(Lansing State Journal, December 4, 1992)
Question submitted by: J.D. Jackson of DeWitt
Let's consider a simple cooking process: boiling water. The energy of water molecules increases as they are heated. When water boils,the more energetic water molecules escape from the liquid as steam.
In normal atmospheric pressure, the boiling point of water is 100 degrees Celsius (212 degrees Fahrenheit) If you put a tight lid on a pot, steam will accumulate under the lid, so the pressure on the top of the water becomes greater than atmospheric pressure. For a water molecule to escape, it must now have more energy than if it had to escape from an uncovered pot.
At high altitudes, we encounter the opposite case; the air is thinner and the atmospheric pressure is less than at sea level. Water molecules can escape as vapor at much lower energies, so water will boil at a lower temperature. The higher the altitude, the lower the boiling point.
In Denver (1700 m above sea level), the boiling point of water can be as low as 95 degrees Celsius (203 degrees Fahrenheit). At the top of Mt. Everest (9500 m above sea level), water boils at 75 degrees Celsius (167 degrees Fahrenheit).
Cooking at high altitudes requires either higher cooking temperatures or longer cooking times due to the lower atmospheric pressure. In baking, increasing the temperature of the oven contracts the lower internal temperature of the food. In cooking on top of the stove, the lower atmospheric pressure means liquids boil at lower temperatures, requiring longer cooking times.