According to Bodner Research Web that is produced by Perdue University, a liquid boils when its vapor pressure is equal to the pressure that is exerted on the liquid by its environment. It is then that bubbles form inside the liquid.
Pressure is caused by the bombardment or collision of particles with a surface. As one moves up a mountain or hill, the number of air particles decreases. Therefore, there is less pressure as the elevation increases up a hill or mountain.
Since the air pressure is less, the water molecules on top of a mountain or hill are not “squished” together as much as they would be at sea level. Therefore, the water is able to expand with greater ease and boil faster at the higher elevation. Thus, at higher elevations, water boils at a lower temperature. For this reason, the amount of cooking time or the temperature at which a food is cooked may need to be increased when made at higher elevation.