1) Lapse rate: The temperature of the atmosphere decreases with altitude at a rate of about 6.4 °C per 1,000 meters (3.5 °F per 1,000 feet). This is because the air at higher altitudes is less dense than the air at lower altitudes, and therefore it can hold less heat. As a result, the air at the top of a mountain is colder than the air at sea level.
2) Adiabatic cooling: When air rises, it expands and cools. This is because the pressure of the air decreases as it rises, and as the air expands it loses heat. The rate of cooling is about 10 °C per 1,000 meters (5.5 °F per 1,000 feet). This means that the air at the top of a mountain is even colder than it would be if it were simply at a higher altitude.
3) Snow and ice: Snow and ice reflect sunlight, which means that they do not absorb heat. This means that the air near snow and ice is colder than the air that is not near snow and ice. As a result, the air at the top of a mountain is even colder if there is snow and ice present.
The combination of these three factors means that the air at the top of a mountain is much colder than the air at sea level. The difference in temperature can be as much as 20 °C (36 °F). This is why it is important to dress warmly when hiking or camping in the mountains, even in the summer.
