Author Topic: Logic for weather station siting recommendations  (Read 364 times)

0 Members and 1 Guest are viewing this topic.

Offline hankster

  • Forecaster
  • *****
  • Posts: 399
    • NFM Weather
Logic for weather station siting recommendations
« on: May 18, 2017, 05:27:36 PM »
I know that it is "recommended" that the temp/hum sensor be 5' above the ground above grass and the anemometer be 33" above the ground, but why were these distances chosen?

 If the anemometer is 33' above the ground, why shouldn't the temp/hum sensor be the same? As it is, being only 5' above the ground means the ground can have a fairly big effect on the readings. Wet grass or snow can keep things cooler than surrounding temps/hum and dry ground can make readings high.

What got me thinking about this was it has been very dry here lately with severe drought conditions, the grass might as well be dirt covered ground. I had to take my Davis Vue ISS down for cleaning and found the readings went up 2 degrees only to return to normal when it was put back up 20+ feet.

IMHO, putting the temp/hum sensors at the same height as the anemometer would eliminate these false readings.

I know some will say that this is the temperature at the ground that you feel. If that is what you wanted then you should mount the anemometer 5' above the ground so you get the wind that you feel.

Just curious and figured it would be a good discussion :)

Offline CW2274

  • Forecaster
  • *****
  • Posts: 2870
    • Conditions @ CW2274
Re: Logic for weather station siting recommendations
« Reply #1 on: May 18, 2017, 05:43:49 PM »
I know some will say that this is the temperature at the ground that you feel. If that is what you wanted then you should mount the anemometer 5' above the ground so you get the wind that you feel.
Well, we do walk on the ground, but to have an anny at 5' would be far too inconsistent unless it's in a completely open area, and dubious even then. Why 33' for the anny? To get it away from possible said ground effect, and probably because commercial/military aircraft control surfaces are generally in that static range of height.
Davis Wireless VP2 SHT31 24hr 67CFM FARS
RW Tipper w/ CoCoRaHS

Offline hankster

  • Forecaster
  • *****
  • Posts: 399
    • NFM Weather
Re: Logic for weather station siting recommendations
« Reply #2 on: May 18, 2017, 06:03:58 PM »
Any proof of those reasons and dates for when they were proposed? I was in the service and other than larger aircraft the vast majority of military aircraft wing surfaces are at 10' or less.

Actually the spec for temperature sensor placement says it should be 100' from any paved or concrete surface and 4X the height from any solid vertical surface. If that is followed for placement of the anemometer than there would be minimal effect from the surroundings. As you said, we do walk on the ground, not 33' above the ground... well, some people think they do :)

Offline CW2274

  • Forecaster
  • *****
  • Posts: 2870
    • Conditions @ CW2274
Re: Logic for weather station siting recommendations
« Reply #3 on: May 18, 2017, 06:41:07 PM »
Any proof of those reasons and dates for when they were proposed? I was in the service and other than larger aircraft the vast majority of military aircraft wing surfaces are at 10' or less.
Proof? No. Common sense? Yes. As far as control surface height, yes many a/c are small, but many are not. For instance, the height of the horizontal stabilizer on a C5 is 65'. When they'd taxi by the control tower I worked in the Navy, it's tail was literally as high as me (very old tower and very short). Grated the extreme, but their are thousands of a/c (including just about every commercial jet in the world) whose control surfaces would not really benefit as much from the wind reading at 5', but the little guy sure can use it at 33'.
Davis Wireless VP2 SHT31 24hr 67CFM FARS
RW Tipper w/ CoCoRaHS

Offline openvista

  • Senior Member
  • **
  • Posts: 65
    • marquetteweather.com
Re: Logic for weather station siting recommendations
« Reply #4 on: May 18, 2017, 06:55:42 PM »
Wind chill is dependent on a 10m placement. The NWS formula multiplies the wind by 2/3rds to get the ground level wind (see: http://meteo.lcd.lu/papers/windchill/newwindchill.html). If you place it lower, your wind chill values will be higher.

