[wxqc] Fog, Dew Point, and RH

[Sandy and David Helms]

MessageI 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&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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