice.

0:52:360:52:42

Each year, the extent of the sea ice is different.

0:52:470:52:50

To see how far it reaches this year,

0:52:500:52:53

I need to travel right to the edge of the sea ice.

0:52:530:52:57

-Want me to bring this?

-Yeah.

0:53:030:53:05

'I meet up with my guide, local hunter Gio Utuaq.

0:53:060:53:12

'His hunting grounds lie right at the edge of the sea ice.

0:53:120:53:16

She's so keen!

0:53:170:53:20

How far do we have to go to get to the hunting grounds?

0:53:210:53:26

20, maybe 25 kilometres.

0:53:260:53:29

After two hours, we reach a huge expanse of sea ice.

0:53:420:53:48

It's impossible to comprehend that the snow we're travelling across sits on ice,

0:53:490:53:54

which sits on the ocean.

0:53:540:53:56

We're travelling across a frozen sea. And look at this!

0:53:560:53:59

This is an iceberg actually trapped within the sea ice.

0:53:590:54:03

It's the most astonishing landscape, or seascape or ice-scape...

0:54:030:54:09

What do you call it?! ..that I've ever seen!

0:54:090:54:12

It's like another world.

0:54:120:54:14

And then, surprisingly quickly, the edge of the ice comes into view

0:54:280:54:32

and I can see the Arctic Ocean.

0:54:320:54:36

For obvious reasons, we make the last stretch of the journey on foot.

0:54:390:54:44

-Are you sure?

-SHE CHUCKLES

0:54:470:54:50

There is something

0:54:500:54:54

very disconcerting

0:54:540:54:57

about walking on sea ice

0:54:570:55:00

when the open sea is so close.

0:55:000:55:04

Yeah, it looks pretty solid. How thick is the ice?

0:55:060:55:10

Like this thick?

0:55:100:55:12

It seems strange to be walking across a frozen sea here in Greenland

0:55:160:55:21

when back at home, the daffodils are beginning to come up.

0:55:210:55:25

But what's even stranger

0:55:250:55:26

is that measurements of the sea ice over the last 50 years

0:55:260:55:30

show that it only reaches its full extent now, in early March.

0:55:300:55:36

So clearly there's a lag between the arrival of the warmth of the sun

0:55:360:55:40

and the melting of the ice. But why?

0:55:400:55:43

It comes down to the properties of water.

0:55:450:55:48

We've already seen that, well into January,

0:55:480:55:52

land continues to lose more heat than it gains.

0:55:520:55:55

Because water radiates heat even more effectively than land,

0:55:550:56:00

the oceans take even longer to start warming up.

0:56:000:56:04

So although the land has been warming since January the 19th,

0:56:040:56:08

the sea is still losing heat and the ice continues to grow.

0:56:080:56:14

Greenland sea ice is at its maximum extent at this time of year, in March.

0:56:140:56:18

But over the next few weeks, the tilt of the Earth towards the sun as it orbits it

0:56:180:56:23

will allow the northern hemisphere to get an increasing amount of solar energy.

0:56:230:56:27

The days will get longer and warmer

0:56:270:56:30

and the sea ice will begin to break up and recede.

0:56:300:56:33

Then the hunting season will be over.

0:56:330:56:35

The existence of the sea ice here in Greenland

0:56:450:56:49

is testament to the complex response our planet has to the sun,

0:56:490:56:54

in whose orbit we travel.

0:56:540:56:56

But it's a very delicate balance

0:56:560:56:59

and no-one is more acutely aware of that than the people who live here.

0:56:590:57:04

Gio tells me that this year,

0:57:070:57:09

there was less ice than in previous years.

0:57:090:57:12

It's part of a trend over the whole of the Arctic.

0:57:120:57:16

The area covered by sea ice has shrunk significantly in the last 20 years.

0:57:160:57:22

A series of warm winters

0:57:220:57:23

have meant that the seas haven't cooled down as much as normal

0:57:230:57:27

so not as much ice has been able to form.

0:57:270:57:32

And there are many scientists who argue

0:57:320:57:36

that the cause of the warmer winters is us.

0:57:360:57:39

Global warming can feel like a myth when, back in the UK,

0:57:410:57:47

we've endured a string of very cold winters.

0:57:470:57:51

But here on the front line, it's a reality.

0:57:510:57:56

Most predictions suggest that the Arctic will continue to warm rapidly

0:58:010:58:05

over the course of this century.

0:58:050:58:08

It could be that we may well prove capable of generating the kind of climate change

0:58:090:58:16

that in the past has been created by changes in the Earth's orbit.

0:58:160:58:20

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