At 8848 meters (29,030 feet) above sea level, Mount Everest is the highest mountain in the world. In 1952 Edmund Hillary, a New Zealand mountaineer, and Tenzing Norgay, a mountain guide from Nepal, became the first humans to reach the top. The climb was very dangerous for several reasons, including the extremely low temperature and low level of oxygen at the summit. When they finally made it to the top, the two men cheered and embraced. Then Hillary did something to prove a point—he took off his oxygen mask. He wanted to show that a person could stay alive while breathing naturally at that high altitude. But after a few minutes, his vision began to fail. When Hillary replaced his mask, his vision improved. Then the two men started back down the mountain.

Figure 1  Edmund Hillary (left) and Tenzing Norgay were the first to climb Mount Everest, the world's highest mountain.

Why did Hillary and Tenzing need to carry oxygen to the top of Mount Everest? And why was it so cold there? The answers to these questions depend on how the air around Earth changes with altitude.