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Scientists Unveil New Scale To Classify Strength Of Atmospheric River Storms

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Researchers have developed a scale to characterize the "atmospheric rivers," the regions in the sky in which water vapor travels from the Pacific Ocean.

The atmospheric rivers can influence the weather in the United States. In the West Coast, for example, atmospheric rivers are the source of most of the heaviest rains and floods, contributing to the area's water supply.

5 Categories Of Atmospheric Rivers

A team led by experts from the Scripps Institution of Oceanography at the University of California, San Diego created five categories of atmospheric rivers. These categories are based on the amount of water vapor being pushed and the duration in which they traversed a certain location.

The categories rank atmospheric rivers from 1 to 5 and use the words "weak," "moderate," "strong," "extreme," and "exceptional."

When an atmospheric river lasts in an area for less than 24 hours, it is demoted to a lower category. When it lasts in an area for more than 48 hours, it is promoted to a higher category.

The scale also assigns warnings to each category. Category 1 (weak), for example, is considered "primarily beneficial." Meanwhile, Category 5 (exceptional) is "primarily hazardous."

"The scale recognizes that weak ARs are often mostly beneficial because they can enhance water supply and snow pack, while stronger ARs can become mostly hazardous, for example if they strike an area with conditions that enhance vulnerability, such as burn scars, or already wet conditions," stated F. Martin Ralph, director of the Center for Western Water and Weather Extremes and an author of the paper published in the Bulletin of the American Meteorological Society.

The researchers explained that unlike the hurricane scale, the atmospheric rivers categories strongly considers duration as a fundamental factor because modest rainfall over a long period of time can also do serious damage.

Weather Forecast

Jonathan Rutz from the National Weather Service said that the scale they developed fulfills the demand for measurement of atmospheric rivers among forecasters in the United States.

"Forecasters in the western U.S. have been using the concept of ARs in their forecasting for a few years now, and many have been looking for a way to distinguish beneficial from hazardous AR storms," he shared. "The scale was designed partly to meet this need, and it is anticipated that it will be used extensively."

The researchers also hope that the new scale will be of use, especially in predicting the weather as the climate continues to change and evolve.

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