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d ~ 82'329'600 km: day 32 of Earth's orbit

Mention the word "weather" to your average Brit, and rain is probably the first image to come into their mind. From refreshing spring showers to gloomy grey downpours, our national relationship with the weather is often distilled down to the question of whether or not we need to take an umbrella when we pop down to the shops.

Water falling out of the sky is a regular occurrence in the UK, and we tend not to worry too much about in the first place.

The one of the driest places on Earth. It's an amazing place, but I think that it's most interesting because it shows how much we take falling water for granted. We live right inside a waterfall the size of a planet, and we almost never notice. Liquid water is heavier than air, and if water was always liquid it would all stay in the oceans. Rain would be impossible and every scrap of land would look like the Atacama Desert.

Fortunately, energy from the sun and the process of evaporation save us from this fate. There are always water molecules evaporating and condensing again at the surface of the ocean. Imagine a boundary between two countries, with people coming and going every day across the border. If the numbers crossing each way stay the same, the total number of people in each country doesn't change even though lots of people have switched places. But if more people travel one way than the other, the total populations do change. The sun provides the energy to do exactly this at the ocean surface. Even a small increase in temperature, causing a small increase in the amount of evaporation, can result in a large amount of extra water ending up in the atmosphere after enough time. During daylight, vast amounts of water move from the ocean to the atmosphere.

Once the water is in the air, wind (also powered by sunlight) carries it higher and often also moves it hundreds of miles sideways. The sun provides an immense amount of energy to lift all this water up very slowly, but we know that what goes up must come down. And if the water condenses and forms raindrops, it comes down an awful lot faster than it went up.

Above our heads are stately rivers of water, either visible clouds or invisible water vapour, being carried around by the wind.

The moving air masses that carry these rivers are part of an interlinked global system. When conditions are right, some of the water from that massive reservoir cascades downwards and we get rain. But over the Atacama Desert, the atmospheric rivers have run dry. While most of the rest of the world experiences the global waterfall at least sometimes, the Atacama is left to parch.

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