[wxqc] Fog, Dew Point, and RH

[Bob]

Actually, that's what's in the AAG humidity sensor. Supposedly, it's
also used for medical applications
http://content.honeywell.com/sensing/prodinfo/humiditymoisture/technical
/faq_hummoisture.stm . What I've done is taken a piece of 2 inch white
PVC. On the upper end, there's a 2 to 3 inch adapter with a 12 volt fan
siliconed into the opening.  There's  two 90 degree bends, the other end
of which attaches to about a foot of 2 inch pipe - think candy cane with
a fan on the short end. The humidity, and pressure sensors are in this
pipe. This whole assembly responds very quickly to temp and humidity
changes - http://www.findu.com/cgi-bin/wxpage.cgi?call=cw3321
<http://www.findu.com/cgi-bin/wxpage.cgi?call=cw3321&last=6> &last=6,
and seems immune to direct sun. 
 
 
 
-----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Sandy and
David Helms
Sent: Friday, April 01, 2005 7:27 AM
To: Discussion of data quality issues
Subject: RE: [wxqc] Fog, Dew Point, and RH



I want one. Them sell 'em at Walmart?  Maybe Honeywell got those stats
in their test chamber for the sensor, but in your backyard YMMV.

-----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net]On Behalf Of Bob
Sent: Friday, April 01, 2005 7:14 AM
To: 'Discussion of data quality issues'
Subject: RE: [wxqc] Fog, Dew Point, and RH


Here's the data on the sensor.
http://content.honeywell.com/sensing/prodinfo/humiditymoisture/009012_2.
pdf

-----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Sandy and
David Helms
Sent: Friday, April 01, 2005 12:19 AM
To: Discussion of data quality issues
Subject: RE: [wxqc] Fog, Dew Point, and RH


Yep,  the fog you describe should give you 100% RH (temp and dew point
same) nearly every time.  But, having fog doesn't mean you will have
100% RH. Sometimes there is a small layer near the surface that is not
quite saturated yet the horizontal visibility can be very low.  Its an
old rule of thumb that you never code fog in an aviation weather (METAR)
observation with dew point depression greater than 7 F (quiz for
tonight, who knows what an AXXX bulletin is?), which is to say, you can
get fog with temp/dew point depressions anywhere from 0 to 7 F degrees.

 
On the Honeywell accuracy of 2%, I'm a bit skeptical.  That kind of
accuracy is typically reached by research quality hygrometers.  I think
most sensor manufacturers won't claim accuracies better than 5%, and
that is a stretch as most hygrometer's performance will "drift"
(degrade) over time.  
 
Looking over your station reports, you track very well with KILM with
temp and dew point so you are doing very well.
 
DaveH
CW0351
 
 -----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net]On Behalf Of Bob
Sent: Thursday, March 31, 2005 10:23 PM
To: wxqc at lists.gladstonefamily.net
Subject: [wxqc] Fog, Dew Point, and RH



If we are in a heavy fog, am I incorrect in believing that the RH is
100%? As I look at my QC data for Today, I note that I am supposedly in
error from 05:00 - 10:00 for reporting a dew point pretty close to
ambient temperature - ie; 100% humidity. During that time period, the
entire area, including the weather service was under an extremely dense
fog.
http://weather.gladstonefamily.net/cgi-bin/wxqchart.pl?date=20050401
<http://weather.gladstonefamily.net/cgi-bin/wxqchart.pl?date=20050401&si
te=C3321> &site=C3321
And the surrounding stations are reporting similar data....
http://www.srh.noaa.gov/data/obhistory/KILM.html
http://www.srh.noaa.gov/data/obhistory/KLBT.html
http://www.srh.noaa.gov/data/obhistory/KFAY.html
 
I have three temperature sensors, and a Honeywell HIH 3610 humidity
sensor in a fan aspirated tube with an intake 5 feet off the grass. The
temperature sensors all read within .2 degrees F, and previously been
verified accurate accurate within .3 degrees F at 32 F, and 212 F, I'm
not likely to be reporting incorrect temperature.  The Honeywell
humidity sensor has a claimed accuracy of 2% Rh. 
 
SO..... If we are experiencing "can't see two feet in front of you" fog,
should I not be reporting ambient temp = RH?

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