From the 'Handbook of Meteorological Instruments': Psychrometers.

From Table II, let's assume that you are dealing with air temperatures
in the range 10 to 25 degC, and a relative humidity 'around' 60 to
80%.

From the table, for a 10% change in the psychrometer coefficient, for
the envelope of values above, then we are looking at 'variations' in
RH of 0.5 to 1.9%.

The psychrometer coefficients (temperatures 0degC / as used in the
UK Met Office) for 'natural' = 0.799 and 'forced' (or aspirated) =
0.666; this represents a difference of 0.133, or roughly 18% of the
mean of these two values. Scaling up from the figures given for the
10% calculations (above) we have roughly a *maximum* "error" of around
3.5 % RH.

I can't imagine, given the other variables in 'field-work' measurement
of temps/wet-bulb, and the fact that cloud-bases are related to the
nearest 100 ft, that this difference is going to make a h'apporth of
difference.

As noted elsewhere, the problem is usually lack of ventilation, not
too much. As long as the bulb *remains* damp whilst reading the dry
and 'wet' bulb, for your purposes, I can't see the problem.

Martin.