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Radiation Shield accuracy

Discussion specific to Davis weather stations
RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Fri Mar 19, 2010 7:12 pm

Pete B wrote:I'm not sure that the more westerly of the 2 stations to the South of me, IWORCSMA1 is a FARS station, unless I've missed something on the website. I've 'spoken' to the owner via e-mail & he didn't mention the FARS. It is a '+' station though, i.e. has both solar intensity and UV intensity sensors on the ISS whereas mine only has the solar sensor so ideally, my ISS requires sunny exposure. His station is also roof mounted, as is the other one to his East so this may affect readings on sunny days.

My mistake Peter. I'd assumed Plus stations came equipped with FARS but it is an optional extra.

Interestingly, today - a windy, cloudy day over this area has resulted in most of the stations on the larger area map all giving very close readings to each other. There is another station on WU very close to me, just to the NE of mine, IWORCEST27. However, I don't know how it's set or exactly which house it is as nothing such as a high mounted anemometer can be seen but the fact that their pressure reading is about 30mb higher than everyone elses doesn't inspire much confidence in it being well thought out or set up for accuracy.

Strangely enough it was also quite cloudy here but there was some sunshine late morning which allowed my station to climb to 14.6C whereas Manchester Airport only managed 13C but they don't include the decimal part so it was probably a bit higher. I can cope with up to 2C difference as I would expect my sheltered garden to retain more heat than an exposed airport. Woodford (3 miles east of me) managed 13.5C. Badly setup stations are best ignored.

...so it may be that there is a genuine mini climate for Malvern due to the unusual (for England) geography with a hill range rising rapidly from a generally flat plain to a height of about 250-300m higher than the plain. Today, my high temp of 15.8C is identical to that of the Pershore station which may be a small private airfield in that area.

I imagine many parts of England will have micro-climates especially those near hills (leeward especially - North Wales being a classic example). Sandy soils like Rickmansworth and Bournemouth are notorious frost hollows. That's why it's so much fun having a weather station!
If I start to get silly readings, i.e. regulars above 32C (90F) on ordinary hot (27 - 29C) days, then I know I need to do something to get meaningful readings. As you say, for winter night minima, it is very difficult to know exactly as lower lying well open areas will always be lower than those in small suburban gardens

Likewise here. My hottest day last year was 30.4C (86.7F) on July 2 and Man Airport recorded 28C (82F). But it felt hotter than 28C.

Lawn cutting is not an issue as I do it myself :D.

But is it awkward mowing around the station? Do you knock the rain guage?

Incidentally, using Google Maps in conjunction with the Google Earth links in WU, you can get a good idea of the location and general area around most stations, i.e. housing density, average garden size & layout etc. E.g, both those roof mounted stations to the South of me can be seen on Google Maps themselves as well as the general area they are in.

Is this using StreetView? That is amazing and you can see the anemometer attached to my chimney. I'll have a look along your street.
Cheers,
Ray, Cheshire.

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Pete B
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Re: Radiation Shield accuracy

Postby Pete B » Sun Mar 21, 2010 1:00 pm

RayProudfoot wrote:I imagine many parts of England will have micro-climates especially those near hills (leeward especially - North Wales being a classic example). Sandy soils like Rickmansworth and Bournemouth are notorious frost hollows. That's why it's so much fun having a weather station!

Yes, and the Malvern micro/mini climate is obvious on some days in various ways. Probably the most notable of all occurs on quiet hot humid summer days with an unstable atmosphere. Air rising from the top of the hills results in convergence of air towards the hills from either side, this further enhancing the process & setting off late afternoon/early evening thunderstorms in situ with little warning other than the rapid cloud buildup. These then, once going, expand & drift North Eastward across the West then into the East Midlands ending up towards Nottinghamshire/Lincolnshire. Fun to watch late evening from the top of the hills, that's once they've well vacated their birthplace. :lol:
On more than one occasion over the years, I have cancelled pre planned summer evening hill walks I have arranged due to the fact that from mid afternoon on such days, it becomes obvious that it's likely to happen.

This will be my first summer with the VP2 so it's going to be interesting to track the temp/humidity/pressure trends during such an episode if we have any this year and compare it with more distant stations away from such activity. We had a few last year.

RayProudfoot wrote:Likewise here. My hottest day last year was 30.4C (86.7F) on July 2 and Man Airport recorded 28C (82F). But it felt hotter than 28C.

