Impacts of High Altitude on Water’s Boiling Point

The siren song of the mountains calls loud and clear to most backpackers, which means we’re often camping well above sea level. Besides impacting our physiology, altitude has an influence on how long it takes to boil water—and, therefore, how long it takes to prepare uncooked food. 

Many outdoor enthusiasts already know the basics, but it’s still easy to get confused about how boiling point, cooking time, and elevation all fit together. So we put together a quick, straightforward primer to help clarify things. Whether you're heading out for a ski tour or planning big summer traverses, here’s what to know about boiling water at altitude.

How Does Altitude Affect Boiling Water? 

The short answer? Water reaches its boiling point faster at higher altitudes because the lower atmospheric pressure reduces the temperature needed for water to boil. Water at sea level boils at 212 degrees Fahrenheit; at 5,000 feet above sea level, the boiling point is 203 degrees F. Up at 10,000 feet, water boils at 194 degrees F.

To understand why, it helps to look at what’s happening with atmospheric pressure as you climb in elevation.  At increasing altitude, atmospheric pressure decreases. Very roughly speaking, you can think of it like this: There’s more air above a point at sea level, which means the atmospheric pressure is greater there than at higher altitudes where there’s less air pressure bearing down above. This fundamental physical rule explains a lot of important weather phenomena, as well as that annoying ear-popping thing when you’re flying in an airplane or driving up or down a mountainside.

The change in pressure impacts the boiling point of water: the temperature at which liquid water begins turning to vapor, which occurs when its vapor pressure equals the atmospheric pressure. At a higher elevation, the lower atmospheric pressure means heated water reaches its boiling point more quickly—i.e., at a lower temperature. 

This is the opposite of what many people suppose: that water takes longer to boil at high altitude. As we’ve just demonstrated, boiling water at altitude is quicker because it requires less energy than at higher pressures. 

But the fact that the boiling temperature is lower at higher elevations means food takes longer to cook, which is where the confusion lies. At 5,000 feet, where water boils almost 10 degrees cooler than at sea level, you need about double the cooking time.

Boiling Water at Altitude Chart

If you don’t have the chart above handy, you can get a rough estimate of the boiling point change by subtracting about one degree Fahrenheit from the boiling temperature with each 500-foot increase in elevation.

Photo by Kristian Keenen

Wind Factor & Other Caveats

Now, keep in mind that outdoor high-elevation cooking sites often contend with wind, and that can affect how long water comes to a boil by whipping that gas flame around and otherwise making it harder to heat your pot. Choosing a sheltered spot and shielding your stove and cooking vessel from the gusts with a windscreen will help ward against this factor. This heat loss due to the wind may also explain why some outdoorspeople think it takes longer to heat water to a boil up in the mountains.

And then there's the fact that different stove fuels perform better or worse at high elevations and in the colder temperatures typically encountered up there. All things considered, liquid-fuel stoves are the best bet for high-elevation camping. At high altitude or subfreezing weather, canister stoves won't work as well, and at best, it may take longer to bring your water to a boil, if it gets there at all.

Photo by Uncage the Soul Productions 

How Does Altitude Affect Water Purification?

Now, what about using boiling temperatures to purify water—how is that impacted by altitude? Boiling is the surefire way to kill pathogens and make water safe to drink. Do you need to worry about the lower boiling temperature of water in the high country?

The Centers for Disease Control & Prevention recommends letting water stand at a rolling boil for a minute at sea level to purify it and extending that to three minutes of a rolling boil above about 6,500 feet.

However, many experts contend that simply allowing water to reach a rolling boil renders it safe, regardless of your elevation. This World Health Organization document, for example, and these posts from Off the Grid and Backpacker support this claim. Even the lower boiling-point temperatures at high mountains are sufficient to kill pathogens. If you want to cover all your bases, by all means, follow the CDC guidelines and boil your water for 3 minutes, but if you’re being fuel-conscious, you’re likely going to be fine simply bringing water to a rolling boil.

(To learn more, explore our Mountain House resources on purifying water and cooking at high altitudes.)

Simplify High-Altitude Cooking With Mountain House

That longer cooking time at higher elevations has a definite impact on backpacking. To cook the same meal up in the mountains or plateaus that you do at sea level, you’ll need to invest in more fuel and more time. On extended trips in the high country, foods that require lengthy and/or multi-step cooking aren’t good choices, because they’ll translate to hauling that much more fuel and weight in your pack.

For this reason, foods requiring long simmer times or multiple steps become impractical on high-elevation trips. Instant-type meals that merely require heating are the better choice, and that includes our just-add-hot-water Mountain House delicacies. In fact, you don’t even need hot water to prepare Mountain House meals: You can use room temperature or even cold water to rehydrate our freeze-dried meals, just give the food about double the cooking time listed on the package.

Stay warm, pack light, and keep mealtime simple with Mountain House on your next high-country adventure.

Pop Quiz

Why does water boil at a lower temperature when making tea on Mount Everest compared to sea level?

• a) Because tea leaves have a special reaction to high altitudes

• b) Because the temperature on Mount Everest is naturally colder

• c) Because air pressure decreases at high altitudes, it affects the boiling point

• d) Because Mount Everest has magical boiling powers

Answer: C

When boiling a cup of tea at a high elevation, why do water molecules behave differently?

• a) Because they become magnetic at higher altitudes

• b) Because they gain energy and move faster as the liquid boils

• c) Because they shrink in size due to low air pressure

• d) Because they hibernate in the cold mountain air

Answer: B

Among different altitudes, which one resembles a pressure cooker in terms of its impact on boiling water?

• a) Sea Level

• b) 5,000 feet

• c) 10,000 feet

• d) 15,000 feet

Answer: C