Diurnal variation of relative humidity over Calcutta (Alipore) and a statistical approach for forecasting minimum relative humidity

A case study on diurnal variation of relative humidity over Calcutta (Alipore) for 1981 reveals (i) Driest period of RH <40% around 1500 IST between mid-December to mid February and also around 1400 IST in the month of November, (ii) Moist period of RH>90 % around 0600 IST during the whole year. The variability of the hourly RH values IS lowest during monsoon. moths In the early morning, however, the highest variability during a day in any month does not coincide with time of occurrence of the minimum RH of that month. Pentad normal of minimum and maximum RH over Alipore through the year have been computed based on data from 1950 to 1980 (excluding 1956). Correlation coefficients between (i) minimum value. of RH of a day with the anomalies of RH corresponding to 03 GMT of the day and 12 GMT of the previous day and (II) 24-hr change of min RH With those values for 03 and 06 GMT shows that there is a good correlation between 24-hr change of RH of 06 GMT and that of the minimum RH of the day.