Which layer of the atmosphere has a very thin layer of air?

Earth's atmosphere is divided into five broad layers based on temperature and altitude — troposphere, stratosphere, mesosphere, thermosphere, and exosphere — and the density of air, that is, the amount of gas actually present, keeps falling as altitude increases, becoming lowest in the layer that lies farthest from the surface.

The exosphere is this outermost layer, stretching from roughly 700 km above the surface out to about 10,000 km, where it gradually fades into outer space. Gas molecules here are spread so far apart that they rarely collide with one another and can drift off into space entirely, so the exosphere holds by far the least amount of air of all the layers.

Checking the other layers by their own defining features:

• Thermosphere: this layer directly absorbs solar ultraviolet and X-ray radiation, which speeds up its gas particles enough to register temperatures beyond 1500°C — here, 'hot' describes particle speed, not the total amount of matter present.

• Stratosphere: this layer holds the ozone layer, which absorbs solar UV radiation so that temperature rises steadily with altitude here; it is stable enough that weather balloons operate within it and commercial flights typically cruise close to its lower boundary.

• Mesosphere: this is Earth's coldest atmospheric layer, with temperatures falling to about −90°C, and it is where friction with atmospheric gas burns up most meteoroids entering from space before they can reach the ground.

So, the atmospheric layer with the thinnest air is the Exosphere.