Simply put, the weight of air on any surface it comes in contact with is called air (or atmospheric) pressure. There's less air pressure at high altitudes because the blanket of air above is thinner than it would be at sea level. As a result, at sea level water boils at 212°F; at an altitude of 7,500 feet, however, it boils at about 198°F because there's not as much air pressure to inhibit the boiling action. This also means that because at high altitudes boiling water is 14° cooler than at sea level, foods will take longer to cook because they're heating at a lower temperature. Lower air pressure also causes boiling water to evaporate more quickly in a high altitude. Therefore boiled foods like dried beans and peas take longer to cook at high altitudes and may require more liquid than at sea level. Meat, poultry and fish usually also require longer cooking times. For deep-fat frying, decrease the fat temperature by 3 degrees for each 1,000 feet above sea level; fry foods for a longer time. Foods stored at high altitudes dry out more quickly than those at low altitudes. That means that an ingredient such as flour is drier and will absorb more liquid. Therefore, slightly more liquid or less flour may be required for cake batters or bread and cookie doughs to reach the proper consistency. Decreased air pressure means that adjustments in some ingredients and cooking time and temperature will have to be made for high-altitude baking, as well as some cooking techniques such as candymaking, deep-fat frying and canning. In general, no recipe adjustment is necessary for yeast-risen baked goods, although allowing the dough or batter to rise twice before the final pan rising develops a better flavor. Increasing the baking temperature by 25°F will help set the crust faster so bread will not over-rise during the oven-spring that takes place the first 10 to 15 minutes of baking.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.