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I am sure that is a question which Mark often asked himself as he cycled the length of the Atacama Desert back in December. You can see where Mark is right now, and what he got 'up to' after the desert cycle on ´óÏó´«Ã½ Cycling The Americas.

The occupies the northern part of Chile and small parts of the neighbouring countries of Peru, Bolivia and Argentina. It is approximately 1000 kilometers in length and about 150 kilometers at its widest. It is the driest desert on earth with some places having never recorded rainfall. This photo shows Mark in the town of Quillagua in northern Chile sitting beside a local resident who has never experienced rain falling. I guess umbrellas don't sell well in Quillagua!

Deserts are described as places receiving less than an average of 250 mm of rain in a year. The Atacama, however, receives less than an average of 1mm a year! This often means that decades pass without any rain at all and then one brief rain storm brings just a few millimeters. The deserts of south west USA and northern Mexico that Mark cycled through in the early autumn are positively 'wet' by comparison.

Why is it that amongst the world's dry places, the Atacama is the driest of them all? The simple answer is that is doesn't rain there. So why doesn't it rain? If we can answer that question, we can explain why the Atacama is the driest desert on earth.

There are basically three reasons why an area may be a desert. Any one of those reasons is enough to cause desert conditions but the Atacama has all three! The diagram below shows how these affect the Atacama.

Firstly the Atacama lies on the wrong side of the Andes with regard to the prevailing winds. At 20 degrees south the most common winds are the south east trade winds which carry in moist air from the Atlantic. As the winds are forced to rise to cross the Andes, they are cooled (remember it gets colder the higher up you go). Eventually, the moisture in the air condenses and turns to rain, falling on... the 'wrong' side of the Andes! This means that the Atacama lies in a rainshadow.

The second reason is a little harder to understand but is related to air pressure. Air generally rises at the equator because this is where maximum heating of the earth's surface occurs. Once it rises into the upper atmosphere, high level winds carry this air towards the poles. The air gradually cools and eventually falls back to earth between 20 and 25 degrees south (exactly where the Atacama is), creating high pressure. Descending air warms up and any moisture in it evaporates into water vapour which is a gas and so doesn't bring rain.

And as if being in a rainshadow and having high pressure wasn't enough, the Atacama lies close to an ocean where a cold current flows northwards up the coastline. The Pacific Ocean is therefore colder than might be expected at this latitude. Any on-shore winds are chilled when they cross the current and don't have enough warmth to pick up moisture from the ocean surface. So, unlike most winds from the seas and oceans, these ones are dry.

There is no doubt that lack of water makes life a struggle for the people who live in the Atacama. However, the extreme dryness is not without its benefits. The Atacama desert is the only place in the world where saltpetre, a type of nitrate which used to be used in explosives and fertilisers, can be found. It's easily dissolved by rain so the Atacama desert is perfect for preserving it. Until the 1950s there were many small mining settlements in the Atacama like the ghost town of Humberstone which Mark cycled through. You can watch a bit of video footage of Mark in Humberstone.

Another benefit is that the absence of clouds makes the Atacama the perfect place to watch the stars! When he was in the Atacama, Mark frequently commented on the clear night skies and said how much he was enjoying star gazing.

"The night sky in the Atacama is the best I have ever seen".

Here is Mark's photo of the Paranal Observatory and what he had to say about it...

"Sitting above 2500m, in a place where it never rains, many miles from the nearest light, makes this spot in the middle of the Atacama desert ideal for star gazing. Despite its remote location, this place must have the best road in Chile!"

I am writing this posting as news of Mark's successful attempt to summit Aconcagua is breaking. He has just climbed the highest mountain in the world outside Asia... but that is a Geoblog for another day.

Val Vannet

Would you expect to find penguins in the desert? Could there be ?