It thins rather than ends
Where Does the Air Stop?
Air has no edge; it simply thins with height until there is effectively none left.
-
Step 1 of 5
Air is squashed by the weight above it.
Every layer of air is pressed down by all the air stacked on top, so the air nearest the ground is packed the most tightly.
-
Step 2 of 5
So the air thins rapidly with height.
Half of the entire atmosphere sits below about five and a half kilometres, which is lower than several mountain ranges.
-
Step 3 of 5
Climbers feel it long before the top.
By the height of the tallest peaks there is only about a third as much oxygen per breath, which is why bottled oxygen is usually needed.
-
Step 4 of 5
There is no boundary, only a convention.
Air keeps thinning without ever stopping, so where space begins is an agreed line at 100 kilometres rather than a physical edge.
-
Step 5 of 5
Even orbit is not completely empty.
The space station flies through the faintest traces of atmosphere, and that whisper of drag slowly pulls it down unless it reboosts.
The space station loses around two kilometres of altitude a month to drag and must be boosted back up regularly.
The short version
Air has no edge; it thins with height until effectively nothing is left, so the boundary of space is an agreed line rather than a real one.
Try it yourself
Take a sealed crisp packet up a mountain or in a plane. It puffs up as the air outside thins and stops squeezing it.
Topics