Setting Up Home Weather Station Instruments – How To Find The Best Weather Station Locations

When setting up home weather station instruments, the most critical part is location, location, location.

Because one of the most important determinants of how accurate the readings will be from your weather station sensors is where you choose to site them.

A wind speed sensor (anemometer) will only measure the wind speed where it is located, if you place it in a sheltered spot then you will get a sheltered reading. An obvious point perhaps, but one that can’t be overstated in importance.

The purpose of this short article is to provide guidance on critical factors that should be taken into account when selecting suitable locations for weather monitoring equipment.

Unless you are very lucky you will find that you will have to make some compromises on the ideal location (the perfect location is seldom physically practical) but this guidance will hopefully still be useful in helping to choose the best available sites that you can. Preferably sites that allow accurate measurement and easy access for maintenance too.

In addition to my own personal experience, I have also used official sensor location guidelines. Where quoted, (in bold italics), recommendations by the World Meteorological Organisation (WMO) are taken from: Guide to Meteorological Instruments and Methods of Observation. Seventh Edition (WMO-No.8).

Home Weather Station Air Temperature Sensor and Humidity Sensors:

These sensors are usually sited together and must be installed in a shaded location. Ideally they should be placed in an enclosure designed to provide shade but also to allow airflow around the sensor. There are two types of these enclosures in common use, the Stevenson Screen and the Radiation Shield (the Radiation referred to in this case is of course solar).

The screen illustrated here is the type and size that would be used at an official weather reporting site, for the typical home it would be much too large. Cut down versions are readily available. The sides of the enclosure are made up of two sets of opposed louvers (i.e. 2 sets of slats aligned in opposition to each other) which allow the ambient air to circulate freely within the enclosure but prevent rapid movement of air over the sensors inside even in strong winds. The enclosure is painted white to reflect the sun’s rays.

The wooden Stevenson Screens are very attractive and are still used by official weather organizations because of their stable thermal properties (even on a sunny day the heating and cooling of a wooden screen is very small compared to other materials, this is of course vital when you are measuring air temperature). The downside of the wooden screen is maintenance. Within a few years these screens will need re-painting (and this is no small job, believe me). There are fiberglass versions of these enclosures around now, they don’t look as good (in my opinion) but they don’t need repainting and there is no evidence that they distort temperature readings.

If you use a Stevenson Screen then it also allows you the space to fit a mercury thermometer alongside your weather station air temperature sensor. This is always useful for an accuracy check.

The Radiation Shield is generally much more compact and is usually supplied with a ‘U’ clamp arrangement so it is easy to mount. The sensor sits in the middle of the pagoda-like column of fiberglass or plastic plates. The structure essentially performs the same function as the Stevenson Screen, it provides shade, reflects solar radiation and keeps the sensing element out of the wind and rain.

Unlike the Stevenson Screen it is almost maintenance free, a clean once in a while is all that is required. There is of course no room for a mercury thermometer but it is undeniably the more practical option for most people.

Choosing a Good Location for Your Air Temperature /Humidity Sensors:

The WMO recommendations are as follows:

“For general meteorological work the observed temperature should be representative of the free air conditions surrounding the station over as large an area as possible, at a height of between 1.25 and 2m above ground level.”

“The best site for the measurements is, therefore, over level ground, freely exposed* to sunshine and wind and not shielded by, or close to, trees buildings and other obstacles”

*”freely exposed” refers to the Stevenson Screen / Radiation Shield, not the sensor itself.

“A site on a steep slope or in a hollow is subject to exceptional conditions and should be avoided.”

“Observations of temperature on the top of buildings are of doubtful significance and use because of the variable vertical temperature gradient and the effect of the building itself on the temperature distribution”

Depending upon the type of sensors that you have you may find that you certain limitations pertaining to cable length. It may not be possible to extend the cables provided (at least not without re-calibration).

The temperature and humidity sensing equipment should be sited at a location that is representative of temperature/humidity for the installation area. For example, at a rural location the sensor should be installed over grass. Similarly in sandy-desert locations they should be sited over sand.

The sensors must be located away from any local heat sources (e.g. central heating vents). Note that the sensors should be as far from any such sources as possible because warm air may be carried down-wind to the instruments. This requirement also means that the surface below the sensing equipment should not be constructed from materials that will reflect/radiate heat. For example, in a garden, the sensors should ideally be located over grass not a patio. The concrete in a patio will spend all day absorbing heat from the sun and during the evening will radiate that heat back into the air. If your temperature sensor is sitting above the patio then you will get distorted readings.

The instruments should be sited in a “free-air” location, i.e. a site that has as unrestricted air movement around and through it as possible.

They should be easily accessible for periodic maintenance.

The Stevenson Screen / Radiation Shield is a relatively fragile construct and so is vulnerable to impact damage. Therefore it should be sited well away children’s play areas.

Barometric Pressure Sensor 

If the weather station that you have purchased is one in which the barometric pressure sensor is intrinsic to the display unit then you have no choice about this, the pressure sensor will be located wherever you place the display. This type of arrangement is generally used with fairly inaccurate sensors however this type of sensor should be perfectly satisfactory for reporting the general pressure trend.