Also one should consider thermal inversions on windless, clear nights. The outgoing long-wave radiation from the Earth's surface cools the immediate "boundary" layer closest to the ground and causes it to de-couple from the air above. The higher you go, the warmer it gets.

That being said, in urban areas, it has been argued that height matters less for measuring temperature than in rural areas, but beware of rooftops. Here's a quote from INITIAL GUIDANCE TO OBTAIN REPRESENTATIVE METEOROLOGICAL OBSERVATIONS AT URBAN SITES by Tim R. Oke (published by the World Meteorological Organization).

(For reference "UCL" is the Urban Canopy Layer which is approximately the level beneath all rooftops and other obstructions in your vicinity)

Quote
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 [urban] canyons show very slight air temperature gradients through most of the UCL, 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. Air temperatures measured above the UCL, using sensors mounted on a tower, are influenced by air exchanged with the UCL plus the effects of the roofs. Roofs are much more variable thermally than most surfaces within the UCL. Most roofs are designed to insulate and hence to minimize heat exchange with the interior of the building. As a result roof surface temperatures often become very hot by day whereas the partially shaded and better conducting canyon walls and floor are cooler. At night circumstances are reversed with the roofs being relatively cold and canyon surfaces warmer as they release their daytime heat uptake. There may also be complications due to release of heat from roof exhaust vents. Therefore, whereas there is little variation of temperature with height in the UCL, there is a discontinuity near roof-level both horizontally and vertically.
Davis Vantage Pro2 with 24hr FARS | https://marquetteweather.com | KMIMARQU13 on wunderground.com | CWOP ID: EW7933

Offline Jáchym

  • Meteotemplate Developer
  • Forecaster
  • *****
  • Posts: 8407
    • Meteotemplate
Re: Logic for weather station siting recommendations
« Reply #5 on: May 18, 2017, 07:03:02 PM »
Temperature

Here we are clearly interested in the temperature at the height we live in. Placing it too close to the ground would create a significant bias. 2m is about the max that is reachable for a human without a ladder or some stairs.
You are right in that the surface can have an effect, but the surface below the station is also described in quite some detail so at a professional station this will not be a problem.

Absolute ideal situation (usually the case of pro-stations at airports)

The site is covered with short grass (height ≥ 4cm and ≤ 10cm). The area will need to be mown at least once a week in the period from April to September, meaning it will be mown about 28 times in the grass growing season. No crops or plants exceeding 0.5 m in height may be grown or placed within a radius of 25 metres around the observation site. No crops or plants exceeding 1.5 m in height may be grown or placed within a radius of 50 metres around the observation site. No obstacles such as trees and shrubs may be placed within a radius of 100 metres around the observation site. No obstacles such as sheds or other buildings and woodland may be placed within a radius of 400 metres around the observation site. 

The sensors for measuring temperature should according to the WMO be situated at a height of between 1.25 and 2.00 metres above flat ground. The terrain above which the measurements are being made should be covered with short mown grass; any snow that may be present need not be removed. The measuring element may not be significantly affected by sunlight, radiation and meteorological phenomena such as precipitation, dew, frost and wind. For this reason, the measuring element is placed in a so-called “dish screen” (shaped like an inverted dish) that protects it against radiation. This casing is white on the outside and black on the inside, to minimize the influence of radiation. The space between the dishes is chosen so that sunlight and radiant heat have no effect on the measuring element and also so that the space inside the screen is suitably naturally ventilated.

Wind

Theoretically yes, you could measure it close to the ground, however this would require an absolutely open space with no obstacles and nothing that could create a bias on the actual ground. This in practice is close to impossible. For this reason it is measured at 10m height where it is not affected so much by obstacles and turbulences near the ground. Also for the planes at the airport the 10m height is more relevant.

