It is colder as you go up in altitude because the atmosphere gets thinner.
Explanation:
As you climb to higher altitudes above sea level there is lower pressure in the sense there are fewer air molecules. When the pressure of a gas is low, the temperature also lowers. In contrast, gas pressure increases will lead to temperature increases. Lower air pressure makes it harder for oxygen to enter our system. Our pulse rate increases in order to get more oxygen to our blood. People who live at high altitudes have been shown to have higher lung capacity and more red blood cells because the body has to work harder to get oxygen. Mountainous areas like the Himalayas and the Andes are home to entire cultures that have adapted to the relatively harsh conditions of living at high altitudes with colder temperatures.
