Stevenson Screen

[Thundery wintry showers]

I was having problems back in November in setting up a Stevenson Screen, with the sun shining on the screen causing temperatures to be somewhat inflated relative to other stations in this area. I got a second set of louvres put on, which solved the problem for the following three months.

Now, frustratingly, I'm back where I started; yesterday all stations in my local area reported maxes of around 9-10C; I was getting 11.3C while the sun was shining on the screen, which shot down to 9.5C when the sun went around the front of the house, thus not shining on the screen.

I have a picture of the screen here:

Can anyone help? I'm starting to think I will never be able to get a recording screen that will function like everybody else's.

[Polar Continental]

I was having problems back in November in setting up a Stevenson Screen, with the sun shining on the screen causing temperatures to be somewhat inflated relative to other stations in this area. I got a second set of louvres put on, which solved the problem for the following three months.

Now, frustratingly, I'm back where I started; yesterday all stations in my local area reported maxes of around 9-10C; I was getting 11.3C while the sun was shining on the screen, which shot down to 9.5C when the sun went around the front of the house, thus not shining on the screen.

I have a picture of the screen here:

Can anyone help? I'm starting to think I will never be able to get a recording screen that will function like everybody else's.

Hi TWS,

The only thing that strikes me with the picture is your screen looks a little close to the fence, maybe heat is radiating off the brown surface which could be affecting your readings now the sun is getting stronger. You may need to place the screen in a more exposed spot if possible.

Another possible source of radiation heat is from the ground and top, are the top and bottom of the screen shielded with double panels? If not, it maybe worth trying to see if that helps.

Paul

[Thundery wintry showers]

Thanks a lot, those suggestions sound like they'd be easy to carry out and I'll certainly give them a try.

[Terminal Moraine]

I'd fully agree with all the above posts. A double top for the screen with an air space between the two layers is essential.

As an example of the influence of lack of ventilation on the temperature within the screen; my own screen is completely open to all directions except west, the ground rises significantly in that direction and there are also several dry stone walls, all helping to reduce the wind speed from that direction

On a day of sunny periods and a light-moderate west wind in spring or summer my screen maxima can be between 0.3c and 1.2c higher than a site fully exposed to the wind only 150 m away.

There is a microclimate effect here and much depends on the length of the sunny spells and how frequently and for how long they coincide with spells of reduced wind speed. The temperature within the screen can be similar to that at the more open site for 90% of the time but it takes only 5-10 minutes of sunshine coinciding with a spell of reduced wind speed to cause a relatively rapid rise in temp' and give an anomalous max'.

Looking at the photo' your site is much more sheltered than mine and the effect will be magnified accordingly if you are comparing your site with others in the vicinity with a more open exposure.

It's one of the vagaries of running an amateur site, it's very interesting but you always need to be aware of local site anomalies when comparing your own data with that of official/other stations.

Having said that, I've visited Met' Office sites where the standard of screen and site maintenance was so poor that the induced errors would outweigh any advantages from the standard open exposure.

T.M

[Osbourne One-Nil]

Doesn't John Holmes do something strange with silver foil?

[Terminal Moraine]

Doesn't John Holmes do something strange with silver foil?

Is this relating to a Stevenson's screen or are you thinking of a hail gauge? Or perhaps neither of these, I've no doubt there are a multitude of 'strange' things for which silver foil is the ideal medium.

Perhaps, if it's not too personal a matter, John can enlighten us.

T.M

[Osbourne One-Nil]

I think he wraps his head in it to stop the voices, but I also thought he made a screen to shade his sensor with it. I could be wrong. I was once...about 1985 in maths.

[Thundery wintry showers]

I tried the suggestions above. It seems that heat radiation from the fence (plus associated lack of ventilation) was the main problem; adding an extra layer to the top didn't make much difference, but moving it further away from the fence has made quite a remarkable difference.

Due to garden constraints I was only able to have it moved approximately 2.5 times further from the fence than it was before, but physics inverse square laws do suggest that if heat radiation was the main issue, the extent of the anomaly would be likely to reduce by a factor of 5, which is consistent with what I am experiencing.

Anyway, thanks a lot for the suggestions, they have been very helpful. I don't mind anomalies of about +/-1 degC relative to Met Office stations; it's probably unavoidable in a sheltered garden setting, and the anomalies would tend to cancel each other out over a significant period of time anyway. That's what I seem to be looking at now, rather than the positive anomalies of 3-5C that I was getting before on sunny days (readings for Thursday-Sunday of 18.8, 12.3, 16.1 and 16.3 are pretty promising)

By the way, if people wonder why I was quoting fairly representative readings during the period November-March, despite the screen not being ideally situated, I have tended to 'cheat' a bit by calibrating anomalous readings relative to my two old, more primitive thermometers' readings.

[johnholmes]

only just seen this thread and yes I'd agree Ian your sensors are all too close to the wattle effect screen it looks like on the photo. The double insulation mentioned by TM should also help.

As to tin foil, yes I use it to try and shield my Davis sensor for mid Spring and similar in early autumn, rather than shift the whole davis set up. It seems to work reasonably well. But its not perfect and as amateurs we have to accept we can never get the near perfect exposure that 'official' sites have to have.

An open airfield site or large field is a much better place than many of us with postage stamp sized back gardens. So we have to be very aware of possible shortcomings, and not afraid to admit them.

Reminds me must get some more tin foil!

John