主播大秀

The second instalment of the series follows the Earth's journey from the start of January to the Spring Equinox in March. Available on iplayer. What did you think?

Kate begins the film on a day with a very significant point in our Earth's journey - Perihelion. Kate climbs Aonach Mor mountain, one of the highest mountains in Scotland, which brings her as close to the Sun as she'll ever be for the entire year.

This however is not because of where she is but because of the point the Earth has reached in its orbit around the Sun. In fact we kick started our blog on this day just over a year ago, when we explored the elliptical shape of our planet's orbit and how significant this was to our understanding of Earth's climate.

Later in the film Helen explains how the proximity of the Earth to the Sun doesn't guarantee warmth - which brings us to the tilt of the Earth (23.4 degrees) - a theme we explore in further detail in episode three.

Throughout this episode Kate and Helen explore the increase in solar radiation and how land and ocean respond to it.

Kate drives over a frozen lake in Canada with an ice road trucker in one of the coldest places in that region and learns how important this ice formation is to connecting communities.

In this film we also tackle ice ages and how over time, as Earth has repeated it's annual journey, it's climate has changed.

Helen dives under water in Belize to discover how sea levels have risen and fallen over time due to ice age - and explores the three cycles that need to be right in order for another ice age to exist.

What did you think of episode two?

(There are a total of three episodes in this series)

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Distance travelled ~ 647'166'400 km

(Peter Gibbs is a 主播大秀 weather forecaster and a regular blogger for 23 Degrees. With all the current Atlantic hurricane activity, here Peter tackles some of the questions flying around as hurricane Katia makes it's way to the UK.)

"Do we get hurricanes in the UK?"

No, it's not possible.

"But what about the 1987 storm, wasn't that a hurricane?"

Well, it did have hurricane force winds, which was why it knocked down 15 million trees, but it wasn't a hurricane.

"How does that work, then?"

I've lost count of the number of times that I've had that conversation during my career, but it's a reasonable question and one that's worth exploring during this very active Atlantic hurricane season.

Hurricanes are creatures of the tropics, they need the warmth and humidity of tropical seas to develop and survive. The core of the storm consists entirely of warm air and it's the release of latent heat as this air rises and condenses into clouds which gives the hurricane its power. It's a bit like a pan of water coming to the boil as you apply heat from below.

Once formed, a hurricane moves through the surrounding atmosphere like a cork floating down a stream, becoming almost a separate entity. The strongest winds form in the lowest layers of the storm, close to the storm's centre just outside the eye.

Move into temperate latitudes and weather works differently. Extratropical storms (more commonly known as mid-latitude depressions) form over much colder waters and get their energy instead from the contrast between masses of warm and cold air. The bigger the contrast, the stronger the storm. Cold air makes up the core, digging under the warmer air mass and lifting it until it forms clouds and rain along a front.

The storm becomes an integral part of the atmospheric circulation, like an eddy in a river. Strongest winds are found high up, in the form of the jetstream, at around 30,000ft while the strongest surface winds tend to occur at some distance from the storm's centre and are spread out over an elongated area. All very different to a hurricane.

Where it gets messy is when a hurricane heads out of the tropics and into higher, temperate latitudes. It goes through an identity crisis as the supply of warmth from below is cut off and cold air is drawn into the circulation, eventually emerging as an extratropical storm after a fuzzy intermediate stage.

Fortunately, the long sea track ensures that any ex-hurricanes reaching the UK have gone through full transition before they arrive. The different amounts of available energy mean that even the most powerful of extratropical storms would barely make it onto the bottom of the hurricane scale.

So that's how it works

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

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

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

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

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

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d ~ 388'492'800 km: day 151

It's easy to forget how large our planet is. If you were to start anywhere on the Equator and follow it around the planet, you'd pass 25,000 miles of mountains, forests, deserts and ocean (mostly ocean) before you arrived back at your starting point. And when you got back, covered in sand, salt, tropical insects and the odd bird dropping, it would be hard to imagine that the weather over a full third of that enormous distance is all connected. But that's what El Ni帽o and La Ni帽a represent - two extreme states of one huge system that covers a third of the Equator.

Digesting that fact might stretch the brain a bit, but it doesn't stop there. The atmosphere and the ocean are equal partners in all this. When the airflow across all those 10,000 miles of the Equator changes, the flow of millions of tonnes of ocean water changes too. Today, we can see that a huge change in the flow of all this air and water is about to happen - a period of La Ni帽a conditions is giving way to "neutral" conditions. But what does that mean?

Let's start in the atmosphere near South America. High up, there's cold dry air travelling eastwards from Australia. It hits the Andes, and it sinks. That pushes down on the air that was already there and this lower air is pushed back westwards along the Equator to Australia. So we have a giant conveyer belt of air that is travelling west along the Equator at the ocean surface. If you blow sideways on a hot cup of tea, you'll see that you push the surface tea away from you, and this is what happens in the ocean. The water at the surface of the ocean is pushed along to the west. But what takes its place? The answer is cold water from deeper down in the ocean, so in normal conditions, the water right at the South American coast is cold, because all the warm water is being pushed along towards Australia. This is a "neutral" state.

happens when that westwards flow of air weakens, and so the warm water can slosh back towards South America. This is where the name "El Ni帽o" came from - it's what Peruvian fishermen called the warm water that sporadically appeared off their coast. When this happens, the cold water never reaches the surface. La Ni帽a conditions are the opposite - the westwards winds get even stronger, and push the warm water even further west. It's a bit like a giant water seesaw. This enormous system of air and water switches between these two states every few years, but we can't really predict when it's going to happen.

During El Ni帽o years, there can be droughts in and Indonesia, and whole ecosystems can be disrupted because the nutrient-rich cold water never reaches the ocean surface. In La Ni帽a years, there is really strong rainfall in Australia and much less rain in South America.

The current data suggest that La Ni帽a is just about to end. This will probably mean fewer extreme weather events in the western Pacific in the next year, and I bet the Ozzies are grateful for that. It's harder to judge whether or not this shift will affect us in Europe directly. Even though it's so large, the Southern Pacific is as far away from us as it's possible to get and still be on Earth. But it's amazing to think that such huge parts of the atmosphere and the ocean are so closely linked, and that a single weather system can stretch right across the largest ocean basin on Earth.

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d ~ 144'076'800 km: day 56

Video update from Chanabaya Chile:

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The team set off to Greenland next week. Keep updated with听our travels via

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