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 own physiology, altitude has an influence on how long it takes to boil water—and, therefore, how long it takes to prepare uncooked food. For many of you, the following will be a no-brainer rehash, but a goodly number of people still get (understandably) confused about the relationship between boiling point, cooking time, and altitude, so we thought we’d run through a quick and basic primer on the subject. Whether you're going ski-touring in the near future or you're spending this winter scheming up awesome high traverses for the summer, read on for a little boiling-water-at-altitude 101!
The Basics of Boiling Water at High Altitudes
At increasing altitude, atmospheric pressure declines. 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 here than at higher altitudes where there’s less air 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.
It also 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. 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.
This is the opposite of what many people suppose: that water takes longer to boil on high. As we’ve just demonstrated, boiling water at altitude is quicker. 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 to roughly double the cooking time.
In terms of a rough boiling point elevation equation, subtract about one degree from the boiling temperature with each 500-foot increase in elevation.
Now, keep in mind that high-elevation cooking sites often contend with wind, and that, of course, can affect how long water comes to a boil by whipping that gas flame around and otherwise making it harder to heat up your pot. Choosing a sheltered spot and shielding your stove and cook vessel from the gusts with a windscreen will help ward against this factor, which 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. (Learn more in our "Choosing a Backpacking Stove" blogpost.)
The Advantages of Mountain House in the High Country
That longer cooking time at higher elevations has an important impact on backpacking. To cook the same meal up in the mountains or plateaus, you’ll need to invest 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 in your pack.
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: You can use cold water to rehydrate our freeze-dried meals—just give the food about double the cooking time listed on the package.
Boiling Point, Altitude, and Water Purification
Now, what about using boiling 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?
Basically, you don’t. 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 rolling boil above about 6,500 feet. Many experts, however, contend that simply allowing water to reach a rolling boil renders it safe, regardless of your elevation—see this World Health Organization document, for example, as well as these posts from Off the Grid and Backpacker. Even the lower boiling-point temperatures at high mountains on Earth is sufficient to kill pathogens. If you want to cover all your bases, by all means, follow the CDC guidelines, but if you’re being fuel-conscious you’re likely going to be fine simply bringing water to a rolling boil.