If your pressure sensor is separate to the display unit then here are some thoughts about where and how to site it.

 

This type of pressure sensor comes in all shapes and sizes. Each will typically have an electrical connection for power and signal (the orange connector block in the bottom right corner of the picture above) and an airline connection port (bottom left).

Ideally you want to be measuring the outside air pressure (this is especially important for air conditioned buildings or other locations where there is a significant variation between the air pressure inside and out). The sensor itself can still be located inside and a plastic airline run from the sensor to a suitable outside location.

The external end of this airline should be terminated in a static pressure head. This is a vented end-piece which performs the dual functions of preventing water ingress into the airline and avoiding a pressure drop within it caused by wind blowing over the top of the open airline (the “Venturi” effect). If you blow over the end of a tube which is open at one end then the pressure within the tube drops. In this instance the open end of the tube is the airline connection port on the pressure sensor. If this sensor was simply placed outside exposed to the elements then any movement of air across the airline connection port would cause fluctuations in the pressure measured by the sensor.

Where I Placed My Own Home Weather Station Pressure Sensor

Personally, my pressure sensor is located within my Stevenson Screen. I do not use a static pressure head (the sensor airline connection port is simply left open within the Stevenson Screen itself). Whether or not this will suffice for any given location will depend upon how windy it is. In a very exposed location you may well find that pressure fluctuates on a windy day even within the Stevenson Screen. If so then an airline from the sensor to a static pressure head is the answer.

When installing the plastic airline it is strongly advised that a large ‘U’ bend in the tubing is incorporated immediately before termination at the static pressure head in order to trap any water that gets in (rather than let it get into the pressure sensor).

Note that the sensor will measure air pressure at the instrument height only, irrespective of where the external airline is run to. The weather system processing equipment / software will then correct this value to give air pressure readings at sea-level (QNH).

Choosing a Good Location for Your Barometric Pressure Sensor:

Depending upon the type of sensor that you have you may find that you certain limitations pertaining to cable length. It may not be possible to extend the cables provided (at least not without re-calibration).

Keep it easily accessible for periodic maintenance.

The static pressure head should ideally be located in free-air with good airflow around it.

The static head should be located away from walls and the edges of roofs, as the wind-rush up the side of the structure can present “Venturi” effect problems. The reliability of the static pressure head in negating the effects of vertical movements of air is less effective than for the horizontal component of airflow.

The static head and associated airline should also be accessible for periodic inspection and maintenance.

Wind Speed and Direction Sensors:

These are probably the trickiest of the sensors for the home user to site and there is certainly no one-fix solution for all cases.

The two key requirements are

1. Unobstructed exposure to the prevailing wind direction.
2. Easy access to the sensor for maintenance.

You want to get the sensor as far clear of any obstacles as you possibly can (without having to install a phone mast in your back garden). When choosing how and where to put the sensor(s) bear in mind that the turbulence associated with any vertical surface (such as the side of your house) extends for many meters above the top of that surface. The wind will hit the side of the house and then travel vertically upwards and over the top of the wall. If you stand on the roof on a windy day you will find a dead area with virtually no wind close to the edge of the wall. This is the dead spot caused by the wind rushing up the wall and over the top of your head. If you place the sensor here you will never get reliable readings. You need either height above the house or distance from it.

Choosing a Good Location for Your Wind Speed and Direction Sensors:

The WMO recommendations are as follows:

“ The standard exposure of wind instruments over level, open terrain is 10m above the ground”

where “open terrain” is defined as:

 “the distance between the anemometer” (wind speed sensor) “and any obstruction is at least 10 times the height of the obstruction”

“Wind observations that are made in the direct wake of tree rows, buildings or any other obstacle are of little value and contain little information about the unperturbed wind.”

“An optimum location is one where the observed wind is most representative for the wind over an area of at least a few Kilometres”

WMO Recommendations Relating Specifically to Wind Measurement at Sea:

“On fixed platforms and ships, it is of the utmost importance that wind sensors be exposed sufficiently high above the platform and its superstructure to avoid the often extensive influence of the platform on the local wind structure”

These recommendations from the WMO are clearly only a wish-list for the home weather station installer but they do at least give a good idea of the important criteria. Before choosing the spot for your wind sensors it would be worth finding out the prevailing wind direction for your area. Then choose your sensor exposure based on this (remember that wind direction is always reported as the direction from which the wind is coming). For most people it will not be possible to site the sensors such that they have free exposure to wind from all directions so your options then are either to site your sensors such that they have free exposure to the prevailing wind direction (that way they will give reliable readings most of the time) or if your weather monitoring system (and budget) are flexible enough you can have multiple wind sensors, each of which has a different exposure providing reliable information whenever the wind direction favours them individually.

As with the previously referred to sensors there may be cabling limitations associated with your particular instruments.

The sensors must be accessible for periodic maintenance.

What Do I Do Next?

If you are looking to get started with setting up a weather station, or you just need additional sensors for your existing kit, take a look at the products on the sidebars. They will provide extra technical specifications and show you the current choices in the marketplace.

Feel free to leave a comment with any questions on setting up your home weather station instruments.