Published: 17 April, 2009

Does being above sea level make water boil faster?

Why does water boil at a lower temperature in Denver (higher altitude) than in New York?

Before we can address your question, we need to understand what "atmospheric pressure" means. Our planet is basically a sphere with a relatively thin layer of gas all around it. This gas is held in place by gravity, the same force that keeps us firmly on the earth's surface!

Pressure is a force per unit area, and the air above us is exerting a pressure on the surface of the planet that can support a column of water roughly 10.3 m high (33.7 ft), or a column of mercury—like in a barometer—760 mm at sea level. We call this pressure 1 standard atmosphere (atm). A diver swimming 10.3 m below the ocean's surface would feel a pressure of 2 standard atmospheres.

Now we need to know exactly what we mean by boiling. You know boiling as the vigorous bubbling of a pot of water when it is heated, but boiling has a specific definition for any pure liquid. What you may not realize is that water always has a vapor—water molecules that are gaseous—associated with it. Water evaporates because small amounts of vapor above the liquid are constantly lost to the atmosphere and replenished from the liquid. Water vapor has a pressure associated with it called "vapor pressure", and at 72°F it is equivalent to about 20 mm of Hg, or 0.026 atm. So if you are near a body of water on a late spring day, it is likely that the atmosphere about you has 2-3% water vapor in it. The actual amount is complicated by a variety of things but this is a rough estimate. As you supply heat to a liquid and increase its temperature, its vapor pressure increases. At its boiling temperature, the vapor pressure of a liquid is equal to the atmospheric pressure surrounding it, or 1 atm at sea level. The bubbles you observe when you boil water are simply water vapor that has been generated in your pot!

Water in Denver (roughly 1 mi above sea level) boils at a lower temperature because its vapor pressure at that temperature, which is equal to the atmospheric pressure, is lower than at sea level!

Now your question&msah;if you will excuse the pun—boils down to why the atmospheric pressure in Denver is less than at sea level. If we go back to our definition of pressure, the force exerted by the air must be less in Denver than at sea level. This could be due to a difference in gravity, but it turns out that the extra "mile high" of Denver's altitude does not change the effect of gravity enough to make much of a difference.

What is different is that the column of air above the pot of boiling water is 1 mi shorter! Since there is less air "pressing" on the water's surface, the atmospheric pressure is less than at sea level, and the vapor pressure needed to boil is achieved at a lower temperature—by about 5°C or 9°F; water will boil in Denver on a "standard day" at 95°C or 203°F.

Be careful though! Atmospheric pressure is changed by a number of factors, and altitude is only one of them!