"Jack Harrison" wrote in message
...

snip


It has been suggested by a retired professional meteorologist that by
blowing air over the bulb and evaporating, I can artificially cool the
reading to be significantly below the true wet bulb - my method is
flawed.

snip

Is my method fundamentally flawed?

Hi, Jack,

Is your method not the aspirated-type psychrometer? The various tables
available vary the psychometric constant to accommodate the type being
used, be it Assman, whirling, Stevenson Screen, aspirated, don't they?
However, I think there needs to be a standard air flow speed over the
wet bulb for the tables to be spot on and the Observer's Handbook gets a
bit complicated as to exposure and time taken. (Doesn't it always!)

As you know, I'm no expert, and who am I to comment on the thoughts of
retired meteorologists, but retired headmasters have an opinion on most
things! One of mine is that the digital hygrometers are seldom more than
a few percent different from the official time-consuming methods, so
perhaps they could be the answer.

HTH,

--
Ken Cook, Copley (5miles north of Barnard Castle), County Durham.
830ft
http://mysite.freeserve.com/copley
(MO climat. site updated before 10Z and 19Z daily)
kencookATcopleydurham.freeserve.co.uk

"Ken Cook"

retired headmasters have an opinion on most
things! One of mine is that the digital hygrometers are seldom more than
a few percent different from the official time-consuming methods, so
perhaps they could be the answer.

I had thought of going down that route but wasn't sure what level of
accuracy they give. What I am ultimately interested in is the convective
cloud base, which is easily worked out from temperature and dewpoint with a
remarkable accuracy of say, +/- 200 feet

Now it would be just as easy to convert relative humidity and dry bulb to
give a cloud base, but I wonder if it will give an equivalent level of
accuracy. I'm not confusing "precision" of measurement with accuracy
(measures to 2% is not the same as accurate to 2%, but you can usually get
some idea of the residual errors)

Any particular instrument that is recommended? I have already done a web
search and some digital ones for £20 give dry temp and humidity, but it
would be very easy to waste money on rubbish.

Jack

"Jack Harrison" wrote in message
...


Any particular instrument that is recommended? I have already done a
web
search and some digital ones for £20 give dry temp and humidity, but
it
would be very easy to waste money on rubbish.


Jack,

I use THGR228N from Oregon Scientific. I have one in my screen and it
compares well with the standard dry and wet bulb method, except in 95%
or so situations when it can take a while to dry out. However, I think
Tudor's advice could mean you do not need to do anything differently.

BTW, I'm in Felixkirk on Sunday, 26th (all being well), close to Sutton
Bank, so might pay you a visit (and bring a digital hygro?). I'm
fascinated by this Pennine Wave.

ATB,

--
Ken
http://mysite.freeserve.com/copley
kencookATcopleydurham.freeserve.co.uk

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.