Ventilation effects on humidity measurements in thermometer screens
Harrison, R. G. and Wood, C. R. (2012) Ventilation effects on humidity measurements in thermometer screens. Quarterly Journal of the Royal Meteorological Society, 138 (665). pp. 1114-1120. ISSN 1477-870X
To link to this article DOI: 10.1002/qj.985
Relative humidity (RH) measurements, as derived from wet-bulb and dry-bulb thermometers operated as a psychrometer within a thermometer screen, have limited accuracy because of natural ventilation variations. Standard RH calculations generally assume a fixed screen psychrometer coefficient, but this is too small during poor ventilation. By comparing a reference humidity probe—exposed within a screen containing a psychrometer—with wind-speed measurements under controlled conditions, a wind-speed correction for the screen psychrometer coefficient has been derived and applicable when 2-metre wind speeds fall below 3 ms–1. Applying this to hourly-averaged data reduced the mean moist RH bias of the psychrometer (over the reference probe) from 1.2% to 0.4%, and reduced the inter-quartile range of the RH differences from 2.0% to 0.8%. This correction is particularly amenable to automatic measurement systems.
Aitken J. 1921. Thermometer screens. Proc. Roy. Soc. Edinburgh 40:172–181. Bolton D. 1980. The computation of equivalent potential temperature. Mon. Weather. Rev. 108: 1046–1053. Harrison RG, Lodge BN. 1998. A calorimeter to detect freezing in supercooled water droplets. Rev. Sci. Instrum. 69: 4004–4005. Harrison RG, Rogers GW. 2006. Fine wire resistance thermometer amplifier for atmospheric measurements. Rev. Sci. Instrum. 77: 116112. Harrison RG. 2010. Natural ventilation effects on temperatures within Stevenson screens. Quart. J. Roy. Meteorol. Soc. 136: 253–259. Harrison RG. 2011. Lag-time effects on a naturally ventilated large thermometer screen. Quart. J. Roy. Meteorol. Soc. 137: 402–408. HMSO. 1982. Handbook of meteorological instruments, Volume 3 (Measurement of humidity). Meteorological Office, 2nd edition, Her Majesty’s Stationery Office. Keil M. 1996. Temperature measurement in Stevenson screens. MSc dissertation, The University of Reading. Met Office 2009. SPOT-ON Observers Guide. Version 4. Met Office, UK. Met Office 2011. UK Meteorological Office Cardington, [McGregor, J.; Price, J.], data taken from NCAS British Atmospheric Data Centre, http://badc.nerc.ac.uk/view/badc.nerc.ac.uk__ATOM__dataent_cardington, (accessed 18 August 2011). Murphy J, Sexton D, Jenkins G, Booth B, Brown C, Clark R, Collins M, Harris G, Kendon E, Betts R, Brown S, Boorman P, Howard T, Humphrey K, McCarthy M, McDonald R, Stephens A, Wallace C, Warren R, Wilby R, Wood R. 2009. UK Climate Projections Science Report: Climate change projections. Met Office Hadley Centre: Exeter. Parker DE. 1994. Effects of changing exposure of thermometers at land stations. Int. J. Clim. 14: 1¬¬–31. Spencer-Gregory H, Rourke E. 1957. Hygrometry. Crosby Lockwood and Son, London. Willett KM, Gillett NP, Jones PD, Thorne PW. 2007. Attribution of observed surface humidity changes to anthropogenic influence. Nature 449: 710–712. WMO. 2006. Guide to Meteorological Instruments and methods of observation. WMO-No.8. World Meteorological Organisation: Geneva.
Repository Staff Only: item control page