Surface Wet Bulb Potential Temperatures in and Near India – Normal values and a Study In Disturbed Weather

Abstract

In the extra-tropical latitudes, it has become customary to identify air masses by the temperature and occasionally humidity characters of the air streams. Though in India various entities like Dry Bulb, Wet Bulb, Maximum and Minimum and their anomalies have been plotted, there has been no critical examination of some concept which could be used. The temperatures obtained by the Radio-sonde observations are still very short of the requirements in the tropics where the space gradient of quantities is small. Surface observations are available over many more stations and over a much longer period that comparisons would be easier. The quantity envisaged is the Wet bulb Potential Temperature introduced by Normand obtained by surface observations. The larger number of observations that are available partly offsets the effect of nearness to the ground.

Normal Wet Bulb Temperature charts for the twelve months are given. During a pre-monsoon cyclonic storm of 1930, the field over India has been plotted for a number of days. From the latter charts, an interpretation is made to reason differing air masses entering into the storm field. Incidentally, it is also shown that secondaries of western disturbances show off as disconnected pools of air which seem to move in some Northeasterly direction. It is suggested that if 24 hour changes in the quantity and plotted, it would help identification of air masses.

A simple analysis is given so that the derivation between Normand’s Wet Bulb Potential and Rossby’s Equivalent or Equivalent Potential Temperatures is brought out and a nomogram connection can be drawn.