Answer:
The air pressure at low altitudes is greatest due to the force of Earth's gravitational pull on the atmospheric gases. The air closest to the center of the Earth is pulled more by gravity and is thus more compact than that in the higher altitudes. This compressed air is more pressurized since the gas molecules are closer together and undergo a myriad of collisions with one another.
At higher altitudes, water boils at a lower temperature than at sea level. This is because the air pressure is much lower and can be more easily overcome by the water molecules. Vapor pressure, or the pressure at which water evaporates, is lower and this means that the temperature at which the water becomes water vapor is lower as well.