CWOP Data variations?

[2-Wheeler]

I'm looking for some help here. I've just started participating in the CWOP data quality program, and my readings seem to be all over the map, based on their expectations. (CW5670). Granted I don't have a lot of data yet, maybe I should wait and take a longer term view?

http://weather.gladstonefamily.net/site/C5670

First, I am still puzzled after reading the threads on the barometer offset and Davis VP2 calculations. At first, my readings were consistently low. I tried the QFE tick-box, but it didn't seem to make any change in the data. Next I added some off-set manually to the Davis Console to match local airport and NOAA settings and it was fine for a while, but the "gain" seems to be off now, while it seems to match with low pressure, at high pressure it seems to be too high now. What to do?   

Next, my temps dive lower than anything nearby as a rule when the air is still and clear because I'm in a depression. So on still clear nights, my temps show as "error".   

Finally, on those same clear-calm nights, (like last night 4/10 or the night before 4/09 for example) my humidity shows significantly higher during the nighttime than their calculations.  Perhaps this is a reflection of my colder microclimate?  Anyway my site is located ideally in an open prairie with natural vegetation, not near any buildings as they suggest would cause the errors.   

Suggestions welcome:
Today's analysis: http://weather.gladstonefamily.net/qchart/C5670?date=20060411
Yesterdays: http://weather.gladstonefamily.net/qchart/C5670?date=20060410

[NorCal Dan]

Did you get the fan aspirated radiation shield (FARS) with your VP2? 

On the CWOP setup screen as you set like in the attached?

[Weather Display]

yes, for when you have a davis VP, definately tick those settings if you live at altitude 

[carterlake]

Also, how is your station setup?

And it looks from your data quality that there is very significant variation in nearby station readings (I hesitate to say error because it may not be.)

This could mean you can't rely on CWOP for quality checking (you'll have to do it yourself). For temps, get an analog thermometer. For rain, get a good analog rain gauge. Etc....

[2-Wheeler]

Thanks for all the quick replies. I'm in Atlanta this week, but when I return home, I'll double check those tick-boxes.

No, I didn't get the fan-aspirated model (no power that far from house), but I would add the Daytime (solar) version if I were convinced my daytime temps were reading too high, but they seem to be very consistent with peak readings nearby - it's only my night time temps that vary, and only when it is not windy at night.

I presently have 4 other outdoor thermometers in various places that track the readings I'm getting within a degree or two. Only one of those (LaCrosse) also has outdoor humidity, but it was only rated from 20-80%.  I also often see as much as a 10 degree rise in temps on my car thermometer over just a mile or two as I drive up and out of the depression on cold mornings. I guess I should look for a mechanical humidity/barometer to compare against?

My station is set-up about 150 feet from the house, 75 feet from nearest pavement in an open field of natural grasses. The ISS is mounted about 5 feet above the ground and the anemometer is mounted 10 feet above that.
Here is the picture (since this was taken I've changed the pole supporting the anemometer to a longer steel pole):

Thanks.
David

[NorCal Dan]

Just wanted to point out that the FARS is solar/battery operated.

No, I didn't get the fan-aspirated model (no power that far from house), but I would add the Daytime (solar) version if I were convinced my daytime temps were reading too high, but they seem to be very consistent with peak readings nearby - it's only my night time temps that vary, and only when it is not windy at night.

I think this is where the FARS would help.

Do the LaCrosse readings, at night, agree with the VP2 readings, especially the humidity? 

[2-Wheeler]

Just wanted to point out that the FARS is solar/battery operated.

I think this is where the FARS would help.

Do the LaCrosse readings, at night, agree with the VP2 readings, especially the humidity? 

Presently, the LaCrosse sensor is located under the eve of the house and near the driveway, so there is more solar mass nearby to hold the heat, but I'll often get frost in parts of the yard, but not near the house.

But I must be a bit confused about how the fan might change things at night. By circulating the air near the sensor, would it somehow raise the temperature or lower the humidity?

[2-Wheeler]

yes, for when you have a davis VP, definately tick those settings if you live at altitude 

Okay thanks. I'm back home  now and I found the boxes were NOT ticked, so I've ticked them as of tonight. I'll check back in a few days and see how it compares over time.

[NorCal Dan]

The idea is to continually exchange the air inside the shield with air from outside.  The end result is hopefully more representative of the actual conditions at the site.

I think all of us at one time or another, or one season or another, have gotten the big red X from the CWOP data checker.  Who's to say the other guys data is more correct than yours?  Maybe all the stations your data is compared with have bad sensors and you have the only correct readings. You could get a sling psychrometer to manually determine humidity and see if your sensor is giving bad readings.   

[2-Wheeler]

Okay thanks. I'm back home  now and I found the boxes were NOT ticked, so I've ticked them as of tonight. I'll check back in a few days and see how it compares over time.

Okay, I made those changes some time ago and there have still been some anomalies, but I've finally got that big green check next to the barometer reading for the past 7 days!   
http://weather.gladstonefamily.net/site/C5670?tile=10;days=7#Data

They still don't like my dewpoint readings much and include the following quote:

The mean error of your daytime readings is significantly different to the mean error of your nighttime readings. In particular, the difference is 3.4 °F warmer during the night. This is often caused by poor siting of the sensor (e.g. under eves, in screen porch, or in thick tree coverage)

But actually, my real temps are lower during the nights and the humidity is higher, making for higher dew points.  My siting is nearly ideal in a grassy field far from anything.

[Weather Display]

thats good news that the barometer correction with a VP is working good (needed if at altitude)
(thanks go to Tinplate for that one)

[Fox_Of_The_Wind]

Now would that fix the problem I am haveing? http://www.weather-watch.com/smf/index.php/topic,12746.0.html All the things he is saying I have the same problems. I just clicked on the 2 boxes now and will wait to see what happens.

[SoftWx]

It depends on your altitude (the higher you are the bigger the affect). After you check those boxes, you may need to make a barometer calibration adjustment to your console, but after that you should be good for a long time.

[Fox_Of_The_Wind]

It depends on your altitude (the higher you are the bigger the affect). After you check those boxes, you may need to make a barometer calibration adjustment to your console, but after that you should be good for a long time.

hmmm. Great no clue how to do that. (better make a backup before I do anything) Thanks for the info.

[SoftWx]

I took a quick look at your CWOP data, and it looks to me like your console is reporting the raw sensor pressure. This happens when the console is set to an elevation of 0 ft.

The easiest way I can describe for fixing this is to exit all weather programs and then run WeatherLink (configured to the right COM port). Select the Setup / Set Barometer... menu item. It will then pop up a dialog that shows your current elevation and barometer. If the elevation is indeed zero, enter 720 (your CWOP elevation) into the elevation and enter the current barometer value into the barometer value. Click ok.

This should get you a LOT closer to where your barometer readings should be. The altimeter feature in WD you should still use. This keeps your reported barometer reading from drifting off where it should be as the seasons change.