[wxqc] CWOP Information for CW4490 (C4490) in Leander, TX US

Sun Jun 1 16:43:37 CDT 2008

I live in the Conejo Valley at the western end of the Santa Monica  
Mountain Range.  The Pacific Ocean is 7 miles to the South as the crow  
flies.

Looking at the MADIS QC graphs for weather stations in the Conejo  
Valley, they all show that the lowest barometric pressure occurs in  
the early morning hours before sunrise.  The peak barometric pressure  
tends to occur midday.  This is contrary to what was stated below.  Is  
it the result of cooler air from the Pacific acting as a container,  
i.e. the heated air near the surface being trapped under a cooler air  
mass resulting in an increase in barometric pressure?

My peak barometric pressure tends to occur an hour or two after the QC  
analysis predicts.  Is it because the weather station is on a western  
facing slope?  The greatest variance with QC analysis tends to occur  
in the afternoon and early evening.  Why?

Merton Campbell Crockett

On 01 Jun 2008, at 12:23:15, Evan Bookbinder wrote:

> > Since the pressure sensor is located in the console, NOT the  
> station, if you see a variation with OAT then almost certainly its  
> coming from a software calculation and not a temperature affect on  
> the sensor its self.
>
> The location of the sensor indoors has nothing to do with the fact  
> the barometric pressure *is* impacted by outside air temperature.  
> Air pressure is the weight of a column of air above a sensor.  
> 99.999% of the air weight is not in your house, it's outside.  
> Barometric pressure is VERY MUCH altered by outside temperature.  
> Because we're not dealing with an enclosed volume of space, when you  
> heat the air, molecules expand and thus there are less of them in a  
> given volume of space. As such, the pressure falls. When you cool  
> the air, air molecules contract, occupying more per unit volume and  
> thus weight more (pressure increase).
>
> In the absence of any significant airmass changes, a diurnal  
> barometric pressure curve will show a MINIMUM in the mid afternoon  
> at peak heating, and a maximum around daybreak during peak COOLING.  
> This is for outdoors, with the greatest impact occuring where there  
> are the most air molecules -- in the lower atmosphere. Changes to  
> air temperature INSIDE your home are generally considered negligeble  
> given the vertical depth of air in your home when compared to the  
> entire atmosphere above you (yes, even with cathedral ceilings).
>
> All things considered equal however, you should not be seeing the  
> barometer rise in the afternoon, unless you are a coastal location  
> experiencing an afternoon seabreeze (which is accompanied by a drop  
> in air temperature and rise in pressure).
>
> Evan
>
>
>
>
>
>
> From: wxqc-bounces at lists.gladstonefamily.net [mailto:wxqc-bounces at lists.gladstonefamily.net 
> ] On Behalf Of Merton Campbell Crockett
> Sent: Sunday, June 01, 2008 2:09 PM
> To: Discussion of weather data quality issues
> Subject: Re: [wxqc] CWOP Information for CW4490 (C4490) in Leander,  
> TX US
>
> Davis Instruments' Application Note 28 explains how the barometric  
> pressure that is reported is derived.  The barometric pressure  
> reported has been reduced to report sea level barometric pressure.   
> On page 11 at the end of the discussion of barometric pressure, a  
> formula is provided for converting the reported barometric pressure  
> to the altimeter reading used for CWOP reports.
>
> I don't understand, exactly, how temperature effects the pressure  
> sensor in the Davis Instruments Vantage Pro2 console but my station  
> tends to report higher barometric pressures in the afternoon.  The  
> console is in a room with cathedral ceilings on the west side of the  
> house.  The room heats up in the afternoon and doesn't cool down  
> until mid-evening.  During this period, the QC graphs show the  
> widest divergence between my readings and the predicted values.
>
> Merton Campbell Crockett
>
>
> On 01 Jun 2008, at 11:31:02, Ted Lum wrote:
