Water boils at a lower temperature in the mountains compared to sea level. At sea level, water reaches its boiling point at 212 degrees Fahrenheit (100 degrees Celsius), the temperature at which liquid water transforms into steam. This shift in boiling temperature is a direct consequence of the physical principle related to the surrounding air.
The Role of Atmospheric Pressure
The air enveloping the Earth exerts a measurable force known as atmospheric pressure. This pressure is the collective weight of the entire column of air from the highest reaches of the atmosphere down to the measurement point. It is a constant, invisible force pushing down on everything, including the surface of the water in a pot.
Atmospheric pressure naturally decreases as elevation increases. This reduction occurs because there are fewer air molecules at higher altitudes, meaning the column of air above that point weighs less.
Consequently, the air exerts less force upon the water’s surface. This change in the downward force alters the temperature required for water to boil.
How Lower Pressure Affects Boiling Temperature
Water boils only when its internal pressure, known as vapor pressure, is strong enough to overcome the atmospheric pressure pushing down on it. When water is heated, its molecules gain energy and begin to escape as steam, creating this vapor pressure inside the liquid. At sea level, the vapor pressure must reach the standard atmospheric pressure of 14.7 pounds per square inch (psi) to initiate boiling.
At higher altitudes, the surrounding atmospheric pressure is lower, meaning the water does not need to generate as much internal vapor pressure to boil. Less thermal energy is required for the water molecules to push back against the lighter column of air, resulting in the water reaching its boiling point at a lower temperature.
The boiling point drops by approximately one degree Fahrenheit for every 500 feet of elevation gain. For instance, in Denver, Colorado, which sits about 5,280 feet above sea level, water boils at around 203 degrees Fahrenheit (95 degrees Celsius). This lower temperature is the highest temperature the water can reach, regardless of how much more heat is applied.
Practical Adjustments for High-Altitude Cooking
The lower boiling temperature affects cooking because food is cooked at a lower maximum temperature. Since water boils at roughly 203°F instead of 212°F at 5,000 feet, foods like pasta, eggs, or vegetables require longer cooking times. A general rule is to increase the boiling time by a few minutes for every 1,000 feet of elevation above 3,000 feet.
Baking also requires adjustments due to the reduced atmospheric pressure. Leavening agents, such as yeast and baking powder, work by creating gas bubbles that expand inside the dough or batter. At lower pressures, these gases expand much more quickly, causing the baked goods to rise too fast and potentially collapse, resulting in a dense or crumbly product.
Adjustments for High-Altitude Baking
To compensate for this rapid expansion, bakers must decrease the amount of leavening agent and sometimes reduce the amount of sugar, which weakens the structure of the baked good. Increasing the oven temperature slightly, often by 15 to 25 degrees Fahrenheit, helps set the structure of the food before the gases over-expand. The drier air and faster evaporation at altitude also mean that increasing the amount of liquid or flour in a recipe is often necessary to prevent a dry final product.
Food safety, particularly in home canning, is a concern at higher elevations. Water bath canning relies on the 212°F temperature to sterilize food and kill harmful bacteria, such as Clostridium botulinum. Since the water cannot reach this temperature at altitude, the sterilization process becomes ineffective.
High-altitude canners must use a pressure canner, which artificially raises the pressure inside the vessel to allow the water to boil at temperatures above 212°F. Alternatively, those using the water bath method must increase the processing time to achieve the necessary pasteurization effect. The required pressure or time increase is directly proportional to the elevation, making careful calculation a necessity for safe food preservation.