Weather Bee | What is the onset of the southwest monsoon?

This brings us to the first measurable characteristic of the monsoon season: rains. Historical averages show that on June 1 there is a sharp increase in rainfall over the Kerala coast, which is also sustained. However, as is the case with all averages, the date of the beginning of such rains was not always June 1 even in the past. This is just the date one gets from calculating the averages of historical rain.

So why is it not raining in the northern half of India even when the monsoon onset this year was two days ahead of the usual date? One reason is that the usual date of monsoon onset over other parts of India is not June 1. Even in the normal course of things, monsoon rains cover all of the Indian landmass only by July 8. This can be seen somewhat in the animated map below. HT calculated the 1961-2010 average of rainfall — a benchmark for rain in India — for each day from June 1 to July 15. At the beginning of June, only the west coast and northeastern states get sustained light or higher intensity rainfall (that of 2.5 mm or more). This is where the monsoon is expected to start first. Such rain becomes sustained over all of India only after the first week of July.

To be sure, the map shown above is a rough approximation of the official criteria used to declare onset over different parts of India. For example, the onset over the Kerala coast is declared after looking at a few more weather features. One such feature is the presence of westerly winds — those blowing from the west — up to the height where air pressure drops to 600 hectoPascals (hPa). Similarly, the amount of cloud cover and water vapour in the atmosphere is measured from the amount of radiation emitted from ground to space (called Outgoing Longwave Radiation). Over other parts of India, the criteria may be just a sharp increase in the rain and the rain sustaining for a few days, but the threshold for each criterion depends on a region’s historical trends. This is why the absolute threshold of 2.5 mm used in the map above fails to show the monsoon’s progress over the southeastern peninsula, where the southwest monsoon season is not the main rainy season.

Is the usual date of monsoon onset being later than June 1 the only reason most of northern India was dry this June? Not really. While monsoon brings sustained rain over these regions, even storms bring occasional pre-monsoon rains to these regions. These storms were muted or absent for most of this June. Moreover, the progress of the eastern arm of the monsoon — the weather system progresses from both the west and the east, as the map above shows — became delayed after reaching sub-Himalayan West Bengal and Sikkim. It is a combination of all these reasons that has kept northern India dry despite an early onset of monsoon over Kerala.

Abhishek Jha, HT’s assistant editor-data, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.