«© Andrew K. Overton 2009 Contents Introduction Siting and Exposure Temperature and relative humidity Precipitation Wind speed and direction ...»
A Guide to the Siting, Exposure and Calibration of
Automatic Weather Stations for Synoptic and
By Andrew K. Overton
© Andrew K. Overton 2009
Siting and Exposure
Temperature and relative humidity
Wind speed and direction
Sunshine, solar & UV radiation
Grass minimum and soil temperature
Wind speed and direction
Solar/UV radiation and sunshine
Cumulative Rounding Errors
Version 3.1 The copyright of this document rests with the author.
Introduction Automatic Weather Stations (AWS) are becoming increasingly popular in many applications owing to their becoming more affordable, reliable, having improved datalogging capabilities and through the growth in personal computing bringing sophisticated data manipulation and archiving within the reach of all. This guide is of relevance to all who manage AWS, but it is primarily aimed at the amateur observer who cannot be tied to a station at 0900 UT due to the requirements of employment and family life, and brings the opportunity to contribute data recorded between the standard climatological times. AWS have the potential to add enormously to the historical weather data in the UK by recording in fine detail meteorological parameters from a wide range and great number of sites. However, they also have the potential to set meteorology back many decades if poorly sited and poorly calibrated.
These two factors are well recognised as fundamental when dealing with traditional instruments but AWS bring new challenges. Although the principles of traditional instruments apply equally to AWS, often the practicalities brought on by the design of equipment mean that compromises are unavoidable and calibration of sensors more difficult. Too often the precision of AWS sensors is mistaken for apparent accuracy.
Accurate many of them have the potential to become, but only with careful exposure and calibration - very few are acceptably accurate 'out of the box'; competitive price restraints on manufacturers prevent the individual calibration to be found on professional grade instruments. However, with careful consideration of exposure prior to installation, and initial and ongoing calibration of sensors, many models of amateur quality AWS can give equipment many times their price a run for their money. Without this application, though, data cannot be relied upon - poor exposure often invalidates it and lack of calibration casts doubt on its accuracy.
Many papers and books have been produced which deal with the application of standard exposure and calibration techniques to AWS but these generally assume access to expensive professional facilities, sites and equipment. These are not available to the amateur, generally operating from a suburban back garden site with only the facilities and equipment affordable by the average person. What this guide sets out to do is to condense the techniques used by the professional and apply them to the amateur environment. Compromises in exposure are generally inevitable and calibration sometimes rough and ready but any effort is better than none at all. A cheap temperature sensor calibrated to no better than ±1 °C is a known quantity and its data can be used with confidence; an expensive one uncalibrated is almost useless. By making the best effort possible (and recording with the data the limitations of exposure and calibration) the amateur gives confidence to those using the data accumulated, who can allow for possible errors and apply corrections where appropriate. Buying an AWS and getting it to work is just the first step on the road to producing climatological and meteorological data which will hopefully be not only interesting in itself but also a future asset to the meteorological community. Siting, exposure and calibration are the essential next stages.
Siting and Exposure The issues of siting and exposure are concerned with ensuring that everyone is recording the same thing, the same way, with the same limitations to allow intercomparison of data. Without some minimum standards of site and exposure it is difficult, if not impossible, to compare one station's data with another as it is specific in nature to the station recording it and can only serve to show changes and variations there. Siting refers to the physical characteristics of the recording location whilst exposure refers to the deployment of the instruments in relation to the elements. When air temperatures are quoted, for example, these apply to measurements taken in accordance with agreed standards of siting and exposure. Should a station which is operating with instruments sited and exposed at variance with these standards attempt any direct comparison of data with another, this is likely to be a difficult exercise. Values of air temperature, in common with most other variables, show marked differences in value at different locations within even the same site, depending on the height at which the observations are taken and their proximity to other objects. Standard siting and exposure of instruments is therefore the bedrock of the entire observing process: neglect these matters and data become much less useful to the wider community. However, before observers with limited sites despair of producing useful data, it should be pointed out that where ideal positioning is not possible and compromise is inevitable, provided detailed notes are kept with the data to indicate the siting and exposure achieved (metadata) the data still has considerable value, as sometimes allowances can be made for limited exposure, etc..
The standard siting and exposures for the different types of recording instrument can be found in The Observer's Handbook, published by The Met Office. If you can meet the criteria within for your own equipment, congratulations! Yours will be very well exposed instruments with your data extremely valuable and you are unlikely to find much of value in the Siting and Exposure section of this guide. These standards are always the ideal to be sought and advice in this Guide is based on these. For those unable to meet the ideal, the following details should allow a good compromise siting. It is to be noted, though, that many AWS have sensors bundled together, e.g. raingauge integral with sensor screen, and the perfect siting and exposure being found for one of these components may mean that the other is poorly sited and/ or exposed. Often it is necessary to choose a location which is ideal for none of them but which provides the best overall compromise. It is sometimes better to have two components moderately well sited / exposed than to have one perfectly well and the other appallingly badly sited / exposed. However, when considering this option judgement must be exercised to ensure that the compromise does not result in the data from one variable being almost useless without considerable manipulation. For example, it would not be a good idea to mount an integral thermometer screen/ raingauge 50 cm above ground level in order to get the latter closer to the ideal height. This would result in recorded temperatures being impossible to compare with other sites without considerable corrections, which would at best only produce approximations. In this case, it is better to accept the undercatch on the raingauge as it will in any case provide a record of the actual occurrence of rain and times., and use another raingauge at a ‘standard’ height to provide the ‘climatological rainfall total’.
