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Humidity and location determine if there will be rain or snow

Researchers at the University of Colorado Boulder have developed a new tool that demonstrates why a particular region receives rain or snow in near-freezing conditions. The experts created a map of the Northern Hemisphere that shows precipitation variability is primarily influenced by humidity and location.

While the air temperature threshold for rain versus snow is commonly considered to be 32 degrees Fahrenheit, the new research indicates that this is not always an accurate predictor of which type of precipitation will fall.

The researchers found that coastal areas have a cooler threshold for rain, which means that below-freezing temperatures may still produce rain. On the other hand, inland and mountainous areas are more likely to receive snow when the temperature approaches the threshold, even at several degrees above freezing.

“In Denver, Colorado, it might be 40 degrees and snowing. But in Charleston, South Carolina, it could be 28 degrees and raining,” said co-author Noah Molotch. “This study shows these fine-grain differences on a hemisphere-level scale for the first time.”

The research was focused on nearly 18 million precipitation observations spanning over 100 countries and four continents across the Northern Hemisphere.

“Snow and rain differ greatly in the ways they affect climate,” said study co-author Ben Livneh. “Snow acts as a water reservoir and reflects incoming sunlight, whereas if the same amount of precipitation falls as rain, that can dramatically change water resource management decisions.”

Up until now, land surface models have mainly predicted rain and snow based on the air temperature threshold of 32 degrees. The new research shows that relative humidity and surface pressure must be factored in as well.

“The rain-snow air temperature threshold is primarily a function of relative humidity and methods incorporating humidity and elevation are more likely to predict rain and snow correctly,” said lead author Keith Jennings. “If you just use 32 degrees Fahrenheit across the board, your estimates will be wrong in lots of places.”

The United States had the most rain-snow variability of any country examined in the study. The research could be used to inform future climate and land surface models, which would allow scientists to make more accurate weather predictions.

The ability to differentiate rain from snow is extremely important for monitoring the Earth’s hydrologic cycle. This information is also critical for water management, particularly in areas that experience a lot of drought.

The study is published in the journal Nature Communications.

By Chrissy Sexton, