How does air pressure affect the temperature at which water boils?
Water boils at lower temperatures at lower air pressures. This is why water boils at lower temperatues at high elevations, such as in the mountains, where the air pressure is lower. As water boils, bubbles of water vapor rise to the surface of the water and escape. In order for these bubbles to make it to the surface, the saturation vapor pressure must equal the atmospheric pressure, otherwise the bubbles would collapse. The saturation vapor pressure is directly proportional to the temperature of the water. In other words, the higher the temperature, the higher the saturation vapor pressure. At higher elevations, the atmospheric pressure is less than at lower elevations. This means that the saturation vapor pressure needed to allow bubbles to escape into the air is less, which means that the temperature water must be heated to in order to bring it to a boil is also less. Meteorology Today by Donald Ahrens has an excellent discussion about this concept. Cooking things in boiling water a
