The boiling point for any liquid occurs when the vapour pressure of the liquid equals the surrounding atmospheric pressure. Heating a liquid at this stage causes some of the liquid's molecules to gain sufficient energy to overcome downward atmospheric pressure and escape as a gas, commonly known as boiling. The boiling point of water at sea level is 100 degrees C, or 100 degrees C. Most people, however, usually live above sea level. Because there is less atmosphere at elevations above sea level, there is less pressure, and water molecules enter the gas phase at a lower temperature. This difference affects cooking times, so knowing the exact boiling temperature is important. Day-to-day changes in local atmospheric pressure also influence the boiling point of water.
Look up a topographic map of the area at a local library or community college. The map will have the elevation (or altitude) above sea level, which is needed to calculate boiling point. Simple point-and-click online applications are also available for finding the approximate elevation for a location. Write down the elevation in feet.
Round the local elevation to the nearest multiple of 500. For example, if the local elevation is 1,648 feet, round down to 1,500 feet.
Determine the standard boiling point of water based on the rounded local elevation. At seal level (0 feet), the boiling point is 100 degrees C. At 500 feet, the boiling point is 99.5 degrees C; at 1,000 feet, the boiling point is 99 degrees C. For every increase of 500 feet above sea level, the boiling-point temperature drops by -17.3 degrees C. Write down this boiling point value.
Check a barometer or a local weather forecast to get the barometric pressure for the area. Write down the current barometric pressure, rounded to the nearest 2/10 inch of mercury (atmospheric pressure in the U.S. is measured in inches of mercury). For example, if the pressure is currently 30.16 inches of mercury, round to 30.2 inches.
Calculate adjustments for the boiling point using the current (rounded) barometric pressure. When the pressure is 29.921, the temperature does not need to be adjusted. At 29.8 and lower, the standard boiling point in Step 3 must be reduced by approximately -17.6 degrees Cor every 0.2 Hg difference in pressure to arrive at the final, correct boiling point. At 30 and higher, the standard boiling point must be increased by approximately -17.6 degrees Cor every 0.2 Hg difference in pressure.
Repeat Steps 1 through 5 when the weather or the location changes.
Adding a little salt to water will raise its boiling temperature, regardless of elevation or atmospheric pressure.
Boiling water is always hot and will cause burns. Cooking at higher elevations will require longer times to kill some pathogens.