The 2023 monsoon onset over the Kerala coast on June 8 was seven days later than usual and is one of only 10 times it has been this late. Its initial progress from Kerala was also slow. These problems seem to have had no impact on the total rain this monsoon has received. As of July 18, monsoon 2023 had a surplus of 3.3% compared to the 1961-2010 average, the benchmark for rain, according to India Meteorological Department’s (IMD) gridded dataset. Does this mean that it has rained faster this year than other monsoon seasons?

The 2023 monsoon onset over the Kerala coast on June 8 was seven days later than usual and is one of only 10 times it has been this late. Its initial progress from Kerala was also slow. These problems seem to have had no impact on the total rain this monsoon has received. As of July 18, monsoon 2023 had a surplus of 3.3% compared to the 1961-2010 average, the benchmark for rain, according to India Meteorological Department’s (IMD) gridded dataset. Does this mean that it has rained faster this year than other monsoon seasons?

The reasons why the question is important also partly answer it. For one, there is no deficit despite a delayed start to the season. While this is a sound reason, it is important to remember that a delayed monsoon onset need not always cause a rain deficit. For example, parts of northwestern India received surplus rain before the monsoon system reached there because of Cyclone Biparjoy. However, it is not always possible for storms to make up for an inactive or delayed monsoon, because they don’t cause as consistent rain as the monsoon does.

The second reason is that the delayed start was followed by slow progress almost for the entire first month of the season. As far as this can be seen in the country-level rain statistics, it answers the question itself. Up to June 25 (that is the first 25 of the 48 days of the season up to July 18) the season had a cumulative rain deficit of more than 20% compared to the 1961-2010 average, the benchmark for rain. By July 1, this number had decreased to a mere 3.8% deficit and the monsoon has maintained a small and consistent surplus consistently since July 7.

Clearly, it rained a lot in a short time to cover big deficits quickly. How fast did it do so? An HT analysis of IMD's gridded data shows that it took just 14 days for 50% of the total rain to fall. This rapidity in this part of the monsoon is also one of the fastest compared to other years: 21st fastest since 1901, the earliest year for which IMD has gridded data. To be sure, this rank partly results from a large number of years being tied together. The fastest 50% of rain took place (from June 1-July 18) in 12 days. Eight other years have achieved this speed. The next fastest speed is 13 days. This was recorded in 12 other years. The 14 days speed was seen in 26 years other than 2023.

While the statistic above shows that this is one of the faster years of rain, it also highlights one of the problems with using this statistic at the all-India level. There aren’t enough unique speeds for 50% rain to fall. There are only seven unique speeds at which 50% of rains have taken place since 1901. Three of them have been listed above and cover 47 years. The other unique speeds are 15, 16, 17, and 18 days which account for the remaining 73 years. This problem does not necessarily decrease for other thresholds, such as 75% rain (10 unique speeds), 95% rain (10 unique speeds), or 100% rain (2 unique speeds). Why does this happen?

This happens because some rain is bound to fall on some or the other place on almost every day of the year in a geographically diverse country such as India, but especially during the monsoon season. The two unique speeds for 100% rain are, therefore, 48 days (recorded in nine years) and 47 days (recorded in all other years) for a 48-day period.

The above discussion clearly suggests that monsoon speed is best measured at the most granular level. Does this show that 2023 is a faster monsoon? Answering this will explain little at this point. It was important to answer it at the country level because a large deficit was covered in a short amount of time.

However, with 70% of the rainiest part of the monsoon (the rest of July and August) yet to come, the trend of a wet phase covering for dry days is yet to happen for most of India. 37% of the country’s area is under a deficit of 20% or more as of July 18. The country as a whole does not have a deficit because surplus rain took place in regions that were not dry to begin with, as HT explained earlier this month. At a more granular level, the speed of rain – even for regions with deficit – will make sense when they have received rain that covers some of their deficit. Such stability is likely to take hold closer to the end of August.