Boiling Point Computation

twoloonscoffee.com

In case you don't have a barometer, you might check out the nearest weather station for your "official" barometric pressure. Once you click the preceding link (which should open in a new window), enter your zip code, and there's the data from the weather station nearest you. (If you leave the Barometric field blank, the standard 29.921" is used.)

At 4524 feet, and 29.921" weather service barometric pressure,
the absolute barometric pressure is 25.34 inches of mercury
The boiling point at that pressure is 203.84° F.

Double, double, toil and trouble
fire burn and cauldron bubble.

Why do we care about the boiling point of water anyway?

The generally accepted recommendation for making French Press coffee is to bring the water to a rolling boil, and then wait x number of seconds before pouring over the grounds. If you live above 4000 feet or so, you may not want to wait at all. Just pour the boiling water directly over your grounds. If you live above 5000 or so, you may actually want to try some cowboy coffee, where you boil the grounds in the water and then strain or filter. As I wait for the gasps of disbelief to subside, remember the reason that boiling coffee is so eschewed is because at 212° the bitter components are so much more easily extracted. At 5000' boiling is around 203°F., right there in the ideal brew range! (assuming you subscribe to the 195°-205° F. range.) So, yes... if you want a brew temp of 203° here, you have no choice but to boil your coffee.
Vac pots, or vacuum brewers, or balance brewers, don't rely on boiling directly, but on vapor pressure. The process of boiling is by definition inextricably linked with vapor pressure, so if you have a vac pot that brews at 200°F at sea level, it'll be brewing at 191°F. when you move to Denver. You might want to grind a little finer, and let the coffee stay up north a little longer to compensate for the cooler brew temp.
Even standard drip machines brew at a lower temperture "at altitude" than at sea level.
Espresso... Nah, I'm not even going there. Suffice it to say that I've brewed excellent espresso at 6151'.
If you have a thermometer that you use for anything critical, like mash, or photography, or anything where you need to know for sure whether your thermometer is "close enough," boiling water can be a good calibration... but only if you know the temperature of boiling water at that particular time and place.

So how are barometric pressure and elevation related to the boiling point of water?

barometer fig.

First off, there are two different meanings of the term "barometric pressure" depending on context, and that's what can make it confusing at times. In order to answer the question at hand, we'll need to find out how the weatherman's definition differs from the absolute pressure.

Originally, a barometer was a very simple device consisting of a glass tube about 32" long, closed at one end, and a beaker of mercury. The glass tube was filled completely with mercury, and then inverted into the open beaker of mercury. Of course there was support and adjustment hardware, but the main point is that the distance between the mercury levels in the beaker and in the tube is equal to atmospheric pressure. As weather fronts come and go, the mercury is pushed up into the tube a bit more, or drops a little back into the beaker.

If you have this kind of barometer, you can forget about elevation, what the weatherman says, etc. Whatever the difference is between the two levels of mercury -- the absolute barometric pressure -- can be entered in this simple absolute atmospheric boiling calculator.

The second meaning of barometric pressure is the one the weatherman uses. Weather patterns are determined by comparing barometric pressures and "connecting the dots" to form isobars, which show what kinds of systems are forming where. If absolute pressure were used, the weather maps would be pretty much useless since higher elevations would always show lower pressures than sea level. Such a map would be much more useful topographically than in predicting weather.

In order to compare readings at sea level with readings at higher elevations, all weather service barometric readings are "projected" to sea level. In other words, when the simple barometer above is at 5000' and reads 24.9" the weatherman makes the sea-level compensation, and then broadcasts the "official" barometric pressure of 29.92".
Since this is a "sea level equivalent" it does us no good in determining boiling points unless we're at sea level... so we have to convert it back to absolute pressure in order for it to be useful in this context.
That's what the calculator at the top of this page does; converts the "official" barometric pressure back to absolute, and then determines the boiling point based on that.

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