The sensors for measurement of wind speed and direction are mounted on a stable metal or plastic mast. The sensor height is 10 metres above terrain that should in principle be flat. The distance from the wind mast to any obstacles in the vicinity must be at least ten times and preferably twenty times the height of the obstacle (applies to all obstacles).
The terrain in the immediate vicinity of the wind mast (radius ≥ 100 metres around the measurement site) is flat grassland or a water surface. The wind observation at an airport must be representative for the wind conditions on the (adjacent) runways for take-off or landing, and in particular for the touchdown zone.

Conclusion:
Most of us will never have a perfect station so no need to worry about it too much ;)

Offline CW2274

  • Forecaster
  • *****
  • Posts: 2870
    • Conditions @ CW2274
Re: Logic for weather station siting recommendations
« Reply #6 on: May 18, 2017, 07:24:52 PM »
The wind observation at an airport must be representative for the wind conditions on the (adjacent) runways for take-off or landing, and in particular for the touchdown zone.
Just about all commercial airports now use sensors at the approach end of all runways. This allows for the detection of low level wind shear with LLWAS, something that became tragically apparent in DFW in 1985 with the micro burst caused crash of DAL191.
Davis Wireless VP2 SHT31 24hr 67CFM FARS
RW Tipper w/ CoCoRaHS

Offline hankster

  • Forecaster
  • *****
  • Posts: 399
    • NFM Weather
Re: Logic for weather station siting recommendations
« Reply #7 on: May 20, 2017, 10:20:40 AM »
My theory is much more simple. My thoughts are these specs were made up maybe a century ago. A time where temperature readings had to be done manually and the only way to do that was have the thermometers close to the ground.

Over time the specs were massaged to make them more accurate. It's been done that way for so long that people will find any reason to defend those specs. Let's remember they have had a century to pound into everyone that is the one and only true way to take temperature and wind readings with 1000's of papers and years of instruction to program everyone. So much so that no one will even question it. Now even with remote sensors available it is not practical to radically change the spec as it would be difficult, if not impossible, to correlate the new data to the old.

So why cause the confusion with a radical change and bring out even more conspiracy theorists when it's good enough and everyone has come to accept it.

Offline openvista

  • Senior Member
  • **
  • Posts: 65
    • marquetteweather.com
Re: Logic for weather station siting recommendations
« Reply #8 on: May 20, 2017, 11:48:08 AM »
It's been done that way for so long that people will find any reason to defend those specs. Let's remember they have had a century to pound into everyone that is the one and only true way to take temperature and wind readings with 1000's of papers and years of instruction to program everyone.

There's ample research, for example, behind optimal wind measurement because of the exploding wind energy and air pollution control industries. The engineers who perform surveys know where "representative" wind layers occur and how to access them. The numbers aren't simply invented nor do they rely on "custom". Lots of investment capital and legal liability at stake.

Bottom line: if you place your anemometer beneath the recommended height for your particular environment or application, you will get erroneous speeds, directions and derived values such as wind chill. If you place it too near a roof or other surface, same result. These are facts, not conjecture.

It's not accurate to say that the siting standards are a century old. For one, the World Meteorological Organization (which determines the siting guidelines for most of the world), didn't exist then. They have contributed significant original research on temperature and wind measurement, among many other subjects since their inception in 1950. A couple of us have already provided snippets of their work which, apparently, you dismissed or ignored to reach the above conclusion. There's much more available if you care to look.

To preserve continuity with historical records there must be some effort to adhere to historical norms. Even now there are criticisms of the differences in how temperature, for example, was recorded a century ago vs now. The instruments, siting, frequency & surrounding environments can be quite different. If official bodies start moving the temperature sensors to whatever height they wish, it will only make it more difficult to establish when records are set or whether "normals" have changed.

As mentioned earlier, everyone's situation is different.  Everyone's requirements for accuracy are different. You don't need to justify your placement to anyone. However, you should understand that there are legitimate reasons for siting guidelines, particularly for official stations, even if those recommendations don't apply to your particular situation or you cannot meet them for whatever reason.
« Last Edit: May 20, 2017, 04:46:38 PM by openvista »
Davis Vantage Pro2 with 24hr FARS | https://marquetteweather.com | KMIMARQU13 on wunderground.com | CWOP ID: EW7933