Evaluation of the influence of rain on air surface temperature measurements

Reliable measurements of surface air temperature are crucial in meteorology and climatology. Thermometers must be shielded to avoid atmospheric influence factors, such as solar radiation and rainfall, compromise their accuracy. In the standard document ISO 17 714, which defines test methods for performance comparison of different screens, potential error sources caused by rainfall are mentioned. Besides the well-known psychrometric effect, an error on the temperature reading related to the difference between air and rain temperature causing heat transfer from water to the thermometer is identified. Despite the high relevance, no studies are available in literature that analyze this effect. The work described in this paper aims at quantifying the influence of the rain on the readings of temperature sensors, with radiation screens of different types. The study was performed by introducing an artificial rainfall generator where the water temperature and rain intensity was controlled and comparing the readings of thermometers under the generated rainfall with respect to a developed reference system not affected by rain. The tests show that the error for especially naturally ventilated thermometers can be unacceptably large, up to -4.0 (0.2) K, whereas for the artificially ventilated air thermometers the errors observed are less significant, smaller than -1.1 (0.2) K. This preliminary study underscores the critical influence of rain temperature and intensity on thermometer readings and highlights the importance of further research to develop standardized testing protocols. The reported methodology can also help to develop future standardized test protocol to qualify the device with regards to the measurement error.