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So you have your new weather station, or are thinking of getting one, the one question we at Mendip Weather always get asked the most is where is the best location to set up the sensors to get the most accurate readings?
This is certainly not always a straight forward and easy question to answer as every customer’s location is unique and it is about assessing the optimum positions for your new weather station sensors to achieve a number of objectives. Which ever location you eventually choose it will always be subject to some limitations whether this be from an overexposed rain gauge, an anemometer which is over shadowed by a nearby tree or a position makes servicing the sensors nearly impossible. There are two principles for collecting good weather data: number one, ensuring that the quality of your readings is as good as possible, mainly by siting your weather sensors with due care; secondly, by having an awareness of the limitations of your data.
Any Meteorological data is scientific data and, if you're serious about collecting good quality weather data of, then your automatic weather stations (AWS) needs to conform, as close as possible to procedures and practices of making good scientific measurements. This isn't normally difficult, but does mean that you shouldn't simply accept the readings of your new weather station at face value.
Optimal Sensor Location.
Highly accurate weather data can only be collected if the weather station sensors are sited and installed with care and consideration. Mounting the sensors of your new AWS at the most convenient location will make for easy servicing but almost certainly lead to significant errors in the data.
Temperature & Humidity
The international standard height for measurements of air temperature is at 1250 mm or roughly four feet above ground level. Temperature can vary by a surprisingly large amount with height above the ground under certain weather conditions, and it is essential that the 1250 mm sensor height is used for comparability with official figures. The sensor must also be protected from both direct sunlight and rainfall if erroneous temperature values are to be avoided, which is usually achieved by placing the sensor inside a Stephenson Screen, a white louvred housing away from any nearby sources of heat such as buildings. The position also needs to have free air circulation, and over a natural surface such as grass, surfaces such as concrete will cause significant errors. As with all sensors, the more care taken over sighting of the sensors the more accurate your observations will be.
Wind speed and direction
Near to the ground wind speed and direction are affected by all physical obstructions, even a flat, unobstructed location, wind speed is markedly reduced by the frictional effect of the ground surface. It is for this reason that the official sensor height for recording wind speed is 10 m above ground level in a clear unobstructed location. In practice, it is almost impossible to achieve anything close to optimum exposure of the wind sensor. The recommendation is therefore to place the anemometer as high as possible given your local circumstances and to accept that the readings will almost certainly be significantly lower than official observations at a similar location. This is easily achieved with the Davis Vantage Pro 2 which has an anemometer which is mountable some distance from the main sensor suit. Often the exposure will be worse from a certain direction and the obvious advice is to aim for maximum exposure of the anemometer to the commonest wind directions.
Note that height above obstructions an anemometer placed just at roof level on a house will often misread because of wind turbulence around the roof structure. The sensor should be placed as high above the roof structure as is safely and economically possible ideally 2-3 meters. No matter how over shadowed your eventually sensor is your reads are valuable as part of the continuing record of weather at that site, but will not be directly comparable with official records of for example maximum wind speed and gusts during a gale.
Rain rarely falls vertically, but is usually blown to some degree by the wind, for accurate measurements it's therefore important that the rain gauge is located in an open area such as a large lawn. As with wind, shadow effects can be surprisingly large but try to ensure that there are no significant obstruction in the direction of the prevailing wind. The recommendation is that the gauge be positioned at a distance corresponding to two to four times the height of any nearby obstruction and away from anything such as vegetation which may cause air turbulence and consequent non-uniform deposition of rain droplets. For Davis Vantage Pro stations the rain gauge rim is automatically at 1400 mm if the temperature sensor is mounted at its official height of 1250 mm.
Ideal field location for a raingauge
Since barometric pressure doesn't vary across a local area at uniform altitude, there are no major concerns about sighting the pressure sensor. However atmospheric pressure reduces by about 1 Mb for every additional 10 meters of elevation, it is therefore essential to know your site altitude accurately.
For more detailed information please visit the RoyalMeteorological Society website , the UK Met Office website and also the faq pages for the uk.sci.weathernewsgroup (see the 'Instrument Siting' section under the 'Observing and Reporting' topic).
All serious commercially available AWS systems such as the Davis Vantage Pro 2 and Vue have published specifications which will show an estimated degree of accuracy for each sensor. It's good practice to look at these specification tables to get an idea of how reliable your data is likely to be.
Although just because your station can display a rainfall value of, of up to two decimal places, doesn't mean that it is accurate to that degree. It should be possible to set up most weather stations to measure weather parameters to 5-10% accuracy, but achieving higher accuracy is difficult.