After floods in California and Pakistan comes a scale for ‘atmospheric rivers’

Clouds carrying as much water as the Mississippi have caused misery in the US. Now meteorologists can compare such events

For skiers it has been an epic winter in California, with more than 16 metres of snow recorded at the Donner Pass in the Sierra Nevada. But for many people the excessively stormy winter has brought misery, submerging homes in snow, and causing widespread flooding and landslides across the state. The source of this string of powerful storms has been an “atmospheric river”, with clouds carrying as much moisture as the Mississippi.

Atmospheric rivers are nothing new, but they do appear to be growing more intense and frequent, driven by warmer temperatures and faster evaporation from the world’s oceans. Now scientists have devised an intensity scale for atmospheric rivers, enabling forecasters to rank the severity and identify extremes. The scale, described in the Journal of Geophysical Research: Atmospheres, mirrors the hurricane scale and runs from AR-1 to AR-5, with AR-5 being the most intense.

The storms that hit California this winter reached AR-4, while the atmospheric river responsible for Pakistan’s devastating floods in 2022 was AR-5. The scale enables meteorologists to compare atmospheric river events around the world, map out the most probable locations and help people better prepare. Strong El Niño years are more likely to have intense atmospheric rivers, and current forecasts indicate an El Niño is likely to develop by the end of this year.

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