As you know, most official stations are set up in the open to reduce very localised effects. But, the more of the home AWS's that get installed, the more the variation in suburban microclimates across quite small distances will begin to show. All very interesting & if so desired, could even be used to select a house with a back garden with a suitable microclimate going on the data fro any nearby AWS's

RayProudfoot wrote:But is it awkward mowing around the station? Do you knock the rain guage?

Right up around the station mounting pole, I will just use hand shears to trim the grass, especially as there is a grass mininmum temperature sensor lying on the grass at the base of the pole.. That keeps the mower away from it.

RayProudfoot wrote:Is this using StreetView? That is amazing and you can see the anemometer attached to my chimney. I'll have a look along your street.

Yes, it is. If you use it on mine, you'll not see anything re the weatherstation as the photo's were taken before I set it up. ;)
I only set my station up at the start of this year so I only have a couple of months of winter of records so far.

Pete B
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Re: Radiation Shield accuracy

Postby Pete B » Sun Mar 21, 2010 2:22 pm

Just to add to any previous information, this article is worth a read regarding any AWS siting, installation & management, not just Davis.

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Sun Mar 21, 2010 4:46 pm

Pete B wrote:Yes, and the Malvern micro/mini climate is obvious on some days in various ways.

Interesting you get such a high incidence of thunderstorms and are able to watch them from a good vantage point. I get less than average and haven't seen a spectacular one for many years.

This will be my first summer with the VP2 so it's going to be interesting to track the temp/humidity/pressure trends during such an episode if we have any this year and compare it with more distant stations away from such activity. We had a few last year.

The most noticeable thing I've observed is the sudden drop in outside humidity in the last couple of weeks. Shows the sun has enough strength to dry out the atmosphere. That's the best indication of spring to me.

As you know, most official stations are set up in the open to reduce very localised effects. But, the more of the home AWS's that get installed, the more the variation in suburban microclimates across quite small distances will begin to show. All very interesting & if so desired, could even be used to select a house with a back garden with a suitable microclimate going on the data fro any nearby AWS's

Assuming they were all setup correctly of course. I've seen some wild variations earlier today. One station 4 miles from Manchester Airport recorded 10F higher at one point! But my 'reference' station - a VP2 with Daytime FARS stayed very close to the airport's temps. I really should write and ask if I can visit.

Right up around the station mounting pole, I will just use hand shears to trim the grass, especially as there is a grass mininmum temperature sensor lying on the grass at the base of the pole.. That keeps the mower away from it.

Thanks for that.

If you use it on mine, you'll not see anything re the weatherstation as the photo's were taken before I set it up.

I looked carefully but saw nothing. Obvious now why.

I did take a couple of readings from the ISS location and another about 8-10ft away and there was a 0.7C reduction in temp. It may not seem very far but the alternative location is further away from that corner thus allowing more air to circulate. It's between its current position and 'B' on the photo on page 2 of this thread. That's all that may be needed. I'm hoping Easter will provide the opportunity to move the station.

Thanks for the link to that document. I read it before I bought my VP2 and it made me decide to locate the anemometer on the chimney. Of all my readings I would say the wind is the closest to official stations (given I'm in an urban area). Rainfall is so variable there's no way to make meaningful comparisons and my temps are acccurate on cloudy days so once I get some air circulating around the ISS on sunny days I'm sure I can get it more accurate.
Cheers,
Ray, Cheshire.

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mcrossley
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Re: Radiation Shield accuracy

Postby mcrossley » Sun Mar 21, 2010 7:46 pm

One station 4 miles from Manchester Airport recorded 10F higher at one point!


Ray, would that be mine? I removed the shield from the sensor today it give it coat of white enamel. I have a feeling this will be a big (and simple) improvement on the standard FO shield. When I removed the shield the temp went up 2.5C when I replaced it with its new paint job it dropped about 4C!

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Sun Mar 21, 2010 8:06 pm

mcrossley wrote:
One station 4 miles from Manchester Airport recorded 10F higher at one point!


Ray, would that be mine? I removed the shield from the sensor today it give it coat of white enamel. I have a feeling this will be a big (and simple) improvement on the standard FO shield. When I removed the shield the temp went up 2.5C when I replaced it with its new paint job it dropped about 4C!