Temperature and Relative Humidity
Air temperature and humidity should be measured at between 1 and 1.5 metres (approximately 4 feet) above the ground in a shaded and ventilated environment; such an environment is usually provided by a whitepainted Stevenson Screen, which acts to keep the temperature sensor out of the warming effects of direct or indirect solar radiation, infrared radiation from all objects in sight, and rain but permits free air flow around the sensor. Ideally the temperature sensors should be positioned in a Stevenson Screen at this height but this is often impossible with makes of AWS which have the raingauge attached to the sensor screen. The problem with relying on the AWS screen is that there are varying patterns, each with their own exposure, and with widely varying responses to solar radiation, wind flow, rainfall etc. The Stevenson Screen has been the standard exposure in the UK and many other countries for over 120 years and, while not perfect in every respect, does provide a benchmark reference by which readings can be compared, not only between stations, but over a long period of climatological record. [Note: Internationally there are many variations in the design and deployment height of Stevenson Screens.] Extensive trials of AWS screens alongside Stevenson Screens to compare exposures have shown differences, and these are to be expected; slavishly attempting to copy one exposure and type of instrument (e.g. mercury-in-glass thermometers in a Stevenson Screen with a fastreacting thermistor in a small plastic screen of much lower thermal mass) will produce only confusion and frustration. It is more likely than not that smaller plastic screens containing thermistors will increasingly be accepted as the standard in the coming decade, but for now the wide variety of screens and systems and fragmentation of standards is more likely than not to introduce doubt about recorded values. During days of high insolation with little wind in summer the small plastic screens provided with AWS can cause an overreading of temperature of 1-2°C compared with Stevenson screens, although well-designed naturally aspirated screens, as well as fan aspirated screens, can greatly reduce this difference. Some screens can even be shown to be cooler than the Stevenson Screen under such conditions; any error in such conditions is likely to be above the true air temperature rather than below.
Ideally, the screen should be fixed in an open place with good airflow on a level surface above short grass at the standard height. It should be no nearer than 30 metres (100 feet) from extensive concrete, aggregate or a road surface. Measure the height of surrounding objects above screen level: the distance from the screen to these objects should be at least two times these heights.
These are the ideals but what of the amateur siting his equipment in a suburban garden? There may be trees in the garden and if it is possible to move the screen far enough away from them it approaches too closely to the walls of the house, garden fence or hedging, or even off the lawn and into the herbaceous border. This is a typical problem and no hard and fast rules can be laid down but an assessment of the various obstructions is necessary to enable a decision to be made over which object will be least detrimental to approach too closely.
For example, moving the screen too close to a south facing brick wall to get it away from a north facing privet would not be the correct compromise due to the greater heat radiated by the wall as opposed to the reduced airflow and increased transpiration near the hedge.
From the above it should be obvious that the siting of screens and sensors on the eaves of the house is totally unacceptable as it will give largely meaningless results, except in windy weather; as is fixing it to the post of a wooden-panelled garden fence, although all of these transgressions have been observed.
Grass minimum temperatures should be measured, as implied, above a grass surface, not soil or other ground cover, unless this is more representative of the locality. Sensors used to measure grass minimum temperature should be of an appropriate design with minimum thermal bulk and good weather-resistance, as these sensors will experience a very wide range of temperatures over a full year and will also be exposed to rainfall, snowfall etc; not many battery-powered wireless sensors will operate reliably for long under such conditions.
The same conditions regarding surrounding objects and siting in respect of screens apply equally to grass minimum temperature sensors, and any overhead or near-overhead obstructions, no matter how apparently insignificant, such as a nearby tall hedge or tree branches, can affect the readings. The grass should be kept short and the temperature sensor fixed so as to be level with the tips of the blades. During periods of lying snow it will be necessary to raise the sensor so as to have it positioned on the snow surface.
Soil temperatures (down to 20-25 cm depth) should be measured in an area of open soil representative of that naturally occurring at the site. Earth temperatures (at 30 cm or below) should be measured beneath a ground cover of short grass. The temperature sensors should be positioned at the required depths and once again should be suitable for the environment in which they are placed; sealed, cabled sensors suitable for burial are essential for an environment which may be waterlogged for several months of the year.
Ideally, the rim of the raingauge should be sited at 30 cm above ground level. Precipitation catch varies with height due to turbulent eddies and there is considerable reduction in catch as wind speed increases above the ground. Severely elevated mountings are also objectionable due to the difficulties with maintenance.
Gauges need regular cleaning to remove silt deposits from tipping buckets and bird droppings. The higher the gauge is mounted, the more attractive it becomes as a perch and the more cleaning required. Surrounding objects also affect airflow and provide a sheltering effect. To mitigate this measure the height above gauge level of surrounding objects: the gauge should be distant at least twice their height above the rim of the gauge and ideally four times. In sheltered sites beware also of splash into the gauge off surrounding surfaces. With some small gauges, often no more than 7-10cm (3-4 inches) tall, ground mounting may cause considerable insplash in heavy rainfall and better results will often be obtained by raising 30 cm (12 inches) or so off the ground.
However, many raingauges are integral with the temerature screen and siting at ground level is not an option: