Cumulus Support

After some temperature spikes when exposed to sun i decided to make myself a Stevenson Screen and copied this design exactly http://aaybee.com.au/StevensonScreen.html , however after a couple of sunny days the temperature does seem to still be a few degrees above what i think it should be. My high today was 19.6c and other stations around recorded 17c and 18.4c. It is placed at approx 5ft above ground level on a slightly raised bed (surrounded by grass) in the centre of my garden above shingle. The problem is i have no area in my garden that is in the shade apart from a north west facing wall, but this is exposed to sun in the middle of summer up to around 10.30 am.

Question really is what do you all do? I can't believe everyone builds big old stevenson screens to record accurate or close to it) temperatures. I want to move it this weekend, but i don't want to have to move it twice, so all input would be gratefully received.

Thanks
Vic

Hi
I had the same problem with a similar screen and managed to find a shady location which fixed the issue, others have used small fans to move the air and it has been said that painting the inside of the bowls mat black also helps.

The bigger the shade the more accurate the reading!

Vic, the unit you have built should really be called a radiation shield - to reduce the sun radiation - a Stevenson screen is really something large and heavy.

One improvement/modification you could make to your radiation shield (to improve its performance) would be to add a solar powered fan to the unit thus making it a daytime fan aspirated radiation shield - during daylight hours, the fan will slowly and continuously aspirate air into the unit thus reducing (or rather averaging) temperature variations due to air stagnation within the unit.

As for the differences of temperature reported by your weather station and the surrounding weather stations ... what makes you think the other reading are right ... your reading are probably the right ones for the area/location where your weather station is located...

I really do not understand this obsession most weather station operators have that the readings from their weather stations must match (almost) exactly the readings of the surrounding weather stations - what is the purpose of having a local weather station (local as near to one's house) if the readings reported by the weather station cannot be accepted as correct local readings (again, local as near to one's house) - then one might has well take down one's weather station and simply rely on the data from a surrounding weather station - hell, one might as well forget the 'also-inaccurate' readings from the surrounding weather stations and simply trust the reading provided by the government weather stations...

gemini06720 wrote:Vic, the unit you have built should really be called a radiation shield - to reduce the sun radiation - a Stevenson screen is really something large and heavy.

One improvement/modification you could make to your radiation shield (to improve its performance) would be to add a solar powered fan to the unit thus making it a daytime fan aspirated radiation shield - during daylight hours, the fan will slowly and continuously aspirate air into the unit thus reducing (or rather averaging) temperature variations due to air stagnation within the unit.

As for the differences of temperature reported by your weather station and the surrounding weather stations ... what makes you think the other reading are right ... your reading are probably the right ones for the area/location where your weather station is located...

I really do not understand this obsession most weather station operators have that the readings from their weather stations must match (almost) exactly the readings of the surrounding weather stations - what is the purpose of having a local weather station (local as near to one's house) if the readings reported by the weather station cannot be accepted as correct local readings (again, local as near to one's house) - then one might has well take down one's weather station and simply rely on the data from a surrounding weather station - hell, one might as well forget the 'also-inaccurate' readings from the surrounding weather stations and simply trust the reading provided by the government weather stations...

Great reply, and yes i do mean radiation sheild

I like the idea of a solar fan, so i will investigate this. One of the reasons i did not attach to a building in the shade is that i want to record the absolute low in winter and decided that if i placed the sensor in the centre of the garden this would reduce any stored residual heat which would affect readings.

Vic, I was not questioning (nor debating) your choice of location for your temperature sensor (or radiation shield) - as you stated, the center of the garden away from any heat-keeping structures is THE best location - it is my belief (understanding) that the temperature/humidity sensors, preferably contained/installed within a fan aspirated radiation shield, should be sited (located) in an as-much-as-possible opened field ... sure ... how many of us now-a-day, have access to acres of opened fields...

