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Instrument siting

The notes below have been written using replies to a survey in the newsgroup and using information extracted from the UK Met.Office handbooks dealing with instruments, and their siting, which in turn are based on internationally agreed standards published by the World Meteorological Organisation (WMO). The notes are only concerned with siting & exposure, not the instruments themselves, or the construction of the screen or a shield in the case of temperature measurements. For advice on these, contact a reputable supplier, or refer to the Observer's Handbook.The BBC Weather Centre web site also has some basic information regarding weather observations etc., and a visit to them would be useful.

Standards are set for a reason: data from many different sites around the world need to be compared one with another, in the knowledge that, as far as possible, the instruments used are exposed in the same way and subject to the same errors. This requirement is particularly important when trying to determine long-term trends in meteorological parameters, both using mean values, and with regard to extremes.

However, even when the WMO recommended standards cannot be met, it is natural that people will want to install weather monitoring kit to enhance their interest in the subject. This note therefore attempts to advise on what is, and what is not possible. At the end of the day it is for the individual user to determine whether the expense involved is worth the outcome. One thing must be made clear though: throwing money into expensive equipment will not improve the exposure!


Basic requirement: For synoptic and climatological meteorology, the temperature required is a representative one of the 'free air' conditions over as wide an area surrounding the observing point as possible, with an internationally agreed height (for the thermometer bulbs, sensors etc.) of 1.25 m above local ground level. A fixed height must be specified, because vertical temperature gradients can be intense: for example on a clear, calm night or around the middle of the day with strong solar heating.
The best site for a screen, or thermometer shield for a land station is therefore over level ground, freely exposed to the sun and wind, but not sheltered by buildings, trees, bushes etc. The temperature sensor must be shielded from direct sunshine (hence a screen or shield) and precipitation (or a dry bulb becomes a wet bulb), and there must be a good circulation of air around the bulb/sensor head. If you have a garden, then the 1.25m above ground level can usually be met with ease. What it usually problematic is gaining sufficient clearance from adjacent buildings, trees etc.

The screen/shield should be positioned over grass (or less preferably, but still acceptable, loose soil), but not compacted soil, tarmac or concrete, as these media absorb and radiate solar energy strongly, and affect the readings quite significantly.

If the garden is not suitable, or you have no garden, then consideration may be given to mounting a screen on a north facing wall. There is a problem in this case with possible contamination from heat energy emitted by the building itself. A practical compromise would be to use such a wall, but carry the screen/shield away from the wall on a bracket - this would allow a free airflow around the equipment. A distance of 20-25 cm for an unshielded sensor has been suggested, and this would certainly minimise any contamination from the walls. For a shielded (or screened) sensor, then 10 cm or so has been suggested as a useful distance. Even a north wall mounting needs watching around the summer solstice, particularly at more northern latitudes, as care needs to be taken to shield the thermometers/sensors from early morning and late evening sunshine with an unobstructed horizon to the northeast or northwest.

The roof is not considered suitable. Not only is the construction of such similar to a solid surface (e.g. tarmac or concrete), and therefore subject to the errors noted above, but a roof is obviously more than 1.25 m above ground level. However, it is worth noting that many of our current crop of weather centres, with London being a notable example, have for many years mounted thermometer screens at a considerable elevation above local ground/street level. If a roof location is all you have, use it, but bear in mind the limitations.


Basic requirement: Rainfall amounts are quoted as a depth of water that would result in any one location on a flat surface after a fall of rain, if there were no run-off, evaporation or percolation. The depth measured in a gauge is assumed to be representative over an area around the gauge, so it is necessary to eliminate as far as possible any local sources of error.

There are many sources of error in rainfall assessment: evaporation, adhesion (sticking of the droplets to the side of the gauge), splash etc., but by far the greatest source of error is due to inadequate exposure. The 'ideal' location is one where objects which might disturb the airflow are some considerable distance away from the gauge, i.e. they are so far away that any perturbations of the wind-flow are so small as to be part of the general 'ground-effect' turbulent flow always present as air passes over the earth's surface.

The recommended standard is that the distance from surrounding objects should be not less than twice the height of such objects, and ideally at least four times. In most suburban gardens, even if the fences are low enough to just about site a gauge to these standards, surrounding trees, neighbour's bushes, and of course, the house (and adjacent buildings) usually are the largest objects, and cannot be realistically circumvented. Most hobby observers cannot meet the latter (4 times) requirement, but something approaching the twice-times-height standard is often attainable.