Hi Mark,

Yes it was. That explains why your readings went sky high! I didn't feel it right to identify you but clearly you know who you are! ;)

I hope it was a worthwhile exercise. Sounds like it will be. I'll watch your readings with interest after that especially on sunny days.
Cheers,
Ray, Cheshire.

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adam5
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Re: Radiation Shield accuracy

Postby adam5 » Tue Mar 23, 2010 7:59 pm

Hi Ray and all,

Been awhile...away doing other things...but thought I would check back to see how your ISS mounting/solar loading issue was coming along. I must say, I am quite impressed with the quest you and others have embarked to get as accurate temp data as possible. Impressive indeed.

Another idea I had, and perhaps it has already been said, you might try giving that 4x4 wooden post the ISS is mounted on a couple coats of bright white paint. You would be amazed how much "cooler" the post will be in full sun afterward. I know...since I just did the same thing with the 4x4 post my temp sensor is mounted on. That darker wooden fence may still be an aritificial source of heat, but might be far enough away to be minimal.

Any chance of painting that fence white...at least in the corner? More work I know, but a thought.

-Brian Adam
http://www.castlewoodweather.net

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Tue Mar 23, 2010 9:04 pm

Hello Brian,

Nice to hear from you again. Yes, I'm afraid I'm like a dog with a bone. I won't give up on this until I've exhausted all practical possibilities.

Someone else mentioned about painting the 4x4 post and it does make perfect sense since any absorbed heat will travel straight up towards the sensor. It's a treated post guaranteed for several years so I hope painting it won't affect its protection. Daft question but I have to ask. ;) It's fairly rough so I'll need a couple of coats of primer and then an external matt finish. Don't think gloss finish would do much apart from dazzle the neighbours.

Painting the fence panels would make them stick out like sore thumbs and probably annoy the neighbours.

I'm a firm believer that to solve a problem you change one and only one thing at a time and observe the result. Given that corner position is probably worse than the unpainted post I'll go for a repositioning first and see how that affects the temps. They're not wildly out from the airport so I'm hopeful I can overcome this irritation.

Thanks once again.
Cheers,
Ray, Cheshire.

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George Richardson
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Re: Radiation Shield accuracy

Postby George Richardson » Thu Mar 25, 2010 7:36 pm

Ray,
You might consider driving a chain link fence post a couple of feet into the ground. this would make a semi-temporary mounting location for your VP2. I find digging a big enough hole to mount a 4 X 4 is awfully close to permanent. If you do decide to go that route, they make a plastic slipcover for 4 X 4s for fences so you don't have to worry about painting.
FWIW
George

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Thu Mar 25, 2010 8:28 pm

George,

It may not be visible on the photos I've uploaded but that post is sitting in a metapost which is about 3ft long. My mate assures me it won't be too difficult to extract and reposition. That's why I chose the metapost rather than using concrete.

However, when we move the ISS to my proposed position one of us will keep it steady for a few minutes whilst I check the temperature on the console. It should become apparent if it's a better location or not quite soon. A sunny calm day is essential for this. Interesting to note that the airport was 2F cooler today even though it was predominantly cloudy suggesting my location will always be warmer.

The plastic cover does sound a good idea. I'll ask at my local DIY store. I don't like painting.

Thanks. :)
Cheers,
Ray, Cheshire.

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Pete B
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Re: Radiation Shield accuracy

Postby Pete B » Thu Mar 25, 2010 11:52 pm

George Richardson wrote:Ray,
You might consider driving a chain link fence post a couple of feet into the ground. this would make a semi-temporary mounting location for your VP2. I find digging a big enough hole to mount a 4 X 4 is awfully close to permanent. If you do decide to go that route, they make a plastic slipcover for 4 X 4s for fences so you don't have to worry about painting.
FWIW
George


Or alternatively to a wooden post, I used a 38mm (1.5in) dia metal pole purchased from a local aerial installer. The advantage is that it will not heat up like a thick wooden post and is probably easier to mount & move if necessary. To anchor it in the ground, I used a suitable metal ground stake, the type used for mounting rotary washing lines. To ensure the post is tight in the metal ground stake, I cut a slot across the stake and down the length to 3cm then used a 'Jubilee' clip around the outside of the stake at the slotted top to clamp tight around the mounting post. I said I was going to post some pictures but don't have any (free) webspace to host them, my ISP doesn't give free server based webspace any longer so unfortunately, I can't at this time!

