After intense heatwaves, the world is now struggling with heavy rainfall and floods. While Western Europe, specifically parts of Germany, Netherlands, and Belgium received upwards of 140 mm of rainfall leading to devastating floods that killed hundreds, areas in China received over 600 mm of rain over a few days, the equivalent of the higher end of rainfall received in a year. Closer home, Mahabaleshwar received 802 mm of rain in just 33 hours.

But what does 802 mm of rain look like? With climate change on a fast track, weather systems are under increasing pressure, leading to more erratic events like extremely heavy rainfall. Understanding how much rain truly is being received is key to understanding the situation on the ground.

What is 1 mm of Rain?

Rain is measured in a few ways. Meteorologists measure rain volume received in a unit of time. This measurement is expressed in terms of “unit of length” per “unit of time.” In countries that use the metric measurement system, this is usually expressed in the form of millimetres (mm) per hour. However, 1 mm of rain refers to the "depth" of rain that would be received in 1 meter² (m²) or a square of one metre in length and width.

So 1 mm of rain translates to 1 litre of water in a single metre square.

So 802 mm of rain would mean that a single metre square would receive 802 litres of rain, 140 mm would mean 140 litres of water received in a single metre square, and so on. With this amount of rainfall received over large swathes of area, the amount of rain received quickly turns to thousands upon thousands of litres.

The average playing area of a cricket stadium is 17,000 m², and with 140 mm of rain, the stadium would be flooded with 23.8 lakh litres of water.

While a significant portion of rain is often drained through the soil and goes to fill up the water table located deep under the ground, it does not always turn out that way during heavy rainfall. For one thing, the soil can only drain so much water at a time. Add to that the fact that most of the soil is covered by cement or concrete in urban living areas, significantly reducing the maximum possible drainage area. For these reasons, urban areas often are the worst hit in events of heavy rainfall, as huge torrents of water have nowhere to go but into the streets and houses.

When Rain is Dangerous…

Countries have established meteorological departments along with other departments like those dealing with natural disasters and flooding that proactively watch out for the possibility of flood-causing rain. Many countries issue flood and weather warnings to inform citizens about upcoming events with the potential to cause damage.

The India Meteorological Department issues colour graded warnings for all weather events. A green alert means that there is nothing to worry about, while a yellow indicates severe bad weather over the next few days. An orange alert is issued for extremely bad weather and a red alert signifies that there is a discernible risk to life due to the weather event.

Current Nowcast at 1645IST today. For details kindly visit: https://t.co/w8q0AaMm0IReport any severe weather at:https://t.co/5Mp3RKfD4yDownload Damini App for Lightning Alerts:https://t.co/IYCSTf9o1Uhttps://t.co/gRs5rUfLW3 pic.twitter.com/62Dtu9JYId

— India Meteorological Department (@Indiametdept) July 23, 2021

(Edited by : Shoma Bhattacharjee)