100 years of Chennai’s rain: What the Data tells us?

What does data tells us about Chennai's rainfall? Image generated by Meta Ai

Chennai's connection with rain is dramatic. There may be a strong downpour one day, followed by dazzling brightness the next day. Recently, I have noticed a peculiar trend of sunny mornings followed by rainy nights. Perhaps the rain does not want to disrupt our daily commute during the daytime. In my previous piece, we have seen the trends of rainfall patterns in Chennai in the last decade. I got this idea of going a little deeper into the 100 years rainfall data to find more exciting insights. Here we go…

Chennai Rainfall by season in the last 100 years. Data source: IMD

Broadly, a few things are quite straightforward:

• February to April is generally dry, with either no or very little rainfall recorded.

• May to September has seen occasional rainfall, serving as a perfect relief from the sweltering heat.

• October to December is when the actual show begins. According to the data, these three months account for about 60% of our yearly rainfall.

Chennaites have experienced both extreme rainfall and dry years over the decades. Take a look at the graph below and see if you can spot any interesting trends.

Annual rainfall trend in Chennai in the last 100 years. Data source: IMD

I am sure you must have identified some interesting trends too—please do share them in the comments to keep the conversation going!

Here are my observations,

1. Extreme variability 

As you can see in the graph, the rainfall pattern is not linear but has been changing constantly over time, and that’s variability. The normal range looks like somewhere between 1,000 to 1,500 mm, yet there are dots both far above and below this range, and that’s extreme variability. The data suggests that in 2003, Chennai received the lowest rainfall of 204 mm, and immediately in 2005, there was extreme rainfall with 2,311 mm. This demonstrates extreme fluctuations in precipitation, with a difference of over 2,000 mm recorded in just two years. This aligns with findings that global warming intensifies extreme rainfall events, creating tail-heavy distributions and shorter duration of rainfall extremes

2. Comparing decadal rainfall

The initial decade shows a somewhat linear pattern, but then between 1940–60, the wave looks erratic, showing both very low and very high rainfall recorded during that period. Then again, there is some calmness and stability, but after 1980, it looks disturbed. After 2000, it looks like it has lost control denoting increasing unpredictability with more extreme rainfall.

Another observation is that from 1901 to 1950, Chennai's average annual rainfall was 1,087 mm, with the highest recorded at 1,742 mm in 1903 and the lowest at 480 mm in 1904. In contrast, from 2001 to 2021, the average annual rainfall slightly increased to 1,097 mm, with a maximum of 2,157 mm in 2021 and a minimum of 290 mm in 2001. These changes echo findings from the Sahel, where the proportion of annual rainfall from extreme events increased significantly from previous decades

This point has led to a month-wise comparative analysis in the last two decades

3. Month-wise recent trends

The variability is extreme in the last two decades, as seen in the above graph, where you can spot two years with less than 500 mm of rainfall and three years with more than 2,000 mm of rainfall.

Heatmap of monthly rainfall pattern in the last two decades. Data source: IMD

The heatmap shows that most of the rainfall in the last two decades has been concentrated in the October–December period. However, there’s more to notice if we look closely. Between 2000 and 2004, rainfall was very low, shown by lighter shades on the map. This changed from 2005 to 2009, with darker shades indicating heavy rains. After that, there were some lighter periods again until 2015, which stands out as a year of extreme rainfall.

Since 2015, we can see a pattern of alternating years like 2016, 2018, and 2020 where rainfall is comparatively low than the subsequent years.

Interestingly, the heatmap also shows unexpected dark spots in February, March, and May (2004, 2010, 2016), which are usually dry months in Chennai. These unusual rains suggest that the rainfall patterns are becoming more unpredictable. 

The changes in rainfall pattern in the last two decades resonate with the predictions of IPCC 6th assessment report. It says, “Monsoon precipitation is projected to increase in the mid- to long term at the global scale, particularly over South and South East Asia, East Asia and West Africa apart from the far west Sahel (high confidence).”

This 100-year rainfall data tells a compelling story of change and unpredictability. The last 20 years clearly indicate climate change with increasing extreme weather events.

This century-long record not only brings to our notice the rainfall pattern of the city but also serves as a wake-up call. With more frequent droughts and devastating floods, these trends underline the urgent need for better water management, urban planning, and climate adaptation strategies.

What are your thoughts on these trends? Have you experienced the unpredictability of rain or any weather patter firsthand? Share your stories and insights in the comments.

Note: As I complete this draft on November 21, 2024, there was a 'super cloudburst' in Rameshwaram with 40 cm of rainfall in 10 hours. By the time I publish this on December 1, 2024, Cyclone Fengal had made landfall in Puducherry and adjacent areas, bringing record rainfall of around 45 cm in Puducherry.

References

1. Moustakis, Y., Papalexiou, S. M., Onof, C. J., & Paschalis, A. (2021). Seasonality, intensity, and duration of rainfall extremes change in a warmer climate. Earth's Future, 9, e2020EF001824.
2. Panthou, G., Vischel, T. and Lebel, T. (2014), Recent trends in the regime of extreme rainfall in the Central Sahel. Int. J. Climatol., 34: 3998-4006.
3. IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change