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RH is the symbol of relative humidity in many textbooks. But, in my experience, U is often used in atmospheric measurements. For example, Vaisala PTU300 means pressure, temperature and humidity. I think U comes from h$u$midity, but this makes it curious why the symbol H is used.

Does anybody know, where the symbol U come from in observation?

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While I cannot add anything to the etymological discussion. I would like to note that capital U is the notation for relative humidity in the standard synoptic code known as SYNOP or FM-12, that is used around the world.

According to WMO Manual on codes known as WMO–No. 306 Volume I.1 – International Codes, Part A – Alphanumeric Codes, The symbols UUU are used as placeholders for relative humidity of the air in SYNOP code, and symbols U₁, U₂, etc. are used in FM-88 code used to report satellite observations. The relative humidity group is defined under section 12.2.3.3.1.

Under unusual conditions, when the dew-point temperature is temporarily unavailable (e.g. because of instrument failure) but relative humidity is available, the group 29UUU shall replace the group 2SnTdTdTd. Every attempt shall first be made, however, to convert relative humidity to dew-point temperature, and the relative humidity included only as a last resort.

So I would say that U is used for relative humidity in instrument descriptions because it has become the established notation in international meteorological data exchange protocols.

Each position in SYNOP is indicated by a single letter possibly with a subscript. One may ask, why meteorologists do not use Rh for relative humidity. I guess this is because they already use R to specify the amount of precipitation. I am almost sure that R here stands for rain as much of modern meteorological notation comes from the codes invented by Sir Francis Beaufort in the XIX century.

The reason dew point temperature is preferred to relative humidity in weather records is that manual observation of humidity is usually performed using a psychrometer, a type of hygrometer composed of two thermometers of which one has its bulb kept dry and another has its bulb wet. Humidity is determined from the pair of readings of the two thermometers.

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Rarely are there standards when it come to using symbols in equations or abbreviations. Having tried Google Translate, Humidity in Latin is generally translated to humiditas, but umor is also, apparently, well used as a word for humidity, fluid & moisture in Latin.

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  • $\begingroup$ There may not be official standards, but there's generally strong consistency in which letter is used for which variables, of course, as that tends to promote understanding $\endgroup$ Commented Nov 5, 2023 at 6:27

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