Cloudbursts: what are the 'rain bombs' hitting India and Pakistan?

Prime Minister Narendra Modi has told his fellow Indians that "nature has been testing us" after cloudbursts caused flash flooding that killed hundreds of people across the north of the country and in neighbouring Pakistan.

What causes cloudbursts?

Usually defined as more than 10cm (roughly 4 inches) of rainfall within an hour over an area less than 30 sq km (11.6 square miles), cloudbursts are caused by a combination of factors characterised by high humidity and low pressure.

When warm, moist air is forced upwards after, for example, encountering a hill or mountain, it cools and condenses, creating large, dense clouds. Once these become over-saturated they burst, releasing their rainfall all at once.

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"Sudden and violent", these intense deluges behave effectively like "a rain bomb", said The Associated Press. They "thrive in moisture, monsoons and mountains", all of which are present in India and Pakistan, "making them vulnerable to these extreme weather events".

Why are they dangerous?

The intense rainfall often triggers deadly flooding and landslides, as happened in northern Pakistan and Pakistan-administered Kashmir last week, killing at least 344 people, according to authorities. The death roll includes 24 people from the same family, who were swept away on the eve of a wedding. In Indian-administered Kashmir, at least 60 people have been killed in flash flooding, with 200 more missing.

Flooding resulting from a cloudburst killed more than 6,000 people in 2013 at Kedarnath in the Indian Himalayas.

Cloudbursts are so dangerous in part because there is "no forecasting system anywhere in the world" that can predict exactly where and when they will occur, said Asfandyar Khan Khattak, an official from Pakistan's northwestern Khyber Pakhtunkhwa province.

Are they caused by climate change?

Cloudbursts are a natural phenomenon, but extreme rain events and their related flash-flooding has worsened in recent years as a direct result of climate change.

A recent report from World Weather Attribution, an international group of scientists who study global warming's role in extreme weather, estimated that the 30-day maximum rainfall in northern Pakistan is approximately 22% more intense than it would have been without the impact of human-induced global warming.

Because a warmer atmosphere holds more moisture, "every tenth of a degree of warming will lead to heavier monsoon rainfall", said Mariam Zachariah, lead author of the study and an environmental researcher at Imperial College London.

A 2006 study published in the journal Science found "significant rising trends in the frequency and the magnitude of extreme rain events" in India in the second half of the 20th century as global temperatures have risen. And a study into the 2013 Kedarnath floods, published in Climate Dynamics in 2015, found more than half of the rainfall was likely to be linked to increases in greenhouse gases and aerosol particles in the atmosphere.

Khalid Khan, a former special secretary for climate change in Pakistan and chairman of climate initiative PlanetPulse, said global warming had "supercharged" the water cycle. "In our northern regions, warming accelerates glacier melt, adds excessive moisture to the atmosphere, and destabilises mountain slopes," he said. "In short, climate change is making rare events more frequent, and frequent events more destructive."