High altitude baking

Altitude changes baking through one physical fact and a pile of consequences that are not physics at all. The fact is air pressure: less of it above you means water boils cooler, and this page works that out properly rather than reading it off a chart — the standard atmosphere gives the pressure at your elevation, and the Antoine equation with the coefficients NIST publishes for water turns that pressure into a boiling point. Colorado State University Extension's own approximate table is printed beside the answer so you can see the two agree. The consequences are a different kind of number. A cake rises faster and sets later at 8,000 feet, and the corrections for that are published guidance built from testing, not a law — so they are given here as Colorado State's bands, with its name on them, and nothing is interpolated between the rows the table actually has.

How it is calculated

P = 1013.25 × (1 − ft ÷ 145366.45)^(1 ÷ 0.190284), then T = B ÷ (A − log₁₀P) − C

The National Weather Service publishes the first relation the other way round, as pressure altitude: "halt = (1 − (Psta / 1013.25)^0.190284) × 145366.45". Rearranged, it gives the standard-atmosphere pressure at a height. The second is the Antoine equation as NIST states it — log10(P) = A − (B / (T + C)) with P in bar and T in kelvin — using A = 5.08354, B = 1663.125 and C = −45.622, the coefficients NIST calculated from Bridgeman and Aldrich for 344 to 373 K. Solved for temperature it gives 100.00 °C at sea level and 94.93 °C at 5,000 feet, against the 212 °F and 203 °F Colorado State prints. None of the cake adjustments are computed: they are read off Table 3 of that publication.

Source: NOAA National Weather Service, El Paso/Santa Teresa weather calculator documentation, "Pressure Altitude" — halt = (1 − (Psta / 1013.25) ^ 0.190284) × 145366.45, with the station pressure Psta in millibars and the answer in feet

Questions people ask

At what temperature does water boil where I live?

Enter your elevation and the first line answers it. Over the range that matters for cooking the drop is close to steady, about a degree Fahrenheit for every 550 feet: 208 °F at 2,000 feet, 203 °F at 5,000, 198 °F at 7,500, 193 °F at 10,000, which is Colorado State's table and within a degree of what the physics gives. The figure here is for the standard atmosphere. Real weather moves it: a deep low over your town lowers the boiling point a little further, a strong high raises it. If you need it exactly — for candy or for canning — boil a pan and read a thermometer.

Does the oven get cooler at altitude too?

No, and this is the most useful thing on the page. Colorado State is explicit that "oven temperatures are not affected by altitude, so sea-level instructions work for oven-roasted meats". An oven is heated air at whatever temperature the thermostat holds; it does not care about pressure. What changes is what happens inside the food: water leaves it faster, and leavening gas expands more. That is why the advice for cakes is to raise the temperature rather than lower it — 15 to 25 °F, to "set the batter before cells formed by the leavening gas expand too much".

Do I need to change my recipe at 2,500 feet?

Probably not. Colorado State says "most cake recipes perfected for sea level need no modifications up to 3,000 feet", and its adjustment table does not begin until 3,500. Between 3,000 and 3,500 feet you are in a gap the publication leaves open, and this page says so rather than filling it in. Its own advice for that stretch is worth more than a number: "do not assume that your sea level recipe will fail. Try it first."

Why are the cake adjustments ranges rather than numbers?

Because they were arrived at by baking, not by calculation, and they depend on the recipe. Table 3 gives 0 to 2 tablespoons less sugar per cup between 6,500 and 8,500 feet, and 2 to 4 tablespoons more liquid, and the text tells you to "try the smaller adjustment first, this may be all that is needed". A tool that printed a single figure there would be inventing precision the source does not have. Sugar and fat both weaken the cell structure of a cake, liquid and egg strengthen it; the ranges are the room you have to trade between them.

What about bread and yeast doughs?

Bread is a timing problem more than a proportion one. Colorado State: "high altitude has its most pronounced effect on the rising time of bread", with the shortened rise interfering with flavour, so the advice is less yeast or an extra knock-back rather than a recalculated recipe. There is a second, quieter effect: "flours tend to be drier and thus able to absorb more liquid in high, dry climates", which is a hydration question — the baker's percentage page is the better place for it.

How accurate is the boiling point here?

It agrees with Colorado State's published table to within about seven tenths of a degree Fahrenheit at every row, and their table is labelled approximate. Two honest limits: the pressure is the standard atmosphere rather than today's weather, and NIST publishes the Antoine coefficients used here for 344 to 373 K, so at sea level the answer sits a fraction of a degree above the top of that window. The page says so on the line itself when it happens.

Sources

The documents this page reads its numbers out of, linked so you can check them yourself.

1. NOAA National Weather Service, El Paso/Santa Teresa weather calculator documentation, "Pressure Altitude" — halt = (1 − (Psta / 1013.25) ^ 0.190284) × 145366.45, with the station pressure Psta in millibars and the answer in feet
2. Colorado State University Extension, "High Altitude Food Preparation" (revised 2013) — Table 1, "Approximate boiling temperatures of water at various altitudes" — sea level 212 degrees F, 2,000 ft. 208, 5,000 ft. 203, 7,500 ft. 198, 10,000 ft. 193
3. NIST Chemistry WebBook, Water, Antoine Equation Parameters — log10(P) = A − (B / (T + C)) with P the vapor pressure in bar and T the temperature in K; A = 5.08354, B = 1663.125, C = −45.622 over 344. to 373. K, coefficients calculated by NIST from Bridgeman and Aldrich, 1964

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