[wxqc] Fog, Dew Point, and RH

[Sandy and David Helms]

MessageYep,  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&site=C3
321
  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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