Mile-high water

There are some really amazing things in our everday lives that we take for granted. For example, the sky is blue because of a phenomenon called Rayleigh Scattering. Just try explaining that to a four-year old.

Consider water. We take for granted that at sea level, water freezes at 0 degrees Celsius (32 Fahrenheit) and boils at 100 degrees Celsius (212 Fahrenheit).

Well, things are different when you’re a mile above sea level. I measured the temperature of ice water and boiling water here, and water still freezes at the same temperature. But it boils at 93 degrees Celsius. I compared that against a table of boiling points that were adjusted for altitude, which predicts something closer to 96 degrees Celsius. Hrm.

Let’s gloss over the fact that either my thermometer isn’t calibrated properly or that something in the water is affecting the measurement. Just consider that the boiling point is less than 100 degrees Celsius. Do you know why?

Liquids boil when the pressure of the vapour is the same as the pressure of the atmosphere. In plain language, that means that water boils when the pressure of the steam matches the pressure of the atmosphere. If the atmosphere is at lower pressure – as it is one mile above sea level – then the steam doesn’t have to reach as high a pressure in order to boil. Therefore, since steam is generated by heating liquid water, you don’t have to heat it to as high a temperature. And that means that water boils at a lower temperature.

Think of it like a cocktail party. The best parties work out when everyone is equally drunk. No one wants to be the loudmouth drunkard who had way too much, and it can be awkward to be the stone sober one when everyone else has had a few martinis. So the object is to maintain a buzz level that is consistent with your peers’. At sea level, the party people drink more (atmospheric pressure is higher), so you have to spend more at the bar (heat to a higher temperature) in order to reach the same level of drunkness  as your peers (reach equilibrium and boil). At high altitude, party people drink less (atmospheric pressure is lower), so you spend less at the bar (heat to a lower temperature) in order to reach the same level of drunkness as your peers (reach equilibrium and boil).

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One response to “Mile-high water

  1. Nicely explained! (But I have a kids who do grok Rayleigh scattering. Bad sample. 🙂 Here in Boulder, we’re dealing with water boiling at around 200-202F, and up where we live, at 198F. It makes for both good news and bad news when cooking, as some things will burn less, and some will take forever to get done. (Btw, your thermo may not be wonky, local atmo pressure affects the boiling point.) –rick

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