why does water boil below 100 c at high elevations
High-altitude cooking High-altitude cooking is the opposite of pressure cooking in that the boiling point of water is lower at higher altitudes due to the decreased atmospheric pressure. This may require an increase in cooking times or temperature and alterations of recipe ingredients. For home cooking, this effect becomes relevant at altitudes above approximately 2,000 feet (610 m). At that altitude, water boils at approximately 208°F (98°C) and adjustments sometimes need to be made to compensate for the reduced air pressure/water boiling point.
Vapor pressure As a general trend, vapor pressures of liquids at ambient temperatures increase with decreasing boiling points. This is illustrated in the vapor pressure chart (see right) that shows graphs of the vapor pressures versus temperatures for a variety of liquids.[6] At the normal boiling point of a liquid, the vapor pressure is equal to the standard atmospheric pressure defined as 1 atmosphere[7] (760 Torr or 101 325 kPa).