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23 Degrees heads to Argentina

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Stephen Marsh Stephen Marsh | 21:30 UK time, Thursday, 27 January 2011

d ~ 69'465'600 km: day 27 of Earth's orbit

Having spent time in the coldest place in North America the 23 Degrees team are now south in Argentina. Their mission is to travel across South America, from East to West following a single line of latitude to discover why there is such a wide range of climates and landscapes across the continent even though they all receive the same amount of solar energy.

They'll begin their journey in one of the most eerily beautiful places on the planet, the cloud forest of Calilegua. It's called a cloud forest because the tree tops grow right up into the clouds and there's a ghostly fog everywhere. It's not that the clouds are low, more that the forest is high up. In fact, it's around 2000 metres up in the foothills of the Andes. It's a very wet environment because as well as the constant clouds it rains here around 150cm a year.

This region is also the thunder and lightning capital of the world. Here, wet air is transformed into giant thunderclouds which unleash incredible rain and lightning, including a particularly strong form called positive lightning. What makes it so deadly is that it can strike kilometres away from the storm, which could well be the origin of the term "a bolt from the blue".

From there the team are driving up over the Andes. On the way they pass through Paso de Jama at the top of the Andes, right on the border between Argentina and Chile. It's at 4,400 metres so they won't stay long because of the risk of altitude sickness.

On the other side they will, visit one of the wonders of the natural world. The lagunas at Salar de Atacama. These briny lakes are home to thousands of Flamingos that come to feed on algae and shrimps. The shrimps are rather special - they have evolved a way of surviving in this hostile place. If there's a drought, their eggs can go into suspended animation for up to 50 years. When there's enough water gain they burst into life and hatch.

The region around the lakes is incredibly dry. It rarely rains here at all. And the reason for this is the Andes Mountains which form a barrier running down the continent. They block the movement of moisture from the east. Any air that does pass over the mountains has its moisture stripped out and dumped as rain on the other side providing moisture for the cloud forests. By the time the air gets here its hot and dry. This drying effect is called a rain shadow.

After the lagunas Kate and the team will be heading to one of the most hostile places on earth - the Atacama Desert. The amount of sun is the same as back in the cloud forest but the terrain is utterly different. Gone are the lakes - and the water. It's as dry as a bone.

To get a sense of just how dry this place is, the team will stop off at a salt mine - they dig out 11,000 tonnes a day here. Salt can only occur on the surface like this if the climate is dry, and I mean really dry. Any rain at all and the salt will dissolve and be washed away. But it hasn't rained here for 400 years, making it the driest place on the planet.

This desert is so arid here because circulation of air in the atmosphere strips away its moisture. Here's how it works. Hot wet air rises at the equator, but as it rises it cools. Cooler air cannot hold as much moisture, so the water vapour condenses and falls as rainfall, creating tropical rain forests. But the air keeps on rising until it reaches around 15 km. Then it flows horizontally toward the South Pole in what's called The Hadley circulation cell. As the air moves away from its main source of heat - the equatorial region it cools down even more. This cooling makes the air denser and so it sinks back to the surface over the Atacama. As the air sinks it creates an area of high pressure full of stable dry air that won't rise or cool and this prevents clouds forming. No clouds mean no rain - creating the super-arid conditions of the Atacama.

Kate's journey finishes at the Pacific Ocean where the climate changes again. Right on the coast is a peculiar natural phenomena - coastal fog that is so persistent that the local get water by "harvesting" the fog, she'll go fishing with the local fishermen to learn how a cold water current just off the coast is the key to the climate of this strange coastal strip.

Kate and the team will have travelled 575 kilometres due west across South America from lush wet cloud forest to arid desert.

Their journey will reveal that even though the land at this latitude gets the same amount of solar energy, the planet itself has the power to transform the climate and create radically different environments.

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