Estimating air humidity from temperature and precipitation measures for modelling applications

Unlike precipitation and temperature, humidity is not widely measured, fostering the development of algorithms that simulate air humidity using other measures. In humid temperate climates, atmospheric water vapour close to the ground often reaches saturation around the time of minimum temperature. This allows models of daily maximum dew-point temperature to be built from minimum air temperatures. Once the dew-point temperature has been estimated, hourly values can be interpolated by assuming that water content in the air remains unchanged during the day, relative humidity being the result of the modulation of air temperature on vapour pressure at saturation. This approach requires certain corrections in order to take into account seasonal and local features. The present work investigated and tested the application of algorithms to simulate relative humidity from minimum daily temperatures in 23 meteorological stations in an Italian alpine region. The basic model is corrected by decreasing dew-point temperature using temperature and precipitation measures, and also requires specific calibration. With respect to errors in the number of hours with high relative humidity, major biases may be generated at some sites. After correction, the number of sites where these techniques yield acceptable results ranges from about a quarter to a half of the total, the lower figure reflecting a stricter acceptance standard in the driest years. The simulation algorithms yield useful results for stations where site-specific calibration can be carried out, encouraging the use of simulated series from selected sites for climatic, meteorological and phytopathological modelling