By Nivedita Khandekar New Delhi, March 5 : The Deep Depression currently hovering near Tamil Nadu coast as on Saturday is only the eighth such cyclonic disturbance since 1891, the India Meteorological Department (IMD) records from 1891-2020 have shown. March, April and May are considered as the pre-monsoon months and generally cyclonic disturbances are found more in May and are not at all common in March. IMD records of 1891 to 2020 have shown that there were just seven such systems in all so many years and this is just the eighth one. Of the seven systems recorded so far in the month of March, only one has crossed Tamil Nadu while the rest were weakened even before it reached the coast. That one had turned into a severe cyclonic storm, the data showed. The current Deep Depression too is set to weaken before it touches the Tamil Nadu coast. As on Saturday evening, it lay centred over the southwest Bay of Bengal at about 370 km northeast of Trincomalee (Sri Lanka), 320 km east-north-east of Nagapattinam (Tamil Nadu) and 270 km east-south-east of Chennai. Explaining the reasons, IMD Director General Mrutyunjay Mohapatra said, "Cyclone genesis depends on a lot of factors such as wind pattern in the atmosphere, the sea conditions among others. A phenomenon called wind shear played a role in this case.." Wind shear means difference of winds in lower layer and upper layer. "When the cyclonic disturbance reaches near the coast, there will be reduction in sea surface temperature and ocean heat content. Plus, there will be incursion from dry air coming from the landmass of India -- dry and cold air is still prevailing - which is preventing further intensification of the system," he told IANS. /IANS