An external view of the Met Office building in Exeter, with people walking down the stairs and approaching the main entrance.

Weather Observation site classification

We do this in order to capture the state of the atmosphere in and around the environments we all work and live in, both in the here and now and also in terms of the long-term climate record. Met Office observations produced at these stations are underpinned by a rigorous quality management system including a longstanding and well-honed site inspection methodology, ensuring that data produced at a site is as accurate and reliable as it can be.

The Met Office inspection scheme assesses a station for each meteorological element in terms of its suitability for use in meteorological and climatological products. This assessment has 4 categories: Excellent, Good, Satisfactory or Unsatisfactory. The scheme looks at instrument conformance, installation, exposure, and observer competence (if applicable). For example, when assessing temperature in a Stevenson Screen, if a screen is mounted too high or too low or if it is too shaded or the opening screen door is not facing north, then the inspection assessment for temperature at site would be classed as Unsatisfactory. The Met Office quality management system has been in use for a long time and the criteria, working practices and inspection procedures have been fine-tuned over the decades. There are minimum siting and exposure standards to adhere to, to ensure conformance and data validity, and this system is designed to minimise data uncertainty. 

The WMO Siting Classification for Surface Observing Stations on Land was formally introduced from 2014, enabling us to make broad comparisons of our weather and climate stations with those around the world. These WMO classifications focus on the exposure of an observed element at a site, with a Class 1 assessment being the highest standard and Class 5 the lowest. These classifications were added to the existing Met Office inspection process, complementing existing assessments around exposure, such as shading and heat sinks/sources, and are now routinely reviewed and updated as part of Met Office site inspection visits.

WMO Siting Classifications were designed with reference to a wide range of global environments and the higher classes can be difficult to achieve in the more-densely populated and higher latitude UK. For example, the criteria for a Class 1 rating for temperature suits wide open flat areas with little or no human influenced land use and high amounts of continuous sunshine reaching the screen all year around, however, these conditions are relatively rare in the UK. Mid and higher latitude sites will, additionally, receive more shading from low sun angles than some other stations globally, so shading will most commonly result in a higher CIMO classification - most Stevenson Screens in the UK are class 3 or 4 for temperature as a result but continue to produce valid high-quality data. WMO guidance does, in fact, not preclude use of Class 5 temperature sites – the WMO classification simply informs the data user of the geographical scale of a site’s representativity of the surrounding environment - the smaller the siting class, the higher the representativeness of the measurement for a wide area.  Indeed, it should be noted that WMO Class 5 is not the same as a Met Office ‘Unsatisfactory’ inspection assessment, which ultimately determines the ongoing use of a site. We use the Met Office grading system to determine record verification because; it has historical relevance, covering a wide range of long-standing criteria at UK observation sites, the equipment, and the exposure in a holistic manner and has clear meaning to what is acceptable or not. It tells us how much confidence we have in the data and permits comparisons.  

While we have mainly talked about temperature here, the Met Office inspection process encompasses an assessment for all observation parameters and their sensors (with WMO siting classifications) within an observation enclosure, e.g. Sunshine/Solar radiation, Wind, Precipitation, etc. The schemes are different for each weather element and there is not a single overall WMO class or overall Met Office rating covering a weather observation site as a whole. The Met Office inspection scheme has longevity and encompasses a wider range of criteria, allowing for comparisons over time, whilst WMO classifications have added a more recent mechanism for international station comparison.