The Pacific Journal of Science and Technology –683– http://www.akamaiuniversity.us/PJST.htm Volume 13. Number 1. May 2012 (Spring) Sharp Depletion of Absolute Humidity associated with Squall over Kolkata (22°34′N, 88°26′E): A Possible Method of Forecasting Squall. U. Saha, M.Sc. 1 ; S.K. Midya, Ph.D. 1 *; H. Sarkar, Ph.D. 2 ; and G.K. Das, M.Sc. 3 1 Department of Atmospheric Science, University of Calcutta, 51/2 Hazra Road, Kolkata-700019, West Bengal, India 2 Centre for Microwave and Millimeter Wave Research and Training, Institute of Radio Physics and Electronics, University of Calcutta, 92 A. P. C. Road, Kolkata- 700009, West Bengal, India 3 Regional Meteorological Centre, Kolkata, West Bengal, India E-mail: drskm2011@gmail.com * upalsaha22@gmail.com ABSTRACT The paper presents the nature of the variation of absolute humidity of the atmospheric medium with time before, during, and after the onset of squall with thunderstorms, only squall event, squall but no thunderstorm days, and normal days having no squall or thunderstorm over Kolkata, West Bengal, India (22°34′N, 88°26′E) during the pre-monsoon period. A critical analysis shows that a sharp depletion of the atmospheric absolute humidity takes place before the onset of squall occurrences which can be a possible method for forecasting the phenomena. Possible explanations are also offered for the said occurrence. (Keywords: absolute humidity, squall, thunderstorm, pre-monsoon, forecast). INTRODUCTION Forecasting mesoscale phenomena have been a challenge to atmospheric scientists since imposing boundary conditions to mathematical equations of mesoscale phenomena (e.g. thunderstorms) are not permitted due to the comparable values of zonal and meridional components. Hence, forecasting these phenomena numerically has been a serious and tedious job. It is well-known that the threat level of only thunderstorm is lesser than those associated with squall. The high-velocity winds in reality are aviation-hazards while aircraft is in flight and very dangerous during landing. A squall is a sudden, sharp increase in wind speed which is associated with thunderstorms or heavy rain showers. Thus, squalls refer to an increase in the sustained winds over a short time interval, usually occurring in a region of strong mid-level height falls, or mid-level tropospheric cooling, which force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake. These high velocity winds whose speed increases by at least three stages on the Beaufort scale – the velocity rising to force 6 (22-27 knots / 41-47 kmph) or more and lasting for at least one minute in duration. Currently, the squall forecasting efficiency is not high enough. This can be partially explained by the fact, as given by Vasil’ev, et al. [1], that the squalls are mainly missed by the sparse observational network, low accuracy of measurements, and indirect estimation of wind speed, as well as higher complexity of the process, its parameterization, and a whole complex of factors which cause formation of squalls. Radar reflectivity in squalls is usually greater at the level of precipitation than that of crystallization initiation and mass crystallization. If the radar reflectivity at the third level is not available, the reflectivity at lower levels should be used in squall forecasting, shown by Ananova, et al. [2]. All the squall lines form in a conditionally and convectively unstable atmosphere which is characterized by strong vertical shear and turning off the shear vector with height at low levels, and weaker shear and only slight turning aloft. There is more variability in the environmental hodographs aloft than at low levels, observed by Bluestein and Jain [3]. Besides Mitra [4] mentioned that there is a close relation between