Evolution and Accuracy of Surface Humidity Reports*

Until recently surface humidity was predominantly measured using psychrometers (wet- and dry-bulb thermometers). In some countries, often in conjunction with increased automation, the measurement technique has changed—usually to capacitive sensors. The scale of the change in instrumentation and the error characteristics of the operational instruments have not been well documented. This paper provides an overview of these operational instruments and their error characteristics, intended to be useful for climate and forecast users of the data. It also includes detailed results from comparisons of psychrometers and capacitive sensors with a chilled mirror reference instrument at a site in the United Kingdom under (near) operational conditions. The psychrometers performed well near saturation but underread at lower humidities; any large errors tended to be positive because of insufficient water supply to the wet bulb. New capacitive sensors perform well but they usually drift to higher values during deployment (except in arid climates); they perform best at lower humidities and need regular adjustment and recalibration. The natural variation of relative humidity and the differences between instruments are larger in daytime than at night. Changes in the U.K. synoptic network are described in order to put the intercomparisons into context. The instruments used in selected other countries are surveyed. There is a need for better documentation and real-time exchange of metadata on the instruments used and any changes. Capacitive sensors are also used on some radiosondes and aircraft; relevant studies are briefly reviewed and some parallels with surface usage are drawn.