Nor’wester over West Bengal and comfortability

Abstract

During the pre-monsoon months of March, April and May the weather over Kolkata is generally very uncomfortable due to high temperature and relative humidity. During this season Kolkata is affected by the nor’wester activity. Generally the nor’wester activity is defined as a disastrous weather activity and is locally called as ‘Kaalbaisakhi’ as it is ‘kaal’ or destruction during Bengali Baishakh (April-May) month over Bengal. However the downdraft associated with the nor’wester brings cool air and temperature sometimes drops by as much as 8 to 9 degree Celsius and the atmosphere becomes comfortable for some period. This is an entirely different and positive aspect of nor’wester.
The human comfortability depends upon many weather parameters apart from human physiological parameters. However the primary weather factors are air temperature and relative humidity especially for tropical areas. Many authors have worked on this and the well accepted formula for thermal index THI or Discomfort Index DI is taken from Thom (1957,1958) which is a function of dry bulb and wet bulb temperature i.e., THI or DI = 0.4 (Ta + Tw) + 15 where temperature is in ºF and is 0.72 (Ta + Tw) + 40.6 where temperature is in ºC. Various stages of comfortability are classified viz., (i) Discomfortable (ii) Partial discomfortable and (iii) Comfortable. Neglecting the effect of wind and further simplifying the equation for DI as a function of (Ta + Tw) the range varies from 27 to 55 in increasing order of temperature.
During the pre-monsoon months when the climate over Kolkata is very sultry and uncomfortable, people generally desire the onset of nor’wester for some temporary relief although it results into destruction. The nor’wester makes the weather comfortable after its occurrence and remains comfortable for 10 to 12 hours and some time even for a day. The present study is made taking six years data over Kolkata, both Alipore and Dum Dum. It is seen that except early morning of March the remaining period of pre-monsoon season is found to be uncomfortable climatologically. In the present study there were 91 occasions of squalls and it is found that most of the squalls occurred during the month of April and May and on most of the occasions nor’wester made the weather comfortable for a large part of the day in the month of April. By the end of May nor’wester activity also brought relief but not to that extent as in March and April to the people of Kolkata and neighbourhood.