It gets thinner (the pressure drops) and colder.
The atmosphere is densest at sea level, so as the density of air goes down, so does the atmospheric pressure. The atmosphere gets less and less dense as you rise higher above the sea level.
A balloon would expand because it is flexable and when the outside pressure gets lower it fills out. An airplane actually shrinks in the exterme cold of the upper atmosphere. It is not going to shrink or expand much because it is made out of metal not rubber.
The air becomes less dense.
it explodes man
it gets coolder
temperature and altitude
In a commercial airliner flying at cruising altitude, the air pressure inside the airplane is considerably higher than the air pressure outside. The air pressure outside is too low for people to breathe comfortably, so the interior is pressurized.
formation of ozone
The Earth's atmosphere becomes thinner the higher it goes, until the vacuum of space is reached.
The air becomes less dense, the higher you go. And generally, the temperature gets colder until you reach the thermosphere, which is the hottest part of Earths atmosphere.
It rises until the density of the atmosphere, which thins as you go higher, is equal to that of the gas. Then the gas disperses into that level of the atmosphere.
Begin to succumb to the power of gravity because of the decrease in lift