And for every day purposes - where one or two degrees error is acceptable - it is that easy.
However, for scientific purposes - the measurement of temperature is actually very difficult.
Later, in science classes, we used the metric scale where water freezes at 0°C (degrees Centigrade) and boils at 100°C.
While researching various subjects related to atmospheric physics, I learned that water freezes at 0.01°C (degrees Celsius) and boils at 99.974°C.
To the casual observer, this makes no difference - water still boils when it boils and the temperature depends more on the atmospheric pressure than on some definition. (In the 3 examples above, the atmospheric pressure must be 1013.25 hPa - by definition.) The following table shows the expected boiling points at various pressures (using the IAPWS formulation [as provided via Vömel 2011] to compute relative humidity).
|828.00||94.41||201.95||Denver, CO 5,470 ft (1.667 km)|
|Note:||Computations of this type can be made with my water vapor programs.|
Without getting too deep into the physics, (See BIPM - Thermodynamic and practical temperature scales for details.)
Below the triple point, the temperature scale is defined by 2 points - zero K and the triple point using water of a specific composition (VSMOW) and a specific pressure. Above the triple point, a different calibration method is used. For (IPTS-68), the boiling point of water at 100°C (and 1 atm) was used. ITS-90 changed the calibration to use the melting points of a number of pure metals. As a result, the boiling point of water became a measured value instead of a defining value.
Stated another way, using atmospheric pressure to define a temperature scale is a bad idea.
The latest standard is adequate for normal usage since most thermometers are only accurate to about 0.1°F and, before the digital age, read to only the nearest half degree. But there is still an issue - basically, temperature is measured on 4 different scales, each with its own calibration and interpolation methods. As a result, when extending the ITS-90 zero to 273.16 K scale, the boiling point of water is about 373.1339 K (99.9839°C). However, when using the scale defined by pure metals, the boiling point of water is 373.124 K (99.974°C). ref While this difference is small, it effects the equations used in many branches of science.
The International Temperature Scale of 1990 (ITS-90)
|Note:||The primary source, Preston-Thomas H., The International Temperature Scale of 1990 (ITS-90), is paywalled at $33 per peak!|