In article ,
Adrian D. Shaw writes:
What's the difference between wet bulb and dewpoint?

Dewpoint is the temperature to which the air would have to be cooled to
become saturated, i.e. to have a relative humidity of 100%. Wet bulb
measures the degree of evaporative cooling, so that the lower the
relative humidity the greater the depression of the wet bulb reading
compared to dry bulb. At 100% RH, the dry bulb, wet bulb and dewpoint
will all be the same. For any lower RH, DB WB and, I think, WB DP.

Dewpoint can clearly
be calculated if you know temperature and humidity (well, my weather
station does it), but can wet bulb be calculated? Your description above
suggests it can be (so why would people bother measuring it?).

I think the attraction of wet bulb temperature is that it can be
measured directly, by having a thermometer whose bulb is encased in wet
muslin, or some similar device, rather than needing to be calculated.
Once you've measured dry and wet bulb temperatures, then you can use
them to calculate the relative humidity and the dewpoint. Your modern
weather station may be able to measure the humidity directly, but I
don't think that "traditional" simple met instruments could do this, so
observers would derive the humidity from the wet bulb reading rather
than vice versa.
--
John Hall
Johnson: "Well, we had a good talk."
Boswell: "Yes, Sir, you tossed and gored several persons."
Dr Samuel Johnson (1709-84); James Boswell (1740-95)