In this study, the variation and trend of specific humidity (SH) of the troposphere (1,000–500 hPa) over Iran was analysed using the monthly European Center for Medium Range Weather Forecasting (ECMWF) Re-Analysis Interim (ERAInterim) gridded data from January 1979 to December 2016. By using nonparametric modified Mann–Kendall test, the trend of values for each gridded point was tested in eight different atmospheric levels at 95% confidence level. The Sen’s slope estimator test was used to estimate the change rate. The findings showed that the annual trend of SH is decreasing in most of Iran and is only increasing on the northern and southern coast of the country. The highest increasing trend of the SH is observed on the southern coast of Iran in 1,000 hPa. The regression line fitting on the standardized time series of Iran’s area average SH showed that the highest decreasing rate occurred in the first layer (1,000–850 hPa) and only the third layer (600–500 hPa) has had a significant increase of SH trend over Iran. Area average annual anomaly of the tropospheric cross sections indicates that the studied period is divided into two periods: a wet period (1979–1998) and a dry period (1999–2016).
