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I live 6 miles north of the official Writtle Met Office station. I have a weather station in my garden which is in the shade next to some trees. Where I live is slightly more rural than Writtle.

Normally my daytime temps are 2 or 3 degrees lower than the Writtle station but my nighttime temps are slightly higher than Writtle.

Is there a meteorological reason for that? Unofficial stations nearer Writtle tend to agree with the official one.

I tried reading temps in different parts of my garden and they're similar, so my weather station position doesn't seem to a factor.

There are quite a few trees in and around my garden though, plus I'm in a very slight valley compared to Writtle. Could these things have an effect?

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I'd say my location is fairly sheltered as it's surrounded by trees and fairly close to my house. I imagine Met Office stations are in fairly exposed locations to ensure minimal influence from trees and buildings.

 

Wouldn't more exposed stations record higher temperatures though, due to greater wind movement?

 

Does a Stevenson's screen have to be in the shade or is the screen itself considered to be enough protection from the sun?

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Not sure which station using but many have an accuracy of +-1c, so that + local factors, position etc could explain the difference.

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Not sure which station using but many have an accuracy of +-1c, so that + local factors, position etc could explain the difference.

I'd wondered if station accuracy was an issue, but the fact my station (a Fine Offset WH1080) is consistently cooler than Writtle during the day but warmer at night suggests accuracy isn't playing a big part. Plus I've  tried other thermometers  in various parts of my garden and they agree with my station.

 

I guess it's local factors which are at play - I find it interesting that quite a small distance (6 miles) can result in a smaller range of temperatures.

Edited by h2005uk
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When I'm in my car, which has a thermometer reading on the dash (not very scientific I know!), I found it incredible the wide range of variation as I was driving around. On a long, regular commute route, I started to notice trends based on where I was. For example, on a cold night, there was a frost hollow which could knock 6c off the temperature. Also, again on a cold night, the temperature could rise 3c on driving between two lakes. The same lakes knocked 1-2c off on a warm day. I reckon it's easier to think of your garden as a slightly different micro-climate, than think there is something up with your readings.

Most official sites are in the middle of fields anyhow, so it's bound to be different in your potentiallysheltered garden.

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Yes agreed unless you are in the middle of a field well away from buildings and water you can always be suseptable to temperature clumps.Here for example is around 2.5 to 3 degrees warmer than where I used to live just 4 miles away wether summer or winter,although I used to live at 340metre and now I am almost in the bottom!

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It's interesting how a relatively small change in location can have such an impact on the weather.

 

Where is the best place for a thermometer sensor in a normal-sized garden?

 

Shade near buildings is inaccurate, plus more exposed areas seem to be inaccurate and have spikes due to the sun (even with a screen/cover over the sensor). I've always gone for shade produced by a tree - with reasonable airflow in the area - to avoid the effect of direct sunlight! I think that's true shade but maybe that's 'too much' shade?!

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Interesting h2005uk I too live close to Writtle (2 miles away) my garden station shows similar maxes but warmer minimums because I live closer to Chelmsford city centre. Mins are 2-3c higher but maxes are similar. The effect of Chelmsford probably helps the maxes be slightly higher than where you are.

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I think it's a legitimate question, which I've been asked by a fair number of people over the years, but there are many reasons.  

 

It's mainly because it makes it much easier to develop a standardised set of conditions that make long-term climatological analysis and comparisons between sites possible.  If you use "sun temperature" then there are variables like the extent to which the sun shines on the equipment, how readily the equipment heats up, how strong the sunshine is, etc, to take into account, and I think this problem is so great that it makes it impractical to use "sun temperature" for any sort of consistent climatological analysis.  Hence the use of shade temperature instead.

 

True, sometimes sites with sheltered exposure can end up reading up to a few degrees higher than Met Office standard sites when using the shade temperature, but if we were measuring temperatures in bright sunlight I would expect differences in excess of 10C to be quite common.

