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Atmospherical Science

Half of the world’s annual rain falls in 12 days

In just 12 days, on average, the world receives half of the rainfall expected for the whole year, according to analyses conducted by a pair of researchers from the Institute for Atmospheric and Climate Science in Zurich, Switzerland, and the US National Center for Atmospheric Research in Colorado, USA (Geophysical Research Letters, October 19). Based on daily average rainfall data collected by 185 meteorological stations around North America, Eurasia, and Australia, they identified variations in annual average rainfall volumes between 1999 and 2014 caused by the warming of the Pacific Ocean (El Niño) and other short-term climate fluctuations. They then cross-referenced this information with data from the same period collected by NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite, finding that in all regions, approximately 75% of annual rainfall falls on the 30 wettest days of the year, while more than 12.5% falls on the two rainiest days and about 8% on the day that it rains the most. The researchers also used climate models to project how rainfall may change in the future, especially between the years 2085 and 2100. In a scenario where the atmospheric carbon dioxide (CO2) level reaches 936 parts per million (ppm)—it is currently approximately 400 ppm—this uneven annual rainfall pattern could become even more accentuated. The study estimates that by the year 2100, half of the world’s annual rainfall could fall in as little as 11 days.