You Can Measure a Mountain by Boiling Water. Here Is How That Works
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Water boils at a lower temperature where the air is thinner, so measuring that temperature accurately gives the air pressure, and from the pressure comes the altitude.
The chain of reasoning
Boiling happens when the pressure of vapour escaping a liquid matches the pressure of the atmosphere pushing down on it. Reduce the atmospheric pressure and the liquid reaches that balance at a lower temperature. Atmospheric pressure falls predictably with height. Therefore measuring the boiling temperature of water gives the pressure, and the pressure gives the altitude. Each step is well understood, and the whole chain converts a difficult measurement of height into an easy measurement of temperature.
How sensitive it is
The numbers make the method practical:
- •Water boils at one hundred degrees at sea level pressure
- •The temperature drops by roughly one degree per three hundred metres
- •So a thermometer reading tenths of a degree resolves tens of metres
- •The instrument is a small boiler with a precise thermometer
- •It needs a fuel source and a few minutes to stabilise
- •The reading must be taken in the steam, not in the water
Why it beat the barometer
A mercury barometer measures pressure directly and would seem the obvious instrument, and it was impractical for the job. A mercury column is a metre long, fragile, heavy and prone to air bubbles entering the tube, all of which make it a poor companion on a mountain or an expedition across difficult country. The boiling method needs only a thermometer, a small vessel and a spirit burner, packs into a case and survives being dropped. Nineteenth century surveyors and explorers carried them routinely, and altitudes in many early accounts were obtained this way.
The other instrument of the same name
Confusingly the word also names a completely different tool used by foresters to measure the height of a standing tree, which has nothing to do with boiling anything. That instrument works by sighting the top and the base of a tree from a known distance and using the angles, or by comparing the tree against a scale held at arm's length. Both instruments measure height, which is where the shared name comes from, and the forestry one is far commoner in modern use. Catalogues and old expedition accounts occasionally cause confusion for exactly that reason.
Where the method fails
The limits are worth knowing because they apply to every altitude measurement based on pressure. Atmospheric pressure varies with weather as well as with height, so a passing depression can shift an apparent altitude by a hundred metres or more, and serious work requires a simultaneous reading at a known reference altitude. Temperature of the air column affects the relationship between pressure and height. The thermometer itself must be calibrated frequently, since small errors translate into large height errors. The same limitations apply to modern pressure altimeters in aircraft, which is why they are reset against reported ground pressure constantly.
The takeaway
Boiling temperature falls with atmospheric pressure, pressure falls with height, so a precise thermometer in steam gives altitude, at roughly one degree per three hundred metres. The method displaced the mercury barometer for exploration because it is small and robust. Weather changes pressure independently of height, so accurate work needs a simultaneous reference reading.