Atmospheres: a planet’s surrounding gases
Gravity helps retain an atmosphere, whose density generally decreases with height. Earth’s atmosphere has layers defined by changes in temperature with altitude, rather than solid dividing walls.
Having an atmosphere does not mean having breathable air. Composition and pressure matter. A visible cloud is not the whole atmosphere: much of the gas around a planet is transparent.
Wonder about this
If a planet has clouds, can people breathe there?
A science clue
Not necessarily. Its gases and pressure may be very different from Earth’s.