Screen Heights

[Richard]

A resent post i made in the 'Frost 2015' thread i mentioned what i believed to be not only a poorly sited NZ Transport Compact Weather Station site but i also mentioned its 3m+ sensor/screen height.
Saturday afternoon while my son's under 18's were warming up i drove out to the MS Culverden site and had a look at its set up there and just as i thought would be the case the screen sits at the top of a approximately 7m high steel pole, the nearest building is 100m away and there are no tall trees within 300m, its in open grassland other than a shingle driveway that passes close by.
According to WMO this does not come close to meeting the recommended specified height and there for is not providing accurate climate data.

Question now is how many other sites around NZ dont meet WMO specification?



World Meteorological Organization - Instruments & Observing Methods

3.2.1 Air temperature
…At non-urban stations the screen height is recommended to be between 1.25 and 2 m above ground level. Whilst this is also acceptable for urban sites it may be better to relax this requirement to allow greater heights. This should not lead to significant error in most cases, especially in densely built-up areas, because observations in canyons show very slight air temperature gradients through most of the UCL, as long as location is more than 1 m from a surface...

[Nev]

7m does sound usually high for a temp sensor. Wouldn't mind seeing a photo.
Standard anemometer height is about 10m.

However, I do believe NIWA have one of their CWS's mounted on a pole in Alexandra township, although that's more likely as a backup to their obs taken manually less than 1 km across town. And of course MS also have their airport AWS.

Btw, this photo is what NIWA refer to as their 'CWS' (Compact Weather Station), possibly with a the anemometer separate, while the standard modern WS typically monitored by NIWA is called an EWS (Electronic Weather Station), and an AWS (Automatic Weather Station) if monitored by MS.

[Richard]

I should have taken the camera,i'll get a photo next time i head to Culverden

[TonyT]

Btw, this photo is what NIWA refer to as their 'CWS' (Compact Weather Station), possibly with a the anemometer separate

The anemometer is the three little sticks on top of the temp screen, its a sonic. Looks like a Vaisala WX520 type sensor which is very widely used under a number of different brand names world wide.

[Nev]

Yes, a 'Vaisala WXT520'. Didn't know about the anemometer though…

[Nev]

Thought I'd repost that WHO quote and include a comment that followed, which is more relevant to this discussion (along with a link if anyone wants to read more )…

WMO - INSTRUMENTS AND OBSERVING METHODS (2006)

3.2.1 Air temperature (pg16)

...At non-urban stations the screen height is recommended to be between 1.25 and 2 m above ground level. Whilst this is also acceptable for urban sites it may be better to relax this requirement to allow greater heights. This should not lead to significant error in most cases, especially in densely built-up areas, because observations in canyons show very slight air temperature gradients through most of the UCL [Urban Canopy Layer], as long as location is more than 1 m from a surface (Nakamura and Oke, 1988).

Measurements at heights of 3 or 5 m are little different from those at the standard height, have slightly greater source areas and place the sensor beyond the easy reach of damage or the path of vehicles. It also ensures greater dilution of vehicle exhaust heat and reduces contamination from dust. ...

Under normal calm conditions I think the greatest air-temp gradient would be in the first half-metre or so, but the gradient between about 1.5m and 5m would be fairly insignificant. Urban environments are a little more complicated, as there is usually a second layer of surface/air-temp mixing occurring at about just above roof-height, which should be avoided.

[Tim S]

Btw, this photo is what NIWA refer to as their 'CWS' (Compact Weather Station), possibly with a the anemometer separate

The anemometer is the three little sticks on top of the temp screen, its a sonic. Looks like a Vaisala WX520 type sensor which is very widely used under a number of different brand names world wide.

That's quite a sloping site to have a weather station, wont be a very good indication of the immediate areas climatic conditions, especially if it is north facing slope??

[Tornado Tim]

Viasala WXT520 stations cannot be WMO certified, as they are an all in one Weather Stations IE: all the weather variables are sampled at one height.

The Same goes for Davis Vantage Vue Weather stations or any other weather station that is a "all in one" and cant be tethered.

I suspect since that the particular site in question is mounted at 7 meters, is due to Niwa wanting the anemometer get a bit more wind exposure.
However there is that Temp/Hum tradeoff as its significantly elevated off the ground, it doesn't reflect the temp and hum nearest the ground.

Most of the Weather Stations I have seen from Metservice (ie: Ministry of Transport sites) are not all in one weather stations.
But they are mounted on a thick concrete base!

[Nev]

I suspect since that the particular site in question is mounted at 7 meters, is due to Niwa wanting the anemometer get a bit more wind exposure.
However there is that Temp/Hum tradeoff as its significantly elevated off the ground, it doesn't reflect the temp and hum nearest the ground.

Yes, but it would seem that there isn't a significant difference between temps at 'standard height' and those at 3 or 5m. Also, I think Richard was referring to MS's Culverden Aero sensors being 7m high.

[Richard]

Here's two photos as promised.

Looking again at the height it more like 6.5m tall

[TonyT]

That is also a WXT520 sensor, with the optional anti-bird fitting. Is it possible there is another temperature sensor inside the electronics box lower down on the mast?

[Richard]

That is also a WXT520 sensor, with the optional anti-bird fitting. Is it possible there is another temperature sensor inside the electronics box lower down on the mast?

Dont know Tony, didnt notice any louvers on the sides of the box though

[TonyT]

Probably not then. Interesting that they set up a separate rain gauge but dont set up a separate temp sensor! The WXT520 head can sense rainfall (acoustically) so they dont need another rain gauge but they put one in anyway, so there must be a data logger in the white box which can interface with more than just the WXT520, yet they dont put a temp sensor at the standard height. Odd really.

[Richard]

Spoke to Steve Knowles who's Network Operations Manager at the MetService, he said that the Culverden site is what they call a ‘full in' recording station and that recording the wind speeds had priority over temperature, unfortunately it all about cost and and its not cheap setting up a fully automated system thats meets WMO specification.

[Manukau heads obs]

it is a shame the WXT510 type station does not have a separate temp sensor

[TonyT]

it is a shame the WXT510 type station does not have a separate temp sensor

But if you look at the photo they have set up a separate rain gauge, so there must be some some datalogging function inside the electronics box, which probably means adding another temp sensor would be both trivial and inexpensive.

[Richard]

Discussions with Steve Knowles has raised the possibility of looking to install and maintaining an additional sensor in conjunction the local council as they have done with other councils around NZ.I'm taking this issue to the major of Hurunui on Wednesday where i'll point out to him that the district only has one AWS that meets WMO standards and as far as i know its the one in Hanmer Springs.