Do I Need A Stevenson Screen?

[Backtrack]

I've been reading around the forum, and noticed that alot of people have Stevenson screens to shield their sensors from the sun.

Say for instance, I had my sensor that in a spot that has 100% shade 365 days a year, is one needed?

Also, by how much could my readings be out by if I don't have a screen (with the sensor in the shade).

Edited July 24, 2010 by Backtrack

[Backtrack]

Just going to bump this. Really need an answer.

[Terminal Moraine]

I've been reading around the forum, and noticed that alot of people have Stevenson screens to shield their sensors from the sun.

Say for instance, I had my sensor that in a spot that has 100% shade 365 days a year, is one needed?

Also, by how much could my readings be out by if I don't have a screen (with the sensor in the shade).

Ideally, Backtrack, a Stevenson's screen sited on an open area of short grass would be the best thing to shield your sensor and would give the best possible readings, subject to the margin of error within the sensor itself.

Stevenson's screens are very expensive, and not everyone has a large area of grass to site their instruments, so the next best thing is to buy a mini-screen and mount it on a white painted arm fastened to a white painted post at about 1.2 m above the ground, preferably grass, in as open an area as you've got.

The main problem with having the sensor in the shade without a screen is that it has to be fastened to something, such as a wall or a fence, and that will provide a certain amount of background radiation which will be particularly noticeable on warm, sunny days with a light wind and on clear, cold nights with little wind. If the sensor is sited under a tree to provide the shade the errors will be even larger due to the sheltering effect from above.

With regard to the margin of error if you don't have some sort of screen, a lot depends on the available area to put the sensor, whether or not it's in a screen.

For instance, if you have a small sheltered back yard which is completely paved and surrounded by walls, buildings fences etc, the margin of error between having a screen and not would be smaller than if you have a relatively open garden. This is because, in the first example the reading error ( compared to an open grassed area outside your yard) coud be as much as 5c too high on a hot sunny day purely due to the shelter and background radiation.

If you've got a fairly open site the error might be 3c too high on a calm sunny day and anything between 2c too high and 3c too low on a calm clear night.

I've gone on about this because there is no simple answer as to how much your readings will be out with no screen. The best idea is to get a mini screen ( if you can find one to take you sensor )and site it as outlined above. If it has to be near a fence or a wall, paint it white if possible to minimise background radiation.

Hope this has been of some use.

EDIT; If you can afford a Stevenson's screen and have the space to put it, all the better.

Edited July 24, 2010 by Terminal Moraine

[Backtrack]

Thank you TM for taking the time to write such a detailed reply!

That's exactly what I was looking for.

I'll take a photo of my pathetic excuse for a garden and post it here. I am unsure where to position my sensor too.

Would a better spot be ontop of the chimney in a screen?

Thanks.

I'll take a picture now.

[Backtrack]

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There is my 'garden'

Where would be the best place to put the sensor? You can see where it is now on the gate.

When I get my new weather station, I can put it on the chimney in a screen. Is that any better?

[4wd]

In that sort of location the best option would be get it up in the breeze on your chimney if you're happy with that for occasional maintenance like battery changes and clearing spider's webs etc.

The surrounding walls are going to create a lot of heated surfaces for much of the year, giving exaggerated maximums and artificially slow cooling overnight.

It would be interesting to have a sensor where you have it now and compare with a more exposed location higher up.

[Thundery wintry showers]

Some very useful insights by Terminal Moraine. I think he is right to say that the difference between having a screen and not having one is smaller when you've got a very small garden.

I originally had my own AWS thermometer situated on a north-facing hut, before I got a custom-built Stevenson Screen for it (assembled from some parts that my dad had lying around- much cheaper than buying a professional one). Having a screened thermometer has led to a substantial improvement in the minimum readings and winter half-year maximum readings that I've been getting, but it hasn't corrected the problem of sunny days in summer being 1 to 2C too high- this is because my garden isn't big enough to neutralise all radiation from solar-heated surfaces.

I think your "garden" is somewhat smaller than mine so I have doubts as to how much benefit you'd get from having a screen. Perhaps, as TM suggested, a mini screen would be a good bet but I think it isn't worth shelling out over £100 for a professional screen in your situation.

[Terminal Moraine]

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There is my 'garden' :lol:

Where would be the best place to put the sensor? You can see where it is now on the gate.

When I get my new weather station, I can put it on the chimney in a screen. Is that any better?

With the limited space you've got, Backtrack, you've got nothing to lose by putting the new sensor in a screen on the chimmney, provided heat doesn't radiate through the chimmney from inside.

Generally speaking the higher up the sensor is placed the higher your minimum temperatures will be, particularly on clear calm nights, relative to what they'd be if the sensor was placed at standard height ( 1.2m ). On the other hand, provided the position on the chimmney isn't in full sun ( north west, north or north east facing would be best )then you'll benefit from increased ventilation for the max' temperatures.

If putting the sensor on the chimmney means it has to be in full sun then I'd try and go for the option of mounting the screen on the fence or a wall down in the yard, preferably facing north. Try and get a gap between the screen and whatever it's mounted on to give all round ventilation.

Your enclosed garden means you'll always have to make allowances when comparing the readings you get with those from other more open sites but, you'll have just as much interest in making them as anyone else. Also, your readings will always be comparable with themselves so you'll have extreme records for your own site which will be equally as valid as those from other sites.

[Backtrack]

With the limited space you've got, Backtrack, you've got nothing to lose by putting the new sensor in a screen on the chimmney, provided heat doesn't radiate through the chimmney from inside.

Generally speaking the higher up the sensor is placed the higher your minimum temperatures will be, particularly on clear calm nights, relative to what they'd be if the sensor was placed at standard height ( 1.2m ). On the other hand, provided the position on the chimmney isn't in full sun ( north west, north or north east facing would be best )then you'll benefit from increased ventilation for the max' temperatures.

