Response times of meteorological air temperature sensors

Guidelines in theGuide to Meteorological Instruments and Methods of Observation(the CIMO guide) of the World Meteorological Organization (WMO, published 2014, updated 2017, section 2.1.3.3,Response times of thermometers) recommend that the 63% response time tau for an air temperature sensor be 20 s, although - as airflow speed influences response time - the minimum airflow speed at which this applies should also be specified in the document. A 63% response time tau(63) = 20 s implies that 95% of a step change be registered within 3 tau(63)or 60 s, the WMO recommended averaging interval for air temperature: rapid air temperature changes on this time-scale are not uncommon, often associated with convective squalls, frontal systems or sea breeze circulations. An alternative way of expressing the effect of the time constant is that in air whose temperature is changing at 0.1 K center dot min(-1)the thermometer would lag by approximately 0.03 K. To assess whether this response time specification was realistic, we have undertaken an experimental and theoretical study of the time constants of meteorological thermometers. Laboratory wind tunnel tests were undertaken to quantify 63% and 95% response times of 25 commercial 100 omega platinum resistance thermometers (PRTs) of various sizes (length and sheath diameter) from five manufacturers. The test results revealed a fourfold difference in response times between different sensors: none of the PRTs tested met the CIMO response time guideline at a ventilation speed of 1 m center dot s(-1)assumed typical of passively ventilated thermometer shields such as Stevenson-type thermometer screens. A theoretical model of the sensors was devised which matched the experimental behaviour with regard to the most important contributing factors, namely ventilation rate and sensor diameter. Finally, suggestions and recommendations for operational air temperature sensor adoption and future sensor development are included.