In our eagerness for monsoon to arrive, any long drizzle may make it seem as though the annual rainy season is finally here. But not all rain spells that arrive in June qualify to be monsoon showers. For example, the sudden showers that greeted a parched Chennai on Monday and Tuesday could not be referred to as monsoon showers, even though they occurred on the dates when the monsoon was estimated to hit India (in Kerala) first.
On June 8, the India Meteorological Department (IMD) announced that the monsoon had finally arrived in India based on the criteria they have formed to determine its onset. Kerala received the kind of heavy, full-day monsoon shower that is characteristic of the seasonal spell of rain on that day.
Explaining the key difference between pre-monsoon showers and monsoon showers, D.S. Pai, head of IMD’s Long Range Forecast division, told The Hindu that pre monsoon showers are usually convective rainfall, that is, they are heat induced rainfall.
“Such showers usually follow a few hot days, and occur generally in the evening or anytime after 12 p.m. But monsoon rain could arrive first in the morning or continue to pour throughout the day,” he said.
Monitoring
In order to define or distinguish between the different types of rainfall received around the time of monsoon onset, 14 monitoring stations situated in Kerala measure various weather indices such as the direction of the wind, its speed, intensity of rain, and atmospheric pressure*. The onset of monsoon over Kerala is announced when from light pre-monsoon showers; the first sudden increase in rainfall is observed.
Criteria
The IMD’s criteria for announcing monsoon arrival requires that 60 per cent of the 14 stations record more than 25 mm. of rainfall for two consecutive days.
Monsoon winds must flow in the south-westerly direction and atmospheric pressure* at the height at which these winds form (approximately 4.5 km above the sea level) should measure about 600 hecta pascal, the unit of measuring pressure. Wind speed should be at least 16-20 knots † and there should be good clouding close to the Arabian Sea,” Mr. Pai explained.
The outgoing long-wave radiation or the amount of electromagnetic radiation emitted as energy from earth and its atmosphere to space in the form of heat is also a criterion for declaring monsoon. Only if at least three of these criteria are fulfilled, monsoon is declared to have arrived.
How pre-monsoon showers differ
Pre-monsoon rain commences and withdraws from peninsular India earlier than in the northern parts, Mr. Pai said. While June is very much a pre-monsoon season for North India, the Southwest Monsoon generally strikes Kerala by June first week. Simultaneously, the monsoon reaches parts of Northeast.
Thus, as you can see, the drizzles that pour before monsoon arrives are fairly different in nature compared to monsoon showers. If you are still unsure whether the downpour you experienced today was pre-monsoon or monsoon shower, some of these additional points of differences might help.
Clouds
Private weather forecaster Skymet notes that clouds are vertical in nature during the pre-monsoon season, but monsoon clouds are stratiform clouds, which are sheet-like continuous layers of clouds. These clouds have less depth but the layers are thick and moisture laden, it says.
Dark monsoon clouds accumulate over Hyderabad . Photo: D. Chakravarthy
Pre-monsoon clouds hover about the sky in Bhopal. Photo: A. M. Faruqui
While pre-monsoon showers are accompanied by squally winds leading to dust storms, winds during monsoon are persistently strong, according to Skymet. Atmospheric pressure is higher before monsoons set in.
Also sea and land breeze remains prominent before the arrival of southwest monsoon in India, but with the available humidity and overcast skies, the breezes are not that marked during the monsoon, Skymet informs.
You are now ready to gaze at the sky and make your own informed forecast!
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* Atmospheric pressure is the force per unit area exerted against a surface by the weight of the air above that surface.
† Knot is one nautical mile per hour.
