The deuterium excess in summer monsoon precipitation, determined from isotopic measurements(δ¹⁸O and <δD) at New Delhi, has been shown to be correlated, negatively, with relative humidity at the 1000 hPa level over the western Arabian Sea. Furthermore, summer rainfall in north-western India is correlated, positively, with the zonal wind speed at the 1000 hPa level over the western Arabian Sea, while relative humidity is correlated, negatively, with zonal wind speed at the same level. It is concluded that evaporation from the western Arabian Sea is a major source of the summer rainfall at New Delhi.