The paper presents the nature of variation of refractive index of atmospheric medium before the onset of squall over kolkata (22 34′N 88 22′E), India. The variation of the refractive index with surface temperature and relative humidity during squall is also presented. It is concluded that sharp decrease of the refractivity may be taken as a possible method of forecasting squall. This kind of study is very much helpful from the standpoint of aviation hazards and communication. Possible explanation for the observed results is also presented. © 2013 Indian Association for the Cultivation of Science.