Edited by Thundery wintry showers
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Official stations have thermometers in a Stevenson's screen placed in an exposed area - that means it'll be in direct sunlight, but the screen is supposed to provide shade. I'd have thought heat could build up in the screen though despite the slats for ventilation, but it has to be done like this to ensure readings across the country are comparable.

 

Shielding that comes with amateur/semi-pro weather stations is seemingly supposed to be designed for use in an exposed area, but my experience is the shielding isn't adequate enough to protect the sensors from direct sunlight.

 

I don't have a Stevenson's screen and don't have enough of an exposed area to warrant one. I therefore prefer shade created by a tree to protect my sensor from direct sunlight. I also have a small 'box' made of white foamboard, with holes and gaps in to allow for ventilation, which I placed over the sensor (instead of using the manufacturer's shield which seemed too enclosed).

 

Shade created by a tree isn't ideal because of  the cooling effect of vegetation - but if your garden has a lot of vegetation (like mine) then you can't  do much about it anyway. It's also preferable to using the shade of buildings which can have fairly significant effects on temperature readings depending on how much heat they're giving off.

 

So at least having some shade means readings can be comparable to official readings to a certain extent. This wouldn't be the case if you placed a thermometer in direct sunlight where the sun could increase readings by 10 - 15C or more.

Edited by h2005uk
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Just spotted this thread. I, too, have noticed that Writtle regularly achieves both the highest max and lowest min for the East of England in the same 24 hour period. I suppose it must just be down to its siting.

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Is there a Met Office guide on what is required for official stations? I had a quick look online but couldn't find anything. Would be interesting to see how standardised the official stations actually are.

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My garden has plenty of vegetation but the thermometer in the shade there still reads 1-2C higher than Shawbury 6 miles away on most summer days. Also it doesn't get as cold on many clear winter nights. In rain and wind or cloudy nights its very similar though.

Wish I could find records from Preston Montford to compare- that site seems quite a bit warmer than Shawbury from the limited data I've seen.

And theres the breeze- Shawbury often reports 10mph or more when theres hardly a breath of wind here. That seems to be part of the reason for its lower temps in summer. It is rural but I wouldn't call it exposed- if anything its sheltered by low hills to the N.

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I've just spotted a pattern which has emerged.

It seems my station is a little cooler than the official Chelmsford one when there are northerly winds. It's more in synch with the official one in southerly winds.

Could it be that southerly winds bring me some warmth from the city area of Chelmsford (which is several miles south of me)?

There's no urban area to the north of me for around 10 miles which may explain my cooler temps (compared to official station) in northerly winds.

Edited by h2005uk
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why are people measuring temp in the shade? its a bit misleading given most of outside is not in the shade.

This is because we're measuring the temperature of the air.

If you try to measure the temperature in the sun you actually measure the temperature of the instrument you're using to do the measuring, not that of the air.

As all objects have differing albedos ( the ability to reflect solar radiation) then any thermometer exposed to the sun will not give an indication of the temperature you actually feel as human skin has a different capacity to absorb solar radiation than a thermometer.

There are special Black bulb in vacuo thermometers ( a matt black thermometer in a matt black shield ) for trying to measure the

temperature in the sun but even these are only comparable with each other and tests have shown that there can be significant variation in the readings of two such thermometers exposed side by side.

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This is because we're measuring the temperature of the air.

If you try to measure the temperature in the sun you actually measure the temperature of the instrument you're using to do the measuring, not that of the air.

As all objects have differing albedos ( the ability to reflect solar radiation) then any thermometer exposed to the sun will not give an indication of the temperature you actually feel as human skin has a different capacity to absorb solar radiation than a thermometer.

There are special Black bulb in vacuo thermometers ( a matt black thermometer in a matt black shield ) for trying to measure the

temperature in the sun but even these are only comparable with each other and tests have shown that there can be significant variation in the readings of two such thermometers exposed side by side.

