Each year, eastern India and Bangladesh experience widespread persistent heavy rain as a result of storm systems that form over the Bay of Bengal. Pakistan, on the other hand, is more arid, and its storms tend to be more fleeting, producing only locally heavy rainfall.

After examining radar data from the Tropical Rainfall Measuring Mission satellite, Robert Houze of the University of Washington and colleagues identified a storm system that had formed over the Bay of Bengal in late July and moved unusually far to the west as the source of the rainfall that caused the Indus River to overflow. As the moisture-laden clouds crossed the Himalaya, they poured abnormal amounts of rain on the barren mountainsides, which then ran into the Indus.

‘We looked through ten years of data from the satellite and we just never saw anything like this,’ Houze said. ‘The satellite only passes over the area a couple of times a day, but it just happened to see these systems at a time when they were well developed.’

March 2011