If putting the sensor on the chimmney means it has to be in full sun then I'd try and go for the option of mounting the screen on the fence or a wall down in the yard, preferably facing north. Try and get a gap between the screen and whatever it's mounted on to give all round ventilation.

Your enclosed garden means you'll always have to make allowances when comparing the readings you get with those from other more open sites but, you'll have just as much interest in making them as anyone else. Also, your readings will always be comparable with themselves so you'll have extreme records for your own site which will be equally as valid as those from other sites.

Thanks alot TM. Much appreciated! :lol:

Say in winter it's a calm night, and the official reading would be -3.0c would my reading be far out from this? (if it was on the chimney)

Some very useful insights by Terminal Moraine. I think he is right to say that the difference between having a screen and not having one is smaller when you've got a very small garden.

I originally had my own AWS thermometer situated on a north-facing hut, before I got a custom-built Stevenson Screen for it (assembled from some parts that my dad had lying around- much cheaper than buying a professional one). Having a screened thermometer has led to a substantial improvement in the minimum readings and winter half-year maximum readings that I've been getting, but it hasn't corrected the problem of sunny days in summer being 1 to 2C too high- this is because my garden isn't big enough to neutralise all radiation from solar-heated surfaces.

I think your "garden" is somewhat smaller than mine so I have doubts as to how much benefit you'd get from having a screen. Perhaps, as TM suggested, a mini screen would be a good bet but I think it isn't worth shelling out over £100 for a professional screen in your situation.

Thank you for the detailed reply TWS. I was looking on a few sites, and the cheapest Stevenson screen I saw was £450.

The weather station I am looking at on eBay at the minute is £100, but comes with a screen already over the temperature sensor.

To be honest I always wondered why my temperature fluctuates so much on a clear night in winter. I understand now.

One minute it would be -1.0c, the next it would be -0.4c. I understand that it was because of the surroundings.

My minimum was -7.9c last winter in that garden. I imagine that would of been closer to -10c?! Amazing.

Say I was to put it on the chimney however, it wouldn't be 1c-2c out would it? I mean surely 5-7 meters can't make a difference more than 0.3c-0.5c on a cold night?

Edited July 25, 2010 by Backtrack

[Terminal Moraine]

Thanks alot TM. Much appreciated!

Say in winter it's a calm night, and the official reading would be -3.0c would my reading be far out from this? (if it was on the chimney)

My minimum was -7.9c last winter in that garden. I imagine that would of been closer to -10c?! Amazing.

Say I was to put it on the chimney however, it wouldn't be 1c-2c out would it? I mean surely 5-7 meters can't make a difference more than 0.3c-0.5c on a cold night?

0

That depends to some extent on the general lie of the land around you. If you're living in a low lying flat area, with no lower ground where cold air can drain into, then the difference will be less than if you live on a slope or on a hill.

It's difficult to put a precise figure on what the difference would be as there are so many variables which could affect it. At a guess I'd say 1.0 to 1.5c higher on a clear, calm night. It would be interesting to keep your old sensor where it is and compare the two when the new one is installed.

You might find that the difference is smaller than I've quoted because there is a small amount of radiant heat from all the walls etc around your garden, and that siting the sensor on the chimmney removes this and gives a reading similar to one exposed at a lower level.

It really is a case of try it and see, even better if you can keep both going over one winter for comparison.

Edited July 25, 2010 by Terminal Moraine

[Backtrack]

0

That depends to some extent on the general lie of the land around you. If you're living in a low lying flat area, with no lower ground where cold air can drain into, then the difference will be less than if you live on a slope or on a hill.

It's difficult to put a precise figure on what the difference would be as there are so many variables which could affect it. At a guess I'd say 1.0 to 1.5c higher on a clear, calm night. It would be interesting to keep your old sensor where it is and compare the two when the new one is installed.

You might find that the difference is smaller than I've quoted because there is a small amount of radiant heat from all the walls etc around your garden, and that siting the sensor on the chimmney removes this and gives a reading similar to one exposed at a lower level.

It really is a case of try it and see, even better if you can keep both going over one winter for comparison.

I will definatley keep them both going.

We don't live on a hill, we are fairly low down too, only being 150 feet ASL.

Should be interesting to see. Thanks for all of your help!

[GulberwickWeather]

I made my own screen out of a large plastic flowerpot (painted white) which works perfectly well and it cost me a couple of quid!!!

[reef]

Its actually quite suprising how even on relatively cloudy days background radiation can give inflated temperatures. For years I ran a LaCrosse WS2300 in a mini-Stevenson Screen with very good results, temperatures were never more than a degree out from official sites all year round. Im now running a Vantage Pro 2 with FARs but thought it would be interesting to have the old LaCrosse WS2300 on the same pole without its mini-screen too see how much of an effect background radiation has.

Perhaps most interesting observation is that the differences are actually just as pronounced at night. Cheaper stations like the WS2300 with just a rain cover for protection have an issue where the cover radiates heat quicker than the surrounding air (like your car or the ground does). This has actually resulted in temperatures that are as much as 1.5C-2.0C too low on cold, clear nights. The station is still located in an area with lots of airflow and little sunshine as before, but temperatures in the summer half year seem to occasionally read a degree or so higher without the shield I was using, so it was definitely having a large positive effect.

The main thing to take into account when setting up your station is not to look too much into other people's stations. A look at amateur stations around here shows some ridiculous maxes and minima equally too low. The MetOffice official sites are a good guide but these are always going to be different to even larger back gardens due to the microclimate effect. Just as long as your station is positioned well and has a good shield then you can be happy - it may even take quite a few attempts to find a desireable place, however most get there in the end.