The boiling point of a liquid decreases with decreased pressure. Since air pressure is lower in the mountains, water will boil at a lower temperature than it does at sea level. This means that it requires less energy and therefore a shorter heating time to reach its boiling temperature. Air pressure is caused by the weight of the atmosphere above us. The higher the altitude, the less air above us so the lower the pressure.
The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals atmospheric pressure. The vapor pressure, which is the pressure exerted by gaseous molecules of the substance at its surface, increases with temperature. Water’s vapor pressure reaches 760 mm Hg at a temperature of 100 C, its normal boiling point. This is the normal pressure at sea level. If the atmospheric pressure is less than 760 mmHg the vapor pressure of water will reach the atmospheric pressure and boil at a lower temperature.
Cooking directions are often altered for high altitudes because the lower boiling temperature of water results in slower cooking.