Climatological standard normals of IRAN, for the period 1981–2010 and 1991–2020: precipitation and temperature

The main functions of climate normals are twofold. They offer a reference point for evaluating recent or ongoing observations and form the basis for various climate datasets that rely on anomalies. Additionally, they are frequently employed to predict the probable conditions that one might encounter in a specific area. The World Meteorological Organization (WMO) advises regularly reviewing climate normals every decade to keep up with the evolving climate. Atmospheric Science and Meteorological Research Center (ASMERC) is proud to release “Iran Climate Normals” for the periods of 1981–2010 and 1991–2020 including a suite of monthly and annual statistics that are based on temperature, precipitation, sea-level pressure, vapor pressure, station-level pressure, snow-depth, wind speed, visibility, soil temperature, relative humidity, dew point, and cloud amount measurements. This study documents the procedures used for quality control, homogenization of daily observations, and calculation of normal values. For each station and each parameter, the results of the outliers due to the error and the homogeneity assessment are reported. Out of all the parameters, the soil temperature has the highest error percentage. However, this does not necessarily imply that it has the most measurement errors; it could be due to the ease of detecting errors for this specific parameter. Of the 143 stations, 56 had a breakpoint recorded in two parameters or more at a specific point in time. According to the analysis of the temperature and precipitation parameters, (a) the new normal of mean, maximum, and minimum temperatures are 0.47, 0.5, and 0.6 °C above the 1981–2010 period; (b) the normal annual precipitation has increased by an average of 5.4 mm in 1991–2020 compared to 1981–2010; (c) comparing the two periods, the changes in precipitation normals vary in different parts of Iran and different months, while the temperature normals increase in all stations across Iran except for four stations (Gorgan, Kerman, Shiraz, Bandar-e Lengeh); (d) changes in the fourth quintile of monthly precipitation are more than average, and minimum temperature changes are higher than maximum and mean temperatures; and (e) generally, the latter period is characterized by a warmer climate almost across Iran, wetter conditions over the Zagros mountain range and the western part of the Caspian Sea coasts, and drier conditions over the east, center, and west of Iran.