This year’s Asian monsoon has been a particularly severe one. Millions were displaced in China and up to a third of Bangladesh was flooded as river levels soared.

In Mumbai, the monsoon has turned streets and highways into river rapids, with strong winds toppling trees. Up 60 percent of the city’s population lives in slums, highly susceptible both to natural disasters, like flooding, and to covid-19.

A recent report found that more than half of residents living in Mumbai’s slums had contracted the novel coronavirus, most of whom were asymptomatic. It is believed that a greater population density and shared facilities in slums makes them a especially susceptible to the virus.

According to meteorological records from the Santacruz Observatory in Mumbai, an astonishing 82.5 inches of rain fell in the city between July 10 and August 7.

Mumbai’s average for the year is about 94 inches. The majority falls during the summer monsoon in June, July, and August.

Around the height of the monsoon, which lasts in Mumbai from June through August, extreme rainfall can occur with rainfall rates topping 4 inches per hour. At least 8 days in the past month have featured more than 4 inches of rain in Mumbai.

Between July 15 and 16, an astronomical 13.26 inches of rain fell. 40.9 inches came down during a five day window surrounding those days.

For comparison, Washington D.C.’s average annual rainfall is 39.7 inches. In Boston, it’s 44 inches, and 62 inches in Miami.

According to The Indian Express, Mumbai has received more than three quarters of its typical monthly rainfall during August in just the first five days of the month.

The magnitude of the flooding was blamed on an aging drainage system, which is 140 years old and was designed when there was much more open space and permeable surfaces to absorb stormwater, which has now been replaced by concrete, according to The Indian Express.

Heavy rains will likely continue for the next few weeks before rapidly diminishing in areal coverage and intensity into September.

The term “monsoon” describes a seasonal wind shift. During the summertime, air over the Plateau of Tibet heats up, as does the atmosphere over the highlands of northern India and into parts of Bangladesh and China. This generates lift and upward motion. As that air rises, it leaves behind a sort of void, or “thermal low,” that draws in moist air from the south, over the Indian Ocean.

That moisture, enhanced by converging onshore winds from the Arabian Sea and Bay of Bengal, produces remarkably heavy tropical downpours.

In the wintertime, that mechanism reverses; the oceans prove warmer and generate lift, drawing in dry air from the north and shutting off the rainfall.

On Wednesday, a photo of a vegetable vendor overcome with emotion while sitting beside a flooded street went viral for capturing the “spirit of Mumbai, tired and beaten.” By Friday, the equivalent of more than $2,000 had poured in to help Ashok Singh, age 45, who was recently forced to pawn his wife’s mangala sutra — or wedding necklace — to purchase food and blood pressure medications for his family. The Mumbai Mirror reports his daughter will soon be married as well.

Human-driven global warming is increasing the intensity of the rainfall associated with India’s monsoon. A 2017 study found extreme rain events in central India tripled between 1950 and 2015. A modeling study conducted in 2019 finds “the frequency of single and multi-day extreme precipitation and flood events are projected to increase substantially in the future over the Indian sub-continental river basins.”