Air is all around us, but we cannot see it. Gravity from the Earth pulls air down - this is called air pressure. We don't feel this pressure because our bodies push an equal amount of pressure outward. This graph shows how air density and air pressure changes with altitude (the distance above sea level). Barometers are used to measure air pressure in milibars.
The atmosphere is divided into different layers depending on how temperature changes. Take a look at the graph below to see how temperature changes with altitude.
As you climb a mountain, you can expect the air temperature to decrease by 6.5 degrees C for every 1000 meters you gain. This is called the standard (average) lapse rate.
If air temperature is 30 degrees C at sea level as shown above, you can expect it to be around 10.5 degrees C at air altitude of 3000 meters because of the lapse rate.
You are going to climb Mount Audubon in Colorado today!