Altitude is an important feature of mountain environments. As you go higher, the air gets colder and thinner. It also tends to rain more, and snow and ice last longer through the year. You notice this in part through the fact that high mountains have a snow line.

Another way of noticing altitude is through the tree line, the height above which trees cannot grow.

Altitude sickness is also known as mountain sickness. It is an illness that affects people when they move quickly from a lowland to a high mountain. It is caused by the lower amount of oxygen that is in the air at high altitudes.