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A new global land surface humidity dataset - HadISDH

Decadal trends in HadISDH surface humidity 1973 to 2013

November 2014 - Our new land surface humidity product HadISDH shows that, overall, the amount of water vapour in the air has increased since the 1970s; but recently mid-latitude regions have become less saturated.

HadISDH is a monthly mean gridded land surface climate data product intended for looking at long-term changes in humidity over large scales. Higher specific humidity (q) means that there is more water vapour available to act as a greenhouse gas, therefore affecting the radiation budget. Water vapour is also important for the energy budget because it is a store of heat. When the water vapour later condenses back into water it releases this heat energy to the surrounding atmosphere. Higher specific humidity is also likely to lead to greater intensity of rainfall in very heavy rain events. Relative humidity (RH) is important in the hydrological cycle, influencing the ability of the air to evaporate and the onset of rain events. It is also relevant to health because it is harder to maintain body temperature when high relative humidity inhibits evaporation of sweat.

HadISDH provides gridded records from 1973 to present for a wide range of humidity related variables:

  • specific humidity (q) expressed in g kg−1,
    the ratio of the mass of water vapour to the mass of moist air;
  • relative humidity (RH) expressed as a percentage (%rh), 
    the amount of water vapour in the air compared to the maximum amount of water that can be held as vapour at that temperature; 
  • vapour pressure (e) expressed in hPa, 
    the partial pressure exerted by water vapour alone;
  • dew point temperature (Td) expressed in ºC,
    the temperature at which the air becomes saturated given the current amount of water vapour, measured by artificially cooling a surface until water condenses onto it;
  • wet bulb temperature (Tw) expressed in ºC,
    the reading of a thermometer cooled by evaporation from a moistened wick where air that is not saturated will allow evaporation of water from the wick, cooling the "wet bulb" thermometer;
  • dew point depression (DPD) expressed in ºC, 
    the amount the air has to be cooled by to reach its dew point temperature; 
  • dry bulb temperature (T) expressed in ºC,
    the reading of a dry bulb thermometer.

In HadISDH, the data were transformed from their raw hourly form to a gridded monthly mean climate product, taking care to ensure high quality. Sometimes station data contain random errors due to mistakes made by observers or instrument malfunction. Sometimes, changes are made to station locations or instrumental set up, to improve the quality for weather forecasting. This results in spurious artifacts in the long-term time series that must not be mistaken for real climatic variations. Care was therefore taken to remove such issues from the data: metrologists at the National Physical Laboratory (LINK) collaborated with us to estimate the remaining uncertainty in the data.

HadISDH reveals a warming world with more water vapour in the atmosphere. The largest increases in water vapour are found over the tropics and the Mediterranean. Over the tropics and the high northern latitudes the surface air over land is becoming more saturated. This means that both the specific humidity and the relative humidity are higher. However, despite increasing water vapour over the mid-latitudes and Mediterranean, the surface air over land is becoming less saturated. This means that although the specific humidity is higher, the relative humidity is lower, because the air has become warmer (panel (a) of illustration).  

This new dataset provides the first observation only long-term monitoring product for looking at a comprehensive range of humidity variables at the Earth's surface over land. This allows us to study both the quantity of water vapour and saturation levels. It is also the first time that a comprehensive uncertainty assessment has been provided for humidity. It will be updated every February to include the previous year's monthly means. The data and various diagnostics are available to download from HadOBS. This work is a Met Office led collaborative project including NOAA NCDC, NPL, CRU and NERSC.

Reference:

Willett, K. M., Dunn, R. J. H., Thorne, P. W., Bell, S., de Podesta, M., Parker, D. E., Jones, P. D., and Williams Jr., C. N.: HadISDH land surface multi-variable humidity and temperature record for climate monitoring, Climate of the Past, 10, 1983-2006, doi:10.5194/cp-10-1983-2014, 2014. Supplementary Material

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