Why Does Water Boil at a Lower Temperature at High Altitude?

Water boils at a lower temperature at high altitude because air pressure is lower there. The USDA says that as atmospheric pressure decreases, water boils at lower temperatures.

1. The short answer

Boiling temperature depends on air pressure. The USDA Food Safety and Inspection Service (FSIS) puts it simply: as atmospheric pressure decreases, water boils at lower temperatures. And air pressure decreases as you go higher.

So the real question is why there is less air pressure up high.

2. Why air pressure drops with height

NOAA explains air pressure as the combined force of countless air molecules striking a surface. Each molecule is too small to feel, but together their collisions add up to a considerable push.

As elevation increases, there are fewer molecules overhead and around you, so the air is less dense and the pressure is lower. NOAA notes that although the atmosphere extends hundreds of miles up, half of all the air molecules in the atmosphere are within the first 18,000 feet (5.6 kilometers).

The FSIS gives the pressure in pounds per square inch at three elevations:

That works out to a drop of about half a pound per square inch for every 1,000 feet, according to the FSIS.

3. How much the boiling point drops

The FSIS gives a rule of thumb: for every 500 feet of elevation, the boiling point of water drops by just under 1 degree Fahrenheit.

It also gives specific values:

You can check the rule of thumb against the 7,500-foot figure. 7,500 feet is 15 steps of 500 feet. Fifteen drops of just under 1 degree each come to just under 15 degrees, and 212 minus about 14 gives about 198 F, which matches.

Using the same rule, a town at about 5,000 feet would be roughly 10 steps up, so its water would boil a little less than 10 degrees below 212, at around 202 to 203 F. That is an estimate from the rule, not a measured value.

4. Why cooking takes longer up high

At 7,500 feet, a pot at a full boil is about 14 degrees Fahrenheit cooler than the same pot at sea level. Food cooked by boiling or simmering is therefore cooking at a lower temperature, so it takes longer to cook. The FSIS states this directly.

A natural reaction is to turn up the stove. The FSIS explains why that does not help: no matter how high the heat, water in an open pot cannot get hotter than its own boiling point. Extra heat just makes the water boil away faster, and whatever is cooking dries out faster.

This also explains why FSIS says high altitude affects cooking in two main ways:

1. Water and other liquids evaporate faster and boil at lower temperatures.
2. Leavening gases in breads and cakes expand more quickly, because there is less air pressure holding them in.

5. How a pressure cooker fixes it

If lower pressure lowers the boiling point, higher pressure raises it. The FSIS describes cooking under pressure as, in effect, increasing the atmospheric pressure inside the pot, which raises the boiling temperature of water. Food then cooks faster and more thoroughly.

That makes a pressure cooker the reverse of climbing a mountain: instead of removing air above the water, it traps steam to push harder on it.

6. How common this is

High altitude is not a rare situation. The FSIS notes that fully one-third of the US population lives at high altitudes. Most cookbooks treat 3,000 feet and above as high altitude, though the boiling point has already dropped to 208 F at 2,000 feet. The FSIS lists Alaska, Arizona, California, Colorado, Idaho, Montana, Nebraska, New Mexico, Nevada, Oregon, South Dakota, Texas, Utah, Washington, and Wyoming as wholly or partly at high altitude.

7. A note on weather reports

If you check a weather report in a mountain town, the air pressure shown will look close to sea-level values. NOAA explains why: weather stations convert their readings to what the pressure would be if the station were at sea level, so readings from different elevations can be compared. Standard sea-level pressure is 1013.25 millibars, or 29.92 inches of mercury. The pressure actually pressing on your pot at elevation is lower than the number in the forecast.

8. Key takeaways

• Water boils at 212 F (100 C) only at sea level; lower air pressure means a lower boiling point.
• Air pressure drops with height because there are fewer air molecules; half of them sit below about 18,000 feet.
• The FSIS rule of thumb: the boiling point falls by just under 1 F for every 500 feet, to about 198 F at 7,500 feet.
• Boiled food takes longer to cook at altitude, and turning up the heat only makes water boil away faster.
• A pressure cooker raises the pressure in the pot, which raises the boiling point and speeds up cooking.