Simply put, the weight of air on any surface it comes in contact with is called air (or atmospheric) pressure. There’s less (or lower) 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 degrees 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. This 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 candy making, 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.
