How Thin the Air Becomes as You Climb

Not every relationship is a straight line. This one is a curve: a cubic metre of air at sea level weighs about 1.2 kilograms, and the weight drops away quickly at first, then more and more slowly, until at the height of Mount Everest it is almost nothing.

The shape is why climbers carry oxygen. At 8,828 metres a cubic metre of air weighs 0.023 kilograms, about a fiftieth of the sea-level figure, so each breath brings in a fraction of the oxygen it would at the bottom. The slider reads the curve at any altitude and the panel gives the weight and how it compares with sea level.