Regarding the accuracy of my station, I recently subscribed to the Climatological Observers Link which gets me a monthly report of all member reporting stations. There is a conventional instrument 'Stevenson Screen' station very close to me and the just received report for the month of February (the first full month I've had mine) that stations max for the month was extremely close to mine and on the same date, as was the monthly minima so I don't seem to be too bad so far. It'll be interesting to see how I compare in the warmer sunnier months.

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steve
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Re: Radiation Shield accuracy

Postby steve » Fri Mar 26, 2010 7:34 am

Pete B wrote:I said I was going to post some pictures but don't have any (free) webspace to host them, my ISP doesn't give free server based webspace any longer so unfortunately, I can't at this time!
You can upload pictures to the forum as attachments (and place them 'inline' in the appropriate part of your message if you want) - they don't need to be hosted elsewhere.
Steve
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Pete B
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Re: Radiation Shield accuracy

Postby Pete B » Fri Mar 26, 2010 1:48 pm

steve wrote:
Pete B wrote:I said I was going to post some pictures but don't have any (free) webspace to host them, my ISP doesn't give free server based webspace any longer so unfortunately, I can't at this time!
You can upload pictures to the forum as attachments (and place them 'inline' in the appropriate part of your message if you want) - they don't need to be hosted elsewhere.

Thanks Steve, I can see how its done now, the attachment is sent up with the message rather than being uploaded to the forum as a separate operation as on some other boards.

When I read the FAQ's and saw the "Why can't I add attachments?" question in the "Posting Issues" section, I misinterpreted the meaning & didn't look any further thinking I had to host elsewhere, then simply include a link to the hosting server in a message. I'll get something sorted & posted over the weekend.

adam5
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Re: Radiation Shield accuracy

Postby adam5 » Fri Mar 26, 2010 5:58 pm

Hi Ray and all,

I also like the metal fence post idea, as a metal post will simply not "heat up" like a wooden post will. I considered the same for my station but ultimately liked the look of a wooden post better, and simply painted the post bright white.

Interesting to note that the airport was 2F cooler today even though it was predominantly cloudy suggesting my location will always be warmer.

That is a very good point worth remembering. The warm bias you see at times (or at least a portion of it) may, after all, be "real" and simply the difference between measuring temperatures in an urban setting versus a wide open airport. I think it is important to not get too caught up in comparing your readings to the nearest airport, or even a close neighbor with a PWS.

Further, everything else being equal, sensor accuracy alone can account for a 1F to 2F difference. Here in the U.S. ASOS temp sensor specs call for +/- 2F to be "in spec." Surprisingly high, although usually the sensors are closer than that. However, one can never be sure of the site conditions or temp accuracy of a nearby "official" site.

Bottom line, again I try not to get caught up in comparing my readings with nearby stations. Just do the best job you can with what you have to work with, avoiding artificial heat sources to the extent possible, and enjoy.

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Fri Mar 26, 2010 7:04 pm

Thanks everyone for your ideas. I've been looking on the website of B&Q here in England and I can't find a suitable aluminium pole. Davis stipulate it must have a diameter of 32mm - 44mm (1.25" - 1.75") but B&Qs are no greater than 20mm diameter.

So I next looked at this site and they stock a 6ft x 37.5mm (1.25") aluminium post. This seems ideal but the only fly-in-the-ointment is how best to position it. Would it be possible to use my 4"x4" metapost? The metapost has two screw holes in the section where the post is inserted and I need to place the aluminium post in that and somehow fix it securely. Got any ideas? Seems tricky.

Alternatively how far into the ground would a hollow aluminium post have to go to support the ISS? Would it be better to buy the 10ft x 1.5" (37mm) and just drive the bottom 2-3ft into the soil? It should go in pretty easily and the weight of the ISS is hardly an issue. That seems a better one and alumimium won't rust.

My other thinking of the longer pole is that I could locate the ISS slightly higher so it's well clear of the fences and would therefore catch more breeze that lower down.

Brian, your suggestion not to compare to the airport is sensible. Instead, I will comapre to the other Davis VP2 with a 24hr FARS 10 miles away. That is probably the better option. But knowing what is natural warming (a warm garden) and what is false warming (post generating heat up to the sensor) is difficult to determine.
Cheers,
Ray, Cheshire.

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