Why do foods take longer to cook at high altitudes?
(Lansing State Journal, November 23, 1994)

At room temperature, most of the water molecules aren't moving fast enough to escape the attractions between them.  As the temperature is increased, the water molecules move faster and faster and more of them are able to escape creating a "vapor pressure" from water.

In order for water to boil, the pressure from the escaping water molecules has to be at least equal to the atmospheric pressure.  At this point the water boils.  The higher the atmospheric pressure is, the hotter the water has to be in order for the molecules to build up enough pressure to boil.

It also works the other way around.  At a lower atmospheric pressure, the temperature doesn't have to be as high for the water to boil.

Food generally contains a certain amount of water.  Under normal atmospheric pressure, water boils at 100 degrees Celsius.  The higher the altitude, the lower the atmospheric pressure.  So , water boiling at a high altitude will boil at less than 100 degrees Celsius.

Since water in the food or the water it is being boiled in are boiling at a lower temperature than normal, the food will  take longer to cook.

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