[wxqc] dew point errors

[Tim Crummel]

Hello, I live in Kalamazoo Mi. my humidity sensor (and I have 4 ) report
errors of maybe only 2 degrees  however there are many times at night when
my errors are as high as 10 degrees off. Where I live (I am surrounded by
several small lakes), and at night you can see fog move in. Thus the
discrepancies. This happens in several spots surrounding the city but not in
the city itself, which sits in a valley or by the airport which is 6 miles
away, and the cities reporting station. (Our Airport has never reported 100%
humidity in the 5 years I have observed the station). Several "ring roads"
around the city have signs post saying "Watch for Fog patches".

            My point is I know that when my sensors hit 98 to 100% humidity
they are correct (for my location) as I have a mirror right next to them and
can watch Dew form. As soon as the mirror starts to dew over all my sensors
indicated 97 to 99. During daylight all are within 5% of each other, and
within 5 degrees dew point of surrounding area stations. Fan aspiration does
improve the errors in the day to about 2 degrees. And also keeps most of the
sensors at 99% in fog.

            My point is, know your location, and if you sensors are fairly
good and new, watch for dew to test the High end, to do the low end you will
either have to get a sling or a very good and well calibrated unit. Make
sure it's shielded from direct sunlight or use a solar shield with a fan to
keep air moving past sensor(s). I replace and / or recalibrate my sensors
every three years (or sooner if calibration gets way off).

 

Tim Crummel  CW0241

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From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Ken Whelan
Sent: Monday, April 17, 2006 9:55 AM
To: Discussion of weather data quality issues
Subject: RE: [wxqc] dew point errors

 

Take a look at my station and its location.   CW5627.   It is sitting on my
boat dock on Beaver lake about 25' in the air.  Its only been operational
for about the last month or so.    As you can tell from looking at the topo,
it is sitting in a pretty good valley.   The reason for this placement is I
want the windspeed and temperature on the lake.   I am also pulling the
water temperature based on an extra temp sensor.

 

For me,  I have dewpoint errors when there is not much wind.    If there is
a good steady breeze the dewpoint(humidity) holds quite well with other
stations in the area.     The Humidity isn't much different up at the house,
(about 120 yards up a hill) than it is on the dock.   The reason for this is
that the whole valley tends to have a higher humidity.   Think of the
mornings when you are on top of a hill and you look out and see fog sitting
in all the valleys around you.   We have that occurance pretty regularly in
my area, and my house is one of the ones under the fog bank.   

 

What I am saying,  is to determine if the lake or perhaps your trees holding
the moisture near your sensors,  watch and see if your errors are worse on
days with no wind.   That will help you determine if you have a sensor
problem or not.

 

 

Please take what I have said here with a grain of salt as I am new to this
weatherstation thing myself.

kw

 

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From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Bill Simm
Sent: Monday, April 17, 2006 12:35 AM
To: wxqc at lists.gladstonefamily.net
Subject: [wxqc] dew point errors

Hi there,

   I have been that getting out of range error message regarding my dew
point continuously for the last couple of weeks.   I thought maybe the
problem was the snowmelt,  but the last of the snow in yard dissapeared
about 2 weeks ago and I am still getting the error message. 

   My thermo-hygrometer is located on the north side of the shed right on
the corner (not directly under the eev - the shed is painted white).   There
are also the two 120 foot spruce trees in the yard.  The result is the
thermo-hygrometer is not exposed to much direct sunlight.   The instrument
has its own litte wind and sun shield on it.  I think the location is pretty
good and combined effect of the all these conditions pretty much amounts to
being in a stevenson screen.      

   I am wondering if the problem has to do with my close proximity to
Cresent Lake?  If you are familiar with Portage la Praire,  you know there
is an oxbow lake in the centre of town.  (It was once part of the
Assignaboine River).    My weather station is about 100 meters from the
water.  

    There are two weather other stations in this area. One,  a CWOP station
at 

Southport (also an Enviroment Canada wx station) it is about 5 kms south of
Portage and not near any bodies of water.   The other is an Enviroment
Canada station at the University of Mantoba Field station in the marsh at
Delta beach along the shore of  Lake Winnipeg about 20 km north of Portage
La Praire.    

    Although close to me neither of these stations are good choice to
compair dew point or humidity readings with.    There are a bound to be
significant differances in our dew point and humidity readings due to the
proximity of  or lack of proximity of large or small bodies of water to each
station.     

    I would be very interested in hearing the opinion of anyone else
regarding my dew point error problem as indicated by the weather quality
report.

 

                                        Many thanks,

                                                                Bill

   

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