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Distance travelled ~ 840'555'200 km

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Being surrounded by sky is not a natural place for a human being. We have evolved to scoot about on the bottom of the atmosphere, stuck to the ground, and we don't often look up. Even when we do, we tend to see the sky as flat - clouds, the moon and aeroplanes move sideways across the sky. It's easy to forget that the sky has depth too, and that air in the atmosphere moves up and down as well as sideways.

Your perspective changes quickly when you're in freefall, three thousand metres above the Earth's surface and travelling downwards at 120 mph.

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Skydivers relish the sense of freedom that falling through the sky brings. There is nothing to get in the way, nothing touching you and a whole extra dimension to play in. For the air in our atmosphere, three-dimensional movement is normal. At the place where I jumped out of the plane, in Arizona, air that starts about 10 miles up is gradually sinking towards the ground. The air doesn't make the squeaking noises that I did, but then it isn't falling nearly as fast - it's a few millimeters per second on average. I was falling through a giant atmospheric waterfall, but a very slow one.

It's not just in Arizona that this happens. Although weather maps tend to show sideways winds, the air making up those winds is all rising and falling as it travels around the Earth. The paths of air parcels weave in and out of each other, making the Tokyo subway map look simplistic by comparison.

Image courtesy of Tokyo Metro

All this is very interesting, but not much comfort to a plummeting physicist. I don't think that I really breathed during the 40 seconds of freefall. Then the parachute opened, everything slowed down, and my brain stopped panicking and started appreciating what was going on around it.

Seeing the layers of the sky is fascinating. Floating down past a cloud is amazing - a fluffy cumulous cloud is telling you that there's been a little puff of air upwards in that location. We can't really see the structure of the atmosphere, but seeing a cloud from the side makes it easy to imagine the turbulent swirls that are mixing all that air up.

After five minutes of sharing the three dimensions of the sky with the clouds, we arrived at the landing zone and my feet touched the ground again. I was very happy to feel something solid under my feet, but there was also a small sense of loss. I was back to crawling around on the bottom of our fabulous three-dimensional atmosphere, and my understanding of the depth of the atmosphere was again limited to hints given away by the clouds. But I remember what it felt like, and my view of the sky will never be quite the same again.

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