Abstract
AbstractGuidelines in the Guide 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 τ 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 τ63 = 20 s implies that 95% of a step change be registered within 3τ63 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·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 Ω 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·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.
Highlights
AND M O T I VAT I O N1.1 Meteorological relevance the relative “sensitivity” of meteorological thermometry was first experimentally examined almost 150 years ago (Symons, 1875), it is perhaps surprising how little recent attention has been paid within the meteorological community to determining and optimising the response times of air temperature sensors
The test results revealed a fourfold difference in response times between different sensors: none of the platinum resistance thermometers (PRTs) tested met the Commission for Instruments and Methods of Observation (CIMO) response time guideline at a ventilation speed of 1 m⋅s−1 assumed typical of passively ventilated thermometer shields such as Stevenson-type thermometer screens
Of particular concern to the meteorological community was the result that the shortest individual τ63 at 1 m⋅s−1 ventilation rate of all sensors tested was 23.6 ± 1.9 s (PRT7, 3 × 100 mm, average of five samples), still some way outside the WMO CIMO recommendation
Summary
The relative “sensitivity” of meteorological thermometry was first experimentally examined almost 150 years ago (Symons, 1875), it is perhaps surprising how little recent attention has been paid within the meteorological community to determining and optimising the response times of air temperature sensors. This is despite acknowledged recognition of the importance of sensor response time on meteorological temperature measurements, maximum and minimum air temperatures, and the implications of differing sensor response times within a heterogeneous meteorological network are significant. Much of the (rather scant) literature on mercury-in-glass or PRT response times concerns industrial or biomedical temperature sensors, some of which require a much wider or much narrower range of operating temperatures than meteorological applications, lower precision and/or much less demanding requirements in terms of long-term calibration stability (years to decades): examples include Chohan and Hashemian (1989), Mackowiak and Worden (1994), Khorshid et al (2005), Kyriacou (2010) and Niven et al (2015)
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