 

Yes precisely this, as early experiments showed the sun temperature can be similar regardless of altitude and bear no similarity to the actual air temperature, as in this description of the the 18th century hot-boxes of Horace de Saussure -

 

The hot box helped de Saussure ascertain why it is cooler in the mountains than in lower-lying regions. His hypothesis was that the same amount of sunlight strikes the mountains as the flat lands, but because the air in the mountains is more transparent it cannot trap as much solar heat. To test the theory, de Saussure carried a hot box to the top of Mt. Cramont in the Swiss Alps. The thermometer in the hot box hit 190º F, while the temperature outside was 43º F. The following day he descended to the Plains of Cournier, 4,852 feet below, and repeated the experiment. Although the air temperature was 34º F hotter than on the mountain, the temperature inside the hot box was almost the same as in the previous experiment.

 

http://solarcooking.org/saussure.htm

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This is because we're measuring the temperature of the air.

If you try to measure the temperature in the sun you actually measure the temperature of the instrument you're using to do the measuring, not that of the air.

As all objects have differing albedos ( the ability to reflect solar radiation) then any thermometer exposed to the sun will not give an indication of the temperature you actually feel as human skin has a different capacity to absorb solar radiation than a thermometer.

There are special Black bulb in vacuo thermometers ( a matt black thermometer in a matt black shield ) for trying to measure the

temperature in the sun but even these are only comparable with each other and tests have shown that there can be significant variation in the readings of two such thermometers exposed side by side.

 

 

excellent reply there TM

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It's interesting how a relatively small change in location can have such an impact on the weather.

 

Where is the best place for a thermometer sensor in a normal-sized garden?

 

Shade near buildings is inaccurate, plus more exposed areas seem to be inaccurate and have spikes due to the sun (even with a screen/cover over the sensor). I've always gone for shade produced by a tree - with reasonable airflow in the area - to avoid the effect of direct sunlight! I think that's true shade but maybe that's 'too much' shade?!

The best place is an open area in a screen painted gloss white or of shiny white plastic, preferably one with double louvres, and even better if it's aspirated.

Tests have shown that standard Stevenson Screens will give higher readings on calm, sunny days in summer than a similar aspirated screen but provided there is a reasonble air flow around the screen the readings should be acceptable and comparable with other local sites, given the usual caveats regarding the accuracy of the sensor you're using.

Siting the thermometer/sensor in the shade of a tree will tend to give lower readings in warm sunny weather due to the cooling effect of transpiration of water from the tree to the surrounding air. Conversely there will be higher readings at night, particularly in clear sky conditions, as the leaves and branches of the tree will interrupt outgoing radiation from the ground and therefore will slow down the cooling of the air; this will apply even in winter when the tree is bare of leaves.

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Thanks for the interesting reply!

Do you know over what area a tree can have an effect, or is the effect only experienced underneath the tree?

Also what sort of effect in degrees terms can trees have on the temperature?

Edited by h2005uk
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It would affect by several degrees depending on size of tree/ shaded area, etc.

Provided your a few metres from the nearest tree you should get reasonable figures on a clear calm night.

My cheaper WS2300 station is adjacent to a large hedge which partly overhangs to shade it, on a clear night it is only a degree warmer at most than my Davis Vue over the lawn. It can read similar to a degree cooler on still, hot days.

Edited by TonyH
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Thanks for the interesting reply!Do you know over what area a tree can have an effect, or is the effect only experienced underneath the tree?Also what sort of effect in degrees terms can trees have on the temperature?

The effect will be out as far as the ends of the branches but the effect will be less the further out from the trunk you go.

I don't have any hard scientific evidence as to the magnitude of the effect on temperature, and it will depend on the size and type of tree to some extent, but I would think that under a large and leafy tree maxima could be around 3c lower than out in the open on a sunny day with light winds.

The effect at night will be greatest on calm clear nights. Even a tree with no leaves, if it's a large tree, could result in minima being 3-5c higher than out in the open in the right conditions.

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