Measurements of natural airflow within a Stevenson screen and its influence on air temperature and humidity records

[摘要] Climate science depends upon accurate measurements of air temperature andhumidity, the majority of which are still derived from sensors exposedwithin passively ventilated louvred Stevenson-type thermometer screens. Itis well-documented that, under certain circumstances, air temperaturesmeasured within such screens can differ significantly from “true” airtemperatures measured by other methods, such as aspirated sensors.Passively ventilated screens depend upon wind motion to provide ventilationwithin the screen and thus airflow over the sensors contained therein.Consequently, instances of anomalous temperatures occur most often duringlight winds when airflow through the screen is weakest, particularly when incombination with strong or low-angle incident solar radiation. Adequateventilation is essential for reliable and consistent measurements of bothair temperature and humidity, yet very few systematic comparisons toquantify relationships between external wind speed and airflow within athermometer screen have been made. This paper addresses that gap bysummarizing the results of a 3-month field experiment in which airflowwithin a UK-standard Stevenson screen was measured using a sensitive sonicanemometer and comparisons made with simultaneous wind speed and directionrecords from the same site. The mean in-screen ventilation rate was found tobe 0.2 m s −1 (median 0.18 m s −1 ), well below the 1 m s −1 minimum assumed in meteorological and design standard references, and onlyabout 7 % of the scalar mean wind speed at 10 m. The implications of lowin-screen ventilation on the uncertainty of air temperature and humiditymeasurements from Stevenson-type thermometer screens are discussed,particularly those due to the differing response times of dry- and wet-bulbtemperature sensors and ambiguity in the value of the psychrometriccoefficient.

[学科分类] 再生能源与代替技术