INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2013) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.3765 Comparative analysis of the influence of solar radiation screen ageing on temperature measurements by means of weather stations G. Lopardo, a * F. Bertiglia, a S. Curci, b G. Roggero a and A. Merlone a a INRiM, Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 73, 10135 Torino, Italy b Climate Consulting S.r.l., Corso Sempione 6, 20154 Milano, Italy ABSTRACT: Solar radiation screens play a key role in automatic weather stations (AWS) performances. In this work, screen ageing effects on temperature measurements are examined. Paired temperature observations, traceable to national standards and with a well-defined uncertainty budget, were performed employing two naturally ventilated weather stations equipped with identical sensors and different only for their working time. Three different tests were carried out employing different aged AWSs: a 5-year-old AWS (AWS5) was compared with a new device (AWS0), a 1year old (AWS1) was compared with both a 3years old (AWS3) and a new one devices (AWS00). Due to solar and weather conditions exposure a degradation of the screen reflective coating is evident for the older AWSs (5 and 3 years old) and so a qualitative estimation of how different conditions of ageing affect the temperature drift was done. During the comparison 0 to 5 and 1 to 3-year- old screens, significant temperature differences were recorded at different times of the day. The differences, wider than the uncertainty amplitude, demonstrate a systematic effect. The temperature measured with the older screen is larger, and the maximum instantaneous difference was 1.63 ◦ C (for 0–5 years comparison) in daytime hours. During night-time the two AWS’s measure the same temperature (within the uncertainty amplitude). This behaviour, increasing with increasing solar radiation intensity and decreasing with increasing wind speed, is attributed to a radiative heating effect. The screen ageing has compromised the shield effectiveness introducing a significant change in the temperature evaluation. The experimental results of a further comparison, between 0- and 1-year-old screens, confirm the same conclusion showing a negligible ageing effect, within the uncertainty amplitude. KEY WORDS radiation screen; ageing; weather station; air temperature; solar radiation; traceability; meteorological thermometer calibration Received 14 December 2012; Revised 13 May 2013; Accepted 19 May 2013 1. Introduction Despite constant improvements in sensors technology, accurate air temperature measurements are still a chal- lenging task. Discussion among the scientific community is open aiming at defining methods and best practice for achieving representative atmospheric temperature records for meteorology and climatology. The representative- ness of the place or area that the measuring stations cover require attention in the characterization, correc- tion and possible minimization of influencing effects. Those effects are due to two main aspects: (1) influ- ences of quantities other than temperature effecting the sensor reading, like sun or wind, humidity content etc. and (2) surrounding characteristics like presence of trees, building, ground coverage etc. In principle, the temper- ature reading of the thermometers embedded in the sta- tions should be limited to the convection effect of the * Correspondence to: G. Lopardo, Istituto Nazionale di Ricerca Metro- logica (INRiM), Strada delle Cacce 73, I-10135 Torino, Italy. E-mail: g.lopardo@inrim.it air surrounding the sensing element of the thermometer. The real measuring conditions are far from this ideal sta- tus, since contact and radiative contributions are always present. Modern automatic weather stations (AWS) are made to minimize contact heat exchange from the body of the station to the thermometers and are equipped with solar screens. The main function of thermometer screens is to prevent the solar direct and reflected radiation, shel- ters the sensor from inclement weather, and allows ade- quate airflow to ventilate the sensor (Van der Meulen and Brandsma, 2008). It follows that the thermometer screens should have a high reflectivity to minimize heat- ing of the plates and subsequent warming of the air as it flows over the plates to the sensor. As a shield material, highly polished, non-oxidized metal or thermally insu- lating plastic-based materials are widely used (Aoshima et al ., 2010), (WMO, 2008). However, the screen reflective coatings are subjected to degradation in time. Due to solar and weather exposure, the screens age and their coatings colour changes from bright reflecting white to sort of light beige, compared to the new ones (Figure 1).  2013 Royal Meteorological Society