The uncertainties associated to classes B, C and D haven’t a so direct link to “official” technical documents, nor typical users’ requirements. Class B uncertainties are derived from the experience of some meteorological services and are a compromise between the best achievable and an affordable system. Class C uncertainties have been defined mainly by expanding the class B uncertainties by a factor of 2. Class D uncertainties are by definition larger than class C uncertainties. The criteria indicated deal with the contribution of the instruments to the measurement uncertainty.
Parameter Class A B Class C Class D Radiation screen High performance radiation screen of known uncertainty Well-maintained radiation screen with known characteristics and over-estimation of Tx (daily max. temperature) < in 95% of cases when compared with a class A radiation screen Radiation screen with known characteristics and of Tx < in 95% of cases. Radiation screen with a large influence of solar radiation (for example, over-estimation of Tx in more than 5% of cases). Air temperature (achievable measurement uncertainty). Temperature probe with uncertainty below or equal (in laboratory conditions, over the measuring range). Uncertainty of the acquisition system < . Class A radiation screen. Yearly calibration in laboratory against a traceable standard having an uncertainty ≤ . Redundant measurements can act as a continuous check on drift. Temperature probe with uncertainty below (corresponds of class A of IEC 751 standard, Pt100 platinum probe). Uncertainty of the acquisition system < 0.1°C. Class B radiation screen Regular calibration against a traceable standard having an uncertainty ≤ . Temperature probe with uncertainty < 0.4°C, with good stability, such as Pt100. Acquisition uncertainty < 0.2°C. Class C radiation screen. Temperature probe designed to be calibrated and regular field verification (at least > Temperature probe and/or acquisition system uncertainty lower than for class C. Or Class D radiation screen. Or Temperature probe/system not designed to be calibrated. Relative humidity 3% (achievable measurement uncertainty). Performance verified over the full range of humidity and a temperature range typical for the location of the station. Acquisition uncertainty < 0.2%. Class A radiation screen. Yearly calibration in laboratory against a traceable standard having an uncertainty ≤ 1%. 6% Sensor specified for 6%, over a temperature range typical for the location of the station. Acquisition uncertainty < 1%. B radiation screen Regular calibration against a traceable standard having an uncertainty ≤ 2%. 10% Sensor specified for 10%, over a temperature range typical for the location of the station. Acquisition uncertainty < 1%. Class C radiation screen. Regular calibration or field verification (at least, 1 point > 10% Sensor with performances or specifications than 10% over a temperature range typical of the location of the station or acquisition uncertainty 1% or Calibration not organized. Or Class D radiation screen.
Error related to precipitation intensity corrected. Daily monitoring of the collecting funnel Yearly calibration. Regular calibration or field verification. Regular monitoring of the collecting funnel. Preventive maintenance planned and applied. or no regular maintenance organized. Precipitation (rainfall) with weighing rain gauges The larger of 5% or . (achievable measurement uncertainty). Reported resolution better than or equal to . Yearly calibration. The larger of 5% or . Reported resolution better than or equal to . Regular calibration or point field verification The larger of 10% or 0.5 mm. Regular maintenance. > 10% or no regular maintenance organized. Global solar radiation 2% for total (achievable measurement uncertainty). Pyranometer of high quality (acc. to CIMO Guide). Ventilated sensor. Calibration two years. Daily control of the sensor (including cleaning if necessary). Sensor heated (if the climatological conditions require it) 5% for daily total. Pyranometer of good quality (acc. to CIMO Guide). Ventilation may be omitted. Regular calibration Regular control of the sensor (including cleaning if necessary). 10% for daily total. Pyranometer of moderate quality (acc. to CIMO Guide) Ventilation may be omitted. Regular calibration. No regular cleaning of the sensor. Uncertainty > 10% for daily total or sensor not using a thermopile. Or Calibration not organized Sunshine duration To be checked by experts The larger of 0.1 h or 2% for daily totals (achievable measurement uncertainty). 120 W.m-2 threshold applied to pyrheliometer’s measurements. Calibration every two years. The larger of 0.5 h or 5% for daily totals. 2% for yearly totals. Use of a sunshine recorder or calculation of direct radiation from the direct measurement of global and diffuse solar radiation. Regular calibration (if use of pyranometers) or regular control of the sunshine recorder. The larger of 1 h or 10% for daily totals. 4% for yearly totals. Use of a sunshine recorder or calculation of sunshine from the measurement of global solar radiation of at least class B. Uncertainty > 1h or > 10% for daily totals or Uncertainty > 4% for yearly totals