Final answer:
The atmosphere gets colder with higher altitude in the troposphere due to decreasing air density and distance from Earth's surface heat. Above the troposphere, temperature increases in the stratosphere due to UV radiation absorption by ozone.
Step-by-step explanation:
Why does the atmosphere get colder with higher altitude? The primary reason is that the air density decreases as you go higher. This means there are fewer air molecules to absorb and re-radiate the heat coming from the Earth's surface.
In the lowest layer of the atmosphere, known as the troposphere, temperature decreases the further away you get from the heat of the Earth's surface, typically at a rate of about 6.5 K per 1 km of altitude.
At the top of the troposphere is the coldest part called the tropopause. Above this layer, in the stratosphere, the temperature starts to increase again because of the absorption of UV radiation by ozone.
The atmosphere is composed mainly of nitrogen and oxygen, with traces of other gases. At higher altitudes, lighter gases like helium are less present due to their ability to reach escape velocity and leave the Earth's gravitational pull.
Most atmospheric gases have smaller root-mean-square (rms) speeds and don't reach escape velocity, ensuring the atmosphere stays largely intact over billions of years.