Meteorologists releasing a weather balloon attached to an upper air sounding system to collect data leading to the forecast of the southwest monsoon at the IMD Met Centre in Thiruvananthapuram on Wednesday.
| Photo Credit: NIRMAL HARINDRAN
After a relatively moderate summer, the southwest monsoon is all set to start its march towards the hinterlands of the country after setting in over Kerala. Though the India Meteorological Department (IMD) has not officially declared the onset, the criteria required for the declaration of the monsoon were met in Kerala on Wednesday.
As per the criteria fixed by the IMD to declare the monsoon onset, 60% of the 14 enlisted weather stations should report rainfall of 2.5 mm or more for two consecutive days. Further, the depth of westerlies should reach up to 600 hPa and the zonal wind speed over a designated area should be of the order of 15–20 knots at 925 hPa. Also, the outgoing longwave radiation value, measured in watts per square metre (wm-2), should be below 200 wm-2 in a defined area.
Speaking to The Hindu, IMD scientists said all the criteria fixed for the declaration of the monsoon were met in Kerala on Wednesday. However, the formal announcement would be slightly delayed as there was a system in the Arabian Sea near the north Karnataka-Goa coasts.
“As per the usual practice, if there is a system in the Arabian Sea or the Bay of Bengal during the monsoon arrival, we will not consider it as monsoon. Once the system is detached, then only the monsoon can be considered,” sources said.
As of Wednesday, it is not clear if the system in the Arabian Sea will intensify into a cyclone. However, the models show its persistence in the Arabian Sea up to May 31, and it may travel onwards between Karachi and Muscat, causing a slight delay in the progression of the monsoon.
Nevertheless, it is expected that the onset will be confirmed within the forecast period between May 23 and 31. The IMD had forecast the arrival of the monsoon in Kerala by 27 May, with a model error of plus/minus four days.
Though this time the monsoon is expected hardly a week ahead of the normal onset of June 1, an early or delayed onset has no relation with the performance of the monsoon in the country or Kerala.
For instance, the earliest onset of the monsoon was on May 11, 1918 and the most delayed onset was on June 18, 1972. In both years, the monsoon was deficient.
The monsoon is basically an air stream laden with moisture. Its arrival is a gradual process beginning with a short period of transition from hot summer to a humid atmosphere with light rain. Similarly, the majority of the sky over Kerala is occupied by stratus-type clouds, which trigger uninterrupted showers for days during the monsoon season.
Published – May 21, 2025 07:20 pm IST