>
>> I don't know. That thread is more than two years old and I don't  
>> know if it was ever resolved. I have not been able to find anything  
>> in the Davis release notes that say they made a change, but that  
>> does not mean they didn't. Since the pressure sensor is located in  
>> the console, NOT the station, if you see a variation with OAT then  
>> almost certainly its coming from a software calculation and not a  
>> temperature affect on the sensor its self. This is only  
>> circumstantial, but your observations seem to imply they have air  
>> density in the calculation. We should find out for sure, I don't  
>> have first hand knowledge. As a Davis owner/operator I want to know.
>>
>> Victor Engel wrote:
>>>
>>> OK. But I thought the latest version of Weatherlink was sending  
>>> altimeter -- and VWS as well. Do I have a wrong understanding there?
>>>
>>> Victor
>>>
>>> On Sun, Jun 1, 2008 at 12:37 PM, Ted Lum <gladstonefamily.net at tedworld.com 
>>> > wrote:
>>> You might want to re-check this thread.
>>>
>>> http://server.gladstonefamily.net/pipermail/wxqc/2005-November/003212.html
>>>
>>>
>>> Sam Drinkard wrote:
>>>>
>>>> Victor Engel wrote:
>>>>
>>>>> I'm going to answer your question specifically and then also  
>>>>> raise a
>>>>> more generic question.
>>>>>
>>>>> First, it looks like your pressure reading is too low. I believe  
>>>>> if
>>>>> your mean error exceeds 2 millibars, you will get the message.  
>>>>> When I
>>>>> first set up my stations (C6155) in 2006, I noticed it tracked  
>>>>> another
>>>>> stations AS425 very closely in all data points, including  
>>>>> pressure.
>>>>> Although we tracked closely, the analysis stated our readings  
>>>>> were too
>>>>> low, so I made an adjustment upwards. My stations has had good  
>>>>> QC for
>>>>> pressure ever since.
>>>>>
>>>>> I wonder, though, about something. All three of us use Davis  
>>>>> Vantage
>>>>> Pro hardware, and all three of us were too low by similar  
>>>>> amounts. Do
>>>>> these stations generally ship with a slight adjustment error?  
>>>>> Could
>>>>> there be an adjustment needed at the factory? Certainly, we can't
>>>>> determine this from just three data points. I'm curious of others'
>>>>> experiences around the country, though, with this hardware.
>>>>>
>>>>> An alternative, of course, is that our hardware is correct and the
>>>>> analysis is wrong. Were that to be the case, though, the airport  
>>>>> would
>>>>> be too high. I think that's unlikely, since aircraft rely on  
>>>>> accurate
>>>>> altimeter readings to navigate.
>>>>>
>>>>> The other thing I've noticed but haven't spent time to check is  
>>>>> that
>>>>> it appears the error in my readings may be correlated to  
>>>>> temperature
>>>>> -- the higher the temperature the lower my reading with respect  
>>>>> to the 
>>>>> analysis. I may just be imagining that, and the magnitude of the
>>>>> effect certainly isn't large, if present. If this really is true,
>>>>> though, I wonder what would cause it.
>>>>>
>>>>> Victor C6155
>>>>>
>>>>>
>>>>     Victor,
>>>>
>>>>     I've noticed the same thing with my station, which is a Davis
>>>> WM-II.  If I look at the long term analysis of pressure, I can  
>>>> see a
>>>> trend in the summer months that *seems* to degrade the sensor,  
>>>> but like
>>>> you, I've not investigated this aspect.  As most of us know,  
>>>> nearly all
>>>> electronic components have some variant or change with changes in  
>>>> the
>>>> component's temperature.  Perhaps it would be a starting point to
>>>> contact Davis and ask if there is any compensation built into the
>>>> pressure sensor, or if ambient air temp changes would in fact,  
>>>> cause the
>>>> sensor to not read in a non-linear fashion.
>>>>
>>>> Sam
>>>>
>>>>
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>
> Merton Campbell Crockett
> m.c.crockett at roadrunner.com
>
>
>
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Merton Campbell Crockett
m.c.crockett at roadrunner.com

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