New Delhi, May 25 -- Monsoon has arrived over Kerala, earlier than the usual date of June 1, the India Meteorological Department (IMD) said on Saturday morning. This year the state sees the "earliest onset over the Indian mainland since 2009, when it began on May 23," according to IMD data. A well marked low pressure area over east central Arabian Sea off south Konkan coast concentrated into a depression and lay centered over east central Arabian Sea and adjoining south Konkan coast. It is likely to move nearly eastwards and cross south Konkan coast between Ratnagiri and Dapoli as a depression during forenoon of Saturday and in association with this system rainfall has increased over Kerala, according to IMD. IMD follows a set of criterion to declare monsoon onset. If, after May 10, at least 60% of 14 stations - Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore - report rainfall of 2.5 mm or more for two consecutive days, the onset of the monsoon over Kerala will be declared on the second day, provided the wind pattern is south-westerly and Outgoing Longwave Radiation (OLR) is low. OLR represents the total radiation going to space emitted by the atmosphere or extent of cloudiness. As the monsoon has arrived in Kerala before time, this marks the earliest onset over the Indian mainland since 2009, when it began on May 23, according to IMD data. HT reported on May 20 that one important factor for a normal monsoon over the country is formation of a so-called 'heat low' over Northwest India. This creates a trough of low pressure that sucks moist air from the monsoon trough. In its absence, the monsoon may be deficient. At present, there is no 'heat low', but the India Meteorological Department and independent scientists have said monsoon will arrive before its normal arrival date....