Official texts completely rule out mounting on a wall or roof (apex or flat), as these features cause marked eddies of wind which grossly distort the passage of falling rain across the mouth of the gauge. However, a flat roof might be a best approximation, if there are no adjacent buildings within the '2 x object height' footprint mentioned above.

The middle of a lawn is all that most of us have .. and provided that it is not grossly shaded, would present a reasonable guide, but unless the wind is very light, under-reading of rainfall (with respect to the 'standard' sites) must be expected. With time, it is usually possible to judge where in a garden is unduly sheltered, and careful note made to avoid these locations. Remember that the 'shadow' changes with wind direction. Try setting up identical collecting receptacles (e.g. the bottom half of plastic lemonade bottles) and note the variation in catch over a period of several months. Another method is to note at the start of any rainfall event which areas become wet first and which stay dry longest. Or perhaps a matrix of collectors, and take an average!

At the end of the day of course, you are measuring rainfall that is of significance to you. Indeed, in extreme rainfall events (such as notable local storms), any measurements are better than none; adjustments and allowances can be made for exposure, and even an 'official' gauge under extreme conditions has difficulty in capturing a 'true' measure of the event. The rain/snow that falls is the amount of rain (to a reasonable approximation) that has fallen in your garden, on your roof or whatever, and as such it is a meaningful record. For this reason alone, it is worth attempting such measurements - the only suggestion is that you don't spend huge amounts of money doing it!


Basic requirement: The wind speed and direction (see ** below) in the first 30 metres or so of the atmosphere varies rapidly with height, due to the varying frictional effect of the general 'surface roughness'. It is greatly affected by undulating ground, and by adjacent obstacles such as trees, bushes, buildings etc. This is a common experience - noted for example within built up areas, major shopping centres etc.

For synoptic and climatological work therefore, a 'standard' exposure is required. That standard is for the wind speed and direction over a level surface to be measured at a height of 10 m above ground level (agl). When these conditions cannot be met, it is permissible to raise the anemometer to give an effective height of 10 m, provided the obstructions are not large, and are distributed uniformly around the instrument site.

It will be immediately apparent, that in the common 'back-garden'/urban development situation, a considerable mast is needed to carry the anemometer clear of these 'ground effect' generating obstacles. For example, consider an outer-suburban garden with houses/trees of approx. height 6m in height, the recommended exposure height would be 6 m (obstruction) + 10 m (standard height)=16 metres. (That's around 50 feet!) This provides problems in maintenance of the sensor, and also there would possibly be planning and structural constraints. To be stable, such a structure would need to be well braced which will not be easy. When considering larger obstructions, such as large blocks of flats, or office blocks, then the sensors would need to be raised even more. An example: For an obstruction of some 15 m in height (a typical large building), which is about 75 m from the site of the intended anemometer site, then the wind vane/cups would need to be about 25 m above ground level.

If such conditions are beyond the scope of your pocket or what the neighbours will allow, then the best compromise would be a fitting a short height above the ridge of a house, provided always that adjacent buildings do not unduly affect the airflow at the sensor level.

[** Wind direction, when quoted in standard meteorological reports such as SYNOP and METAR, are given in terms of deviation from TRUE north (degT). This should be remembered when setting up a weather station that includes an anemometer: align the directional head such that the read-out gives degT - to do this you will need to allow for the local magnetic deviation of your compass from true north: many websites are available that will help with this subject.]

As will be appreciated from the above, the best advice we can give when 'standard' conditions cannot be met is to think seriously whether its worth the cost and effort. By all means mount a relatively inexpensive anemometer just above the roof level of your house etc., but treat this simply as a monitor of the conditions for your site. The reading you get will not be of use to compare with adjacent 'standard' instrumentation, or even with someone a few streets away with similar problems. However, it is a record at the point you have installed the anemometer head, and as such does provide interest. You will find though that the poorer the exposure, the greater the variability in wind direction.

And finally ... it is pertinent to note that there are occasions when limitations of exposure are a positive advantage. For example, a useful field study for students is to set up a series of temperature recording devices within a stand of trees, both horizontally and vertically. Readings from such an array would obviously be used for the study of the heat/humidity budget of the wood and any need to 'standardise' as above is not a factor, apart from ensuring of course that the thermometers or other devices are correctly calibrated and 'zero-referenced' for the range of values required. And there are specialised applications where the sensor site must depart from the WMO standards: for example, temperature and wind sensors set adjacent to a major motorway route are there precisely to monitor the disturbed airflow and heat characteristics consequent upon heavy traffic flow passing a couple of metres away. For these specialised applications, advice should be sought from the manufacturers of the equipment and other relevant authorities.