PERCEPTION STUDY OF NORWESTER – A RECENT RESEARCH TREND TO UNDERSTAND THE EARTH SYSTEM DYNAMICS

                                                                 2

Ms. Saswati Roy,Junior Research Fellow, Visva Bharati, Santiniketan,India

 Prof. Malay Mukhopadhyay, Visva Bharati, Santiniketan,India

During the pre monsoon season i.e. from March to May (now also in June) the eastern and north eastern states of the subcontinent  like West Bengal , Assam, Bangladesh, parts of Bihar , Jharkhand and parts of Orissa experience dramatic appearance of a special type of thunderstorm. Strong heating of landmass during mid day initiates convection over Chhota nagpur plateau which moves south east and gets intensified by mixing with warm moist air mass and generate thunderstorms (Science Plan, 2005).These thunderstorms are locally named as “kalbaishakhi” which means calamities in the month of Baisakh(the monsoon month of the Bengal calendar). In the other parts of the country these storms are designated as “Norwester” as they move from North West to south east.

        A) Characteristics of kalbaisakhi storm

i) Norwesters in Bengal starts as early as March. The number gradually increase reaching maximum in April and May and decreases thereafter along with onset of monsoon.(Mishra,2006)

ii) Chhotanagpur plateau is considered as source region of Kalbaishakhi.

iii) Usually Kalbaishakhi strikes in the afternoon.

iv) It originates from cumulonimbus clouds.

v) Kalbaishakhi has three stages of development- cumulus, mature and dissipating.

vi) Norwester is accompanied by rainfall, thunder, lightning, hail and squall line.

vii) The storm has a wind speed of 55-80 km/hour but in some cases it may exceed 100km/hour.

viii) Duration of Kalbaishakhi over a station varies from a few minutes to an hour.

B)      Mechanism of kalbaishakhi storm

Thunderstorms are highly localized weather phenomena and are randomly distributed. Thunderstorms are classified on the basis of its mechanism. Among little type of thunderstorms Kalbaishakhi is a local heat thunderstorm. This local heat thunderstorm form because of thermal convection produced by intense surface heating during daytime. Conditionally unstable air is very much necessary for their occurrence. They mostly originate as isolated convection cells and are usually smaller in size. The equatorial belt of calm with warm and moist air provides the most favorable conditions for their development (Lal D.S., 2004). Meteorological studies reveal that during the summer months there is normally moist southerly air in the lower levels over Bengal and dry westerly or north-westerly air in the upper levels .This distribution of air masses gives rise to latent instable conditions which are prime necessity for the occurrence of Norwester over the Bengal basin in summer months (Mishra,2006). There are 3 stages in the development of a thunderstorm.

i) Cumulus stage: This stage is characterized by updrafts of air.

ii) Mature stage: This stage is marked by both updrafts and downdrafts and the severity of thunderstorm reaches at its peak. In this stage surface weather is characterized by rainfall, lightning, hailstorm, high velocity winds.

iii) Dissipating stage: The disappearance of updrafts marks the beginning of the dissipating stage.And finally this mechanism designates the end of the storm.

Scientific forecasting of kalbaishakhi

•       Highly localized and difficult to predict or forecast.

•       Forecasting techniques based on data collected by AWS (Automatic Weather Stations)  and RMS(Regional Meteorological Station)

•       The STORM (Severe Thunderstorm Observation and Regional Modeling) project has been undertaken by the Govt. of India, 2007, to understand the nature of the pre monsoon storms.

•       The project is assisted by the working of 2 important components - AWS and RMS. The AWS is equipped with sensors to measure the following parameters:

Air temperature at 1.5 m height, Relative humidity, Atmospheric Pressure, Wind speed, Wind direction, solar radiation, Rainfall, Soil temperature at 10cm & 30 cm depth, Soil moisture at 10cm & 30cm depth.   

Forecasting of kalbaishakhi by means of people’s perception

It is very much difficult to forecast any kind of atmospheric phenomena but it is extremely important. The loss of life and damage to properties can be considerably reduced if protective measures are taken in time which requires timely and reasonable accurate prediction to storms. In case of Kalbaishakhi it is much tougher to forecast in compare to others. The forecast can be issued in the morning for small areas as most of Kalbaishakhi occur in the afternoon and is difficult to anticipate the changes likely to occur during the day and to estimate the conditions in the afternoon. At this stage it does not appear to be possible to state definitely what conditions in the morning are sufficient to give rise to Norwesters in any part on Bengal, Bihar or Orissa. Predicting the weather has been a human passion for ages. Farmers wanted to know what kind of a season was forthcoming -- would there be enough rain or would there be drought? Would the winter be cold or mild? How could they tell if a violent storm was coming? Not having Doppler radar in their log cabins, they relied on natural signs to help them predict the weather. Sometimes the most convenient barometer was their farm animals.

Scientific relevance behind people’s perception

Today, forecast is done with the help of satellite imagery weather data gathered from different meteorological stations. But in this paper present authors have attempted to draw out the latent science in the people’s perception of forecasting the Kalbaishakhi. Several such perceptions arose out of prominent changes observed in the environment and animal behavior. Yearly occurrences of such changes installed within the local people resulted in some consolidated practices that got infused within their tradition. The practice of uttering folklores related to such observations became a part of their customs.  Thus, by looking into these performances the modern man must bear their preparedness through:

•       Local level forecasting through different observations and folklores.

•       Observation of animals, as they have stronger instincts and are often more sensitive towards change in weather parameters.

•       Synergism that must be attempted between science and perception to draw out risk mitigation applications.

 Following are the perception and the relevant science within:     

Physical components and people’s perception

*Red sky at night, sailor's delight/ Red sky in morning, sailor takes warning.

        If there is a red sky during sunset (looking to the west), there is a high pressure system with dry air that stirs dust particles in the air, causing the sky to look red. A red sky in the morning (in the East, where the sun rises) means that the dry air has already moved past, and what follows behind it (on its way towards the observer) is a low pressure system that carries moisture. Presence of red clouds in the western horizon with gaps indicates that the storm will come within 5-6hours.

  *Knots of thread get tighter due to high relative humidity before storm

       * If salt is sticky and gains in weight/ it will rain before too late. Salt being a good absorbent of humidity becomes sticky and gains weight immediately before storm as there is high relative humidity in the air.

          *If temperature falls immediately with rainfall during storm, then lightning will be less in number and vice versa

           * When the ditch and pond offend the nose/then look out for rain and stormy blows.

Ponds and ditches stink and smell stale before Kalbaishakhi. Plants release their waste in a low pressure atmosphere, generating a smell like compost and indicating an upcoming rain. Swamps release gasses just before a storm because of the lower pressure, which leads to unpleasant smells. Even a proverb says "Flowers smell best just before a rain." Scents are stronger in moist air, associated with rainy weather.