World Meteorological Organization

The Siting Classification for Surface Observing Stations on Land is the first common ISO/WMO standard. It was published by ISO as ISO standard 19289:2014 (EN), and by WMO in the WMO Guide to Instruments and Methods of Observation (WMO-No. 8), Part I, Chapter I, Annex 1B.

Below, it is referred to as "Siting Classification".

This webpage is meant to provide guidance on how to implement the classification and to share tools used by NMHSs in implementing this classification (or similar classifications) in their services. It contains:

• A list of questions and answers, and
• Tools used by NMHSs and contact persons who can provide clarifications on their implementation.

The CIMO Expert Team on Standardization, at its first session (26-29 Nov. 2013, Geneva, Switzerland) decided to develop a list of questions and answers to clarify the use and the purpose of the siting classification. It is based on the questions raised during the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (Brussels, Belgium, 16-18 October 2012) during which a discussion session was organized on the subject of the siting classification. During this session a number of NMHSs reported on their experience with the implementation of the classification in their services. It was agreed that this classification was needed and useful, but that there was a need for some clarifications on how to implement it.

This list is a living document. Further questions and answers will be added following requests for clarifications. It is not expected that this list will be formally published.

Should your question not be covered by the replies below, you may send a query to:

• Mr Yves-Alain ROULET (yves-alain.roulet@meteoswiss.ch) with copy to Mrs Isabelle Rüedi (iruedi@wmo.int).

Questions and answers

1. What is the purpose of the Siting Classification (SC)?

• It gives an estimation of how well the siting of an instrument meets the siting recommendations provided in the WMO-No. 8.
• To make network planners and installers consider good practice under circumstances where compromise is needed. It aims to give some acceptable "middle ground" in between perfect and unacceptable.  It should be stated that it is intended to be quite simple, as detailed metadata and quality assurance techniques exist to handle the very granular and dynamic items. In addition it is the general trend to declare the outcomes of processes in use. It is more beneficial to share the classification of parameters rather than imply that all meet the conditions of the guide.
• It enables NMHSs to rapidly assess the value of stations from partner networks.
• Users, such as climate researchers using the data get a quick idea of how representative the data may be of the region. They also get an idea of the history of the station. The higher the number, the more detailed examination of the metadata will be required to ascertain the usefulness of the data for the desired purpose.

2. Evidence of impact

Work is emerging to clarify the impact suggested by 'uncertainty' suggested in the SC.  These may lead to changes in future as the evidence improves. For example, there is well developed literature on the impacts of wind shading but less on other topics. The SC has prompted some studies into the impacts of sites with respective scores and the details will take some time to be clear. (see literature references below).

The suggested ‘uncertainty’ is given to document the order of magnitude of the errors which may arise for a given class. It doesn’t mean that all the measurements are affected by such an error. Some influence factors may minimize the errors due to siting, such as moderate or high wind for temperature measurements, zero or low winds for precipitation measurements. Nevertheless, other meteorological parameters are not taken into account for the SC, to keep it simple and static (see also 5.).

3. What is the significance of the numbering system?

There has been some debate on the use of numbers in the SC.  This is a good and convenient system. A colour system or one based on text only could have been chosen. The numbers should not be taken to mean that higher class stations are of low value, as there may be very good reasons for the site exposure depending on the purpose for which that station was established (specific vs general purpose, mountain stations, agricultural stations, safety reasons, …). However, we acknowledge that the use of numbers can easily lead one to suggest a ranking. This is not the purpose and should be avoided.  For some time the measurement experts have taken different requirements for different users, and this may be more pronounced in emergency circumstances when higher (number) classes may still be highly valuable for some applications, the SC reflects this. Because many sites have been chosen to serve the needs of many users, it is likely that many sites will not be class 1 for all parameters.

4. What tools and software can be found to help making SCs practical and efficient?

The SC-MINT Expert Team on Surface and Sub-surface Measurement will gather, review and link to tools and software, that have been found useful by users.

5. Should some parameters be considered in conjunction?

This has been well discussed, but it has been decided that the SC should remain quite simple and these will not be used. Also, considering parameters in conjunction would make it a dynamic rather than a static classification, depending on the conditions on the site.

