How does the elevation and air pressure affect the boiling point of water?

It depends only on the pressure

The only factor that determines the boiling point of water at any altitude is the barometric pressure (the altitude itself doesn't actually matter, it's just that at higher elevation, the barometric pressure usually drops). However, water will boil at two different temperatures at the same elevation if you are in a high pressure weather system instead of a low pressure system.

See the two Web Links listed to the left of this answer to find a chart of the standard barometric pressure at different altitudes (SableSys.com). Also, I've linked a very easy to use calculator to find the boiling point of water at any given barometric pressure (PrimoGrill.com). Just enter the pressure (the absolute pressure, not relative) and it gives you the boiling point!

As an example, here are the numbers under standard conditions for 7000 ft:

At 7000 ft, the standard barometric pressure is 23.088 inches Hg, and so water boils at 199.3 degrees Fahrenheit or 199.3 °F, which is equal to 92.94 degrees Celsius, or 92.94 °C. However, the pressure at 7000 feet will not always be exactly 23.088 in Hg (for instance, if a weather pattern changes the barometric pressure). It is more precise to specify the barometric pressure and ask the boiling point. Here is an equation to calculate the boiling point of water at any barometric pressure:

boiling point = 49.161 * Ln (Pressure in inches Hg) + 44.932

(where "Ln" is the natural logarithm in base e)

Here are other values for the boiling point of water a various barometric pressures and the approximate altitudes that corresponds to under normal conditions.

31 in. Hg: 214 °F or 101.1 °C (at approx -1000 ft or -305 m below sea level)

30 in. Hg: 212.15 °F or 100 °C (at approx sea level)

29 in. Hg: 210.3 °F or 99.06 °C (at approx 1000 ft or 305 m above sea level)

28 in. Hg: 208.44 °F or 98.02 °C (at approx 2000 ft or 610 m above sea level)

27 in. Hg: 206.59 °F or 96.99 °C (at approx 3000 ft or 914 m above sea level)

25 in. Hg: 202.89 °F or 94.94 °C (at approx 5000 ft or 1524 m above sea level)

23 in. Hg: 199.19 °F or 92.88 °C (at approx 7000 ft or 2134 m above sea level)

21 in. Hg: 195.48 °F or 90.82 °C (at approx 10,000 ft or 3048 m bove sea level)

19 in. Hg: 191.78 °F or 88.77 °C (at approx 12,000 ft or 3658 m above sea level)

17 in. Hg: 188.07 °F or 86.71 °C (at approx 15,000 ft or 4572 m above sea level)

10 in. Hg: 175.11 °F or 79.51 °C (at approx 27,000 ft or 8230 m above sea level)

5 in. Hg: 165.85 °F or 74.36 °C (at approx 42,000 ft 12,802 m above sea level)

NOTE: These pressure values are absolute values, as opposed to relative. Many types of pressure gauges measure relative pressure (such as the ones used to measure the pressure in bicycle or car tires). However, a barometer measures the absolute pressure (as well as most electronic pressure gauges). If using a relative gauge, you to find the absolute pressure, you must subtract the outside air pressure from the relative reading (so you must already know the outside air pressure using a barometer).




 

More Info

The boiling point of any substance is the temperature at which the liquid phase changes to the gas phase. The phenomenon called boiling is the formation of vapor bubbles within the liquid. Since heat must be applied from the environment to change a liquid into a gas, the container is hotter than the contents, and pockets of gas are generated which then rise to the surface and are dispersed.

The ambient air pressure affects how fast the molecules of the liquid must be moving to leave the liquid phase and enter into the gas phase. As the pressure drops, the speed, therefore the amount of energy, needed gets less, so the temperature at which the substance boils lowers

By the way, the reason a microwave oven can boil water without the cup getting very how is that the microwave energy heats the water molecules directly. So, rather than heating the container which transfers energy to the contents of the pot, the contents heat to boiling and then transfer energy to the container