As I keep saying/writing over and over, one can only install one's weather station within the restrictions of one's area, one can only expect so much from one's weather station - the quality and reliability of the weather data is directly proportional to the quality and reliability of the weather station - one cannot expect to get the same quality and reliability from a lower priced weather station than from a more expensive (and in some countries, overpriced) weather station - one should not expect the data produced by one's weather station to be identical to the data provided by surrounding (different unit, different quality, different price) weather stations...

One final suggestion Vic ... and it is not concerning your weather station ... when quoting a message, you do not have to leave all the text of the quoted message ... you can edit and leave only the important part of the message ... as long as you do not remove the opening and closing quote tags ''...

Tau Bootis wrote:Hi
I had the same problem with a similar screen and managed to find a shady location which fixed the issue, others have used small fans to move the air and it has been said that painting the inside of the bowls mat black also helps.

And probably the worst thing you can do is to use black paint (mat, gloss, satin - whatever - anywhere near the temperature sensor - why do you think all sensor supplied are white?

As for the previous comment about the 'big Stevenson Screen' - they are big for a reason, not just because someone decided to make them that big. As Ray points out - unless you want to become an Official Meteorological Recorder, then you (and I am using the Royal You here) have to start accepting that:

a) The instruments you/we use are NOT scientifically accurate enough for precise measurements
b) The readings you get from them are the readings at YOUR location, not at the Official Site 10 miles down the road or even the neighbour 200 yards away.
and finally
c) If you feel the information your sensors are providing you then you have two choices - buy more accurate equipment (and maybe spend thousands doing so) and, in the process, make sure the instruments are situated and mounted correctly or secondly, stop using any instruments at all.

serowe wrote:

Tau Bootis wrote:Hi
I had the same problem with a similar screen and managed to find a shady location which fixed the issue, others have used small fans to move the air and it has been said that painting the inside of the bowls mat black also helps.

And probably the worst thing you can do is to use black paint (mat, gloss, satin - whatever - anywhere near the temperature sensor - why do you think all sensor supplied are white?

If you re-read what you quoted, the poster is talking about painting the INSIDE black. There is solid theory why this might help, although the impact might not be large enough for you to notice the difference. It certainly won't hurt.

Painting the OUTSIDE black is another matter entirely, which nobody is discussing.

Actually I never mentioned painting 'outside' I said 'black'. The design of Stevenson Screens is white inside and out for a very sound reason - having ANY black areas WILL start to absorb heatand retain it, skewing any readings.

So to reiterate what *I* said - there is no scientific basis for painting ANY part of a screen (Stevenson or otherwise) black.

As for 'might not be large enough for you to notice the difference' - what is 'large enough'? Skew the temperature by even a degree and your readings become suspect.

Not sure what the official specs are for a Stevenson Screen but the UK Met Office appears to be using SS which are black inside. http://www.metoffice.gov.uk/media/pdf/k ... No._17.pdf The link shows the SS and states "It is constructed of plastic, coloured white outside and black inside, with double roof, floor and side louvers that serve to reduce the amount of heat that would otherwise be conducted from the outer wall to the inner enclosure, especially in strong sunshine."

gemini06720 wrote: you can edit and leave only the important part of the message ... as long as you do not remove the opening and closing quote tags '

Code: Select all

'...

Yes, know all about editing quotes thanks, just didn't do it on this occasion.

I'm actually toying with the idea of building a single louvred wooden box that just fits outside the existing radiation shield for evaluation in essence this would become a double louvred design and from jotting down a few ideas seems like it is very simple to do

done and so far the readings reflect a davis station up the road from me

should be a resounding success

Sounds interesting Steven, can you post a picture of your finished article.

I made up this screen to hold the original screen...



Zip tied in and also has a computer fan in the top for warmer temps. I have a 12 volt solar panel that I'll mount on the mast when it re-warms here (already had 81F!).

Used the fan last year with a car battery. Batt lasted > 1 month but not very practical.

Se the rest of it here:

https://cumulus.hosiene.co.uk/viewtopic.php?f=16&t=6069