How are Dew points measured in winter? I would think the old hydr
ometer with the wet sock would record 0 and freeze. Or are they just theor
etical values based on other measurements.

I have done it many times. My wet bulb thermometer is fixed and I use a
fan to aspirate the wick. I keep the wick inside when it's below freezing
outside. Once the wick is placed on the thermometer bulb I must wait until
the temp. fall below freezing by natural cooling before turning on the fan.
On calm nights it can take 45-50 minutes before fan time. On a very cold
night like tonight (12°F) with a 7mph breeze as soon as the "wet bulb"
would reach the 20's the fan could be turned on and the correct "wet bulb"
would be measured. I can't explain how or why but it does work. If the fan
is turned on too soon it will go right to the freezing mark.

Wayne R.
"Matthew Guest" wrote in message
...
How are Dew points measured in winter? I would think the old hydr
ometer with the wet sock would record 0 and freeze. Or are they just theor
etical values based on other measurements.

"Wayne Roop" wrote in message
...
I have done it many times. My wet bulb thermometer is fixed and I use
a
fan to aspirate the wick. I keep the wick inside when it's below freezing
outside. Once the wick is placed on the thermometer bulb I must wait
until
the temp. fall below freezing by natural cooling before turning on the
fan.
On calm nights it can take 45-50 minutes before fan time. On a very cold
night like tonight (12°F) with a 7mph breeze as soon as the "wet bulb"
would reach the 20's the fan could be turned on and the correct "wet bulb"
would be measured. I can't explain how or why but it does work. If the fan
is turned on too soon it will go right to the freezing mark.


A lot of heat is released as water freezes. This will result in the
measured temperature holding at the freezing point from the time the wick
starts freezing until all is completely frozen. Until this last state is
reached you cannot get any sort of realistic sub-zero dewpoint.

But would not putting on the fan straight away still speed up this process?
It would be necessary to wait until the "ice bulb" had reached a steady
temperature below the measured dry-bulb, but this should still be quicker
than letting nature take its course on a calm night. The observed behaviour
of the "wet bulb" dropping rapidly to freezing then holding steady is due to
the latent heat release as above. But once the freezing process is
complete, the temperature measured should drop to the required "ice bulb"
temperature.
--
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In article ,
Matthew Guest wrote:

How are Dew points measured in winter? I would think the old hydr
ometer with the wet sock would record 0 and freeze. Or are they just theor
etical values based on other measurements.

Officially, in Canada, dew points are measured in the following
manner...

Above zero - use a wick that draws water from a bottle of distilled
water.

From 0C to -12C - a muslin sleeve is put on the bulb of the
thermometer. About 5 minutes prior to reading, one dips the bulb into
water and rotates the thermometer, until the reading drops below zero,
and the burst of latent heat of evaporation rescedes back below zero.
You rotate the thermometer to ensure an even layer of ice on the bulb.

Below -12C - a bare bulb is all that is required as the water freezes
quite quickly at that temperature. Procedure is the same as the muslin
sleeve for applying an ice coating.

When the temperature falls below -37C, mercury freezes, so the dry
bulb, wet bulb and maximum thermometers have to be brought in. One then
uses just the minimum thermometer to get the dry bulb only. Mimimum
thermometers are filled with alcohol with a much lower freezing
temperature. When this happens, and a dewcel is not available, the dew
point would be transmitted as missing.

Of course, most offices these days have Dewcel's, which give the
reading remotely and do not need any prepping.

--
Pete Ware
Thomasburg ON
Remove the obvious when replying by e-mail - I'm fed up with the latest virus!

Matthew Guest wrote:
How are Dew points measured in winter? I would think the old hydr
ometer with the wet sock would record 0 and freeze. Or are they just theor
etical values based on other measurements.

You can still measure a wet-bulb temperature even below freezing.
Evaporation (better: sublimation) and cooling occur even when the wick
is frozen.

Actually, there are two possibilities: the wick could be merely
supercooled (i.e., below 0 deg. but still liquid) or it could be frozen.
In the first case, nothing changes in how you compute dewpoint and/or
RH from readings (same table or formula applies). If the wick is
frozen, you need to use an alternative relationship. In the days when
we used circular slide rules (so-called psychrometric computers, aka
'psychedelic confusers'), there were two scales: one for a wet wick
(regardless of temp.) and one for frozen wick.

Under some circumstances, standard procedures dictate that you force a
supercooled wick to freeze by briefly touching it with a piece of ice.

In my experience, there are two problems that crop up when making
psychrometer measurements at very cold temps.:

1) extremely long equilibration times (which tends to be annoying when
you're using a sling psych. at -20!), and

2) as a result of (1), a tendency for the wick to dry out before you've
gotten your final reading.

The Canadian procedure of coating the wick or bare bulb with a fairly
heavy coating of ice (as described elsewhere in this thread) seems like
a sensible solution to the second problem. Probably the only solution
to the first is to use a fan-ventilated psychrometer and park it on a
fence post for 20-30 minutes while you keep warm in your idling car
(parked downwind, of course) or other shelter.

Grant