title: Metrological challenges for measurements of key climatological observables, Part 4: Atmospheric relative humidity.
abstract: Water in its three ambient phases plays the central thermodynamic role in the terrestrial climate system. Clouds control Earth's radiation balance, atmospheric water vapour is the strongest "greenhouse" gas, and non-equilibrium relative humidity at the air-sea interface drives evaporation and latent heat export from the ocean. In this paper, we examine the climatologically relevant atmospheric relative humidity, noting fundamental deficiencies in the definition of this key observable. The metrological history of this quantity is reviewed, problems with its current definition and measurement practice are analysed, and options for future improvements are discussed in conjunction with the recent seawater standard TEOS-10. It is concluded that the International Bureau of Weights and Measures, (BIPM), in cooperation with the International Association for the Properties of Water and Steam, IAPWS, along with other international organisations and institutions, can make significant contributions by developing and recommending state-of-the-art solutions for this long standing metrological problem, such as are suggested here.
authors.affiliation: Measurement Standards Laboratory (MSL), Lower Hutt, PO Box 31-310, New Zealand.
authors.forename: J W
authors.initials: JW
authors.lastname: Lovell-Smith
authors.affiliation: Leibniz Institute for Baltic Sea Research (IOW), D-18119 Warnemünde, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Feistel
authors.affiliation: National Institute of Standards and Technology (NIST), Boulder, CO 80305-3337, USA.
authors.forename: A H
authors.initials: AH
authors.lastname: Harvey
authors.affiliation: Leibniz Institute for Tropospheric Research (TROPOS), D-04318 Leipzig, Germany.
authors.forename: O
authors.initials: O
authors.lastname: Hellmuth
authors.affiliation: National Physical Laboratory (NPL), Hampton Road, Teddington, Middlesex, TW11 0LW, UK.
authors.forename: S A
authors.initials: SA
authors.lastname: Bell
authors.affiliation: MIKES Metrology, VTT Technical Research Centre of Finland Ltd, Tekniikantie 1, FI-02151 Espoo, Finland.
authors.forename: M
authors.initials: M
authors.lastname: Heinonen
authors.affiliation: Queen Mary, University of London (QMUL), Mile End Road, London, E1 4NS, UK.
authors.forename: J R
authors.initials: JR
authors.lastname: Cooper