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
U₂, etc. are used in FM-88 code used to report satellite observations. The relative humidity group is defined under section 220.127.116.11.1.
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
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.