6. How much work will it take?  

This will depend on how readily it can be fitted to other new or existing work. However, currently it has been suggested (personal communication UK) that for a new site it may take an add 20 minutes on top of the previous work of about 3 hours. In Météo-France, the typical time needed to classify a station is two hours. It may be proposed that existing systems will have a long window to classify. The amount of time depends also strongly on the equipment used to assess the site.

(If other institutes want to share experience, please provide it to SC-MINT Expert Team on Surface and Sub-surface Measurement.)

7. Some surfaces have little or no vegetation - are these included?

The SC states “ground covered with natural and low vegetation representative of the region”. What is important is to have ground representative of the natural state of the region, including the low vegetation of that region. If you are located in a region where there is no vegetation, then the ground surface in the vicinity of the site has to be representative of that region, with no vegetation.

8. Sometimes the sensor changes height above the surface when snow accumulates, what about these?

As long as the snow does not bury the sensor and screen, and height information is available this has no impact on the SC. The class of a site is intended to be a static number (during a year). If there is a risk that the sensor could be buried under snow, it should be mounted higher.

9. Would it help to declare the purpose of the site?

(For example this is a highway site).This has been discussed but it was thought too complex for the SC, many sites have multiple uses, these will increase over time. This information may be available in other metadata of the site.

The class helps to know whether the data are likely to be representative of a larger area.

10. Very few sites will be class 1 for wind since the SC requires a clear radius of 300m.

This is understood but evidence shows that a wind impact can be detected at this range. Amendment of the WMO-No. 8 has been agreed by CIMO ET-Standardization (28-Nov-2012) to clarify that.

11. Will the SC depend on the assessor?

It should not. To reduce subjectivity in site assessments, staff need to be trained to ensure consistent applications of the SC. (See also point 4 above on tools and software)

12. If large snow piles can be made near the site - how is this assessed?

The SC is a static parameter so it is assessed as frequently as annually but not changed on a seasonal basis.  Metadata and quality assessment techniques should be used.

Different methods can be used to assess a site and define the classes of each sensor. Indeed various methods were reported during TECO-2012. The CIMO Expert Team on Standardization, at its first session (ET-Standardization, 26-29 Nov. 2012, Geneva, Switzerland) therefore decided to share the various practices in use by WMO Members rather than developing a generic document on the subject.

ET-Standardization members and other experts from NMHSs implementing the classification agreed to share the material (procedures, tools, software, etc.) they have on how they assess a site and encouraged other NMHSs to also share their expertise, as it becomes available, so that it could be posted on the WMO website for the use of all WMO Members, and to provide the contact details of a person who could provide clarifications on the method presented in these documents.

The documents posted below are the result of this process and were reviewed by ET-Standardization before posting on the website. However, please note that these practices may not fully comply with the siting classification as published in the WMO-No. 8 and available above and may be subject to changes.

Nordic countries Mareile Wolff et al. mareilewmet.no Temperature siting classification in Nordic Countries Canada Brian Howe brian.howeec.gc.ca AWS sensor siting classification Sensor Siting Classification Assessment Forms United States Bruce Baker bruce.bakernoaa.gov Decision document for site Rio Blanco Lake Station Score log

Sites Classification
M. Leroy, in: WMO (2010), Papers Presented at the WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO–2010), Instruments and Observing Methods Report No. 104

TECO-2016, Madrid, Spain, 16-18 Oct. 2012 - Discussion session on the Siting Classification Scheme:

• Joel Fisler et al., Switzerland, Integrating third party data from partner networks: Quality assessment using MeteoSwiss' Meteorological Certification Procedure

TECO-2012, Brussels, Belgium, 16-18 Oct. 2012 - Discussion session on the Siting Classification Scheme:

• Rodica Nitu, Canada, Automatic Weather Station: Sensor Siting Classification
• Bruce Baker, USA, Siting Classification
• Kumamoto Mariko, Japan, Classification of JMA manned stations
• Mareile Wolff, Norway, Site Classification at the Norwegian Meteorological Institute
• Mike Molyneux, UK, Site Classification