Articles | Volume 10, issue 6
Research article
21 Nov 2014
Research article |  | 21 Nov 2014

HadISDH land surface multi-variable humidity and temperature record for climate monitoring

K. M. Willett, R. J. H. Dunn, P. W. Thorne, S. Bell, M. de Podesta, D. E. Parker, P. D. Jones, and C. N. Williams Jr.

Abstract. HadISDH.2.0.0 is the first gridded, multi-variable humidity and temperature in situ observations-only climate-data product that is homogenised and annually updated. It provides physically consistent estimates for specific humidity, vapour pressure, relative humidity, dew point temperature, wet bulb temperature, dew point depression and temperature. It is a monthly mean gridded (5° by 5°) product with uncertainty estimates that account for spatio-temporal sampling, climatology calculation, homogenisation and irreducible random measurement effects. It provides a tool for the long-term monitoring of a variety of humidity-related variables which have different impacts and implications for society. It is also useful for climate model evaluation and reanalyses validation. HadISDH.2.0.0 is shown to be in good agreement both with other estimates and with theoretical understanding. The data set is available from 1973 to the present.

The theme common to all variables is of a warming world with more water vapour present in the atmosphere. The largest increases in water vapour are found over the tropics and the Mediterranean. Over the tropics and high northern latitudes the surface air over land is becoming more saturated. However, despite increasing water vapour over the mid-latitudes and Mediterranean, the surface air over land is becoming less saturated. These observed features may be due to atmospheric circulation changes, land–sea warming disparities and reduced water availability or changed land surface properties.

Short summary
We have developed HadISDH, a new gridded global land monthly mean climate montitoring product for humidity and temperature from 1973 to then end of 2013 (updated annually) based entirely on in situ observations. Uncertainty estimates are provided. Over the period of record significant warming and increases in water vapour have taken place. The specific humidity trends have slowed since a peak in 1998 concurrent with decreasing relative humidity from 2000 onwards.