Different shades of Onset of Southwest Monsoon in India

Normal date for the onset of Monsoon in mainland of India (Kerala) is June 1. The month of June is the onset period for the four month-long Monsoon season. By the end of June, most parts of the country (barring west Rajasthan) are covered by the Southwest Monsoon. Different shades of weather activity generally precede and follow the onset of Monsoon in a particular place. Alternatively, we can say that different shades of onset phase are witnessed in various parts of the country.

Onset of Monsoon in India

The onset phase of Monsoon in Peninsular and Northeast India differs greatly from the onset phase in Northwest Plains. As Monsoon arrives, vigorous rainfall activity is witnessed along the West Coast of India and Northeast India. In fact, even places along the East Coast of Peninsular India receive heavy showers.

The Western Ghats receive approximately 800 mm of rain in the month of June, while the adjacent interiors of Peninsular India receive just about 100 to 150 mm of rainfall. Northeastern states also receive good showers of 500 mm. Monsoon showers taper down, as we move from east to west with West Bengal receiving about 250 mm of rain, Bihar 150 mm, Uttar Pradesh 100 mm. The Northern states of Punjab, Haryana and Delhi receive about 50 mm of rainfall each, while west Rajasthan records less than 15 mm of rain.

Looking at the above figures we can conclude that vigorous Monsoon activity cannot be expected when Monsoon arrives in the Northern plains.

Prerequisites for declaring onset of Monsoon

In places where the monthly average rainfall itself is less, Monsoon surge becomes active but very heavy showers are not noticed. Onset of Monsoon is generally declared in these places even after 5 or 10 mm of rain, given the fact that other prerequisites for Monsoon onset are met. Before the arrival of Southwest Monsoon in a particular place, the following conditions are witnessed:
a.Temperatures come down considerably.
b.Cloud cover persists throughout the day.
c.Rainfall pattern changes, as in rainfall is witnessed during late night and morning hours.
d.With easterly winds blowing over the area, humidity rises considerably.

Monsoon onset in North and East India

The Monsoon trough, throughout the season, lie below the states of Punjab, Haryana, Delhi, Uttar Pradesh and Bihar. Moreover, most of the Monsoon systems developing in the Bay of Bengal and the Arabian Sea weaken by the time they reach the northern half of the country. No singular weather system can bring very heavy showers. Only the presence of Western Disturbances in the hills of North India, along with induced cyclonic circulations in the plains can bring vigorous activity during the onset phase of Southwest Monsoon.

Also read, Promising week ahead for Monsoon in India

Image Credit: wsj.com