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 air 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 2000 feet (600 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.
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Boiling
At sea level water boils at 212°. For each 500' increase in elevation boiling point is lowered by just under 1°. For 7,500' elevation, water boils at 198°. Boiling as a cooking method must be adjusted or alternatives applied. Vegetables and some starches will simply take longer to cook while rice and legumes (beans) need a pressure cooker. Pasta will need a pressure cooker.
Methods used at high altitudes
From pressure cooking: A pressure cooker is often used to compensate for the low atmospheric pressure at a very high elevation. Under these circumstances water boils at temperatures significantly below 100 °C (212 °F) and, without the use of a pressure cooker, may leave boiled foods undercooked, as described in Charles Darwin's The Voyage of the Beagle (chapter XV, March 21, 1835 books.google):
Having crossed the Peuquenes [Piuquenes], we descended into a mountainous country, intermediate between the two main ranges, and then took up our quarters for the night. We were now in the republic of Mendoza. The elevation was probably not under 11,000 feet [...]. At the place where we slept water necessarily boiled, from the diminished pressure of the atmosphere, at a lower temperature than it does in a less lofty country; the case being the converse of that of a Papin's digester. Hence the potatoes, after remaining for some hours in the boiling water, were nearly as hard as ever. The pot was left on the fire all night, and next morning it was boiled again, but yet the potatoes were not cooked.
Lightweight pressure cookers as small as 1.5 liters weighing 1.28 kg are available for mountain climbers. Sherpas often use pressure cookers in base camp.
Boiling point of pure water at elevated altitudes
Based on the standard sea-level atmospheric pressure (courtesy, NOAA):
Source: NASA