CYCLONES

A tropical cyclone is  an intense  low pressure area or a whirl in the atmosphere over tropical or sub-tropical waters, with organised convection (i.e. thunderstormactivity) and winds at low levels, circulating either anti-clockwise (in the northern hemisphere) or clockwise (in the southern hemisphere).  From the centre of a cyclonic storm, pressure increases outwards.  The amount of the pressure drop in the centre and the rate at which it increases outwards gives the intensity of the cyclones and the strength of winds. As per the criteria adopted by the World Meteorological Organisation (W.M.O.), India Meteorological Department classifies the low pressure systems in the Bay of Bengal and in the Arabian Sea   into 7 classes. Low pressure areas with maximum sustained surface winds of speed between 31 & 61 km.p.h. (17 to 33 knots) are called tropical depressions