GUWAHATI, May 21 � The frequency of thunderstorm during the pre-monsoon season in the current year is not abnormally high even though the frequency of squall (in the instant case Bordoichila) days in April this year was higher compared to the previous years. However, to link these developments with the phenomenon of climate change we need to examine the climatological figures for a longer period, (not less than 100 years) said Regional Meteorology Centre (RMC) sources here.
During this year, thunderstorm days over Guwahati airport are comparable with those of the previous years. For, in 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 thunderstorms were recorded on 45, 40, 33, 48, 34,36, 39, 26 and 37 days respectively, said the sources, adding, hails and squalls are generally associated with severe thunderstorms.
Month wise also, thunderstorm incidents were spread over the three months of March, April and May (up to May 17). The same conclusion can be derived in the case of hailstorm also.
But the frequency of squall days during this year was significantly higher specially during April (10 days). To investigate the reasons we have to look forward to the normal comprehensive situations of the season and the prevailing weather condition during this year, said the sources.
Usually, the main seasonal feature during April and May is the existence of a trough of low pressure (on the lower level up to 0.9 km above sea level) over Uttar Pradesh and north Bihar, with its axis or trough line running in a West-Northwestern - East-Southeastern direction. Around this trough, West-Southwestern /Westerly current normally blows over Assam (NE India) in the lower levels.
With the passage of western disturbances or otherwise, the seasonal trough often gets accentuated and extends south-eastwards into the Gangetic West Bengal and neighbourhood or eastwards into North-Bay of Bengal causing incursion of moist southwesterly to southerly air from the Bay of Bengal into Assam. These situations are responsible for thunderstorm activities.
During this year, the North-South trough from Sub-Himalayan West Bengal (SHWB) to Gangetic West Bengal or North-Bay of Bengal is observed almost every alternate day with or without a cyclonic circulation over SHWB and neighbourhood areas. Also the incursion of moisture in lower levels from the Bay of Bengal over our region is almost regular.
Again the availability of local moisture due to excess rainfall activity over Assam during the season (March � 20 per cent above normal, April � 93 per cent above normal, May � 46 per cent above normal) seems to be responsible for development of severe thunderstorm activities. At present we may attribute the above situations to severity of thunderstorm in terms of squall during this year, said the sources.