Air temperature decreases as altitude increases. Air is a mixture of gases, and at higher altitudes, air pressure decreases, which cools any gas. Air pressure on Earth is estimated to be about 14.7 pounds per square inch at sea level.
Air pressure drops to about 1.6 pounds per square inch at 50,000 feet above sea level. Air pressure is created when gravity acts on the many miles of air molecules comprising the atmosphere. Air pressure is not uniform worldwide; it can change over time. Desert regions typically have lower air pressure than other areas because the hot desert air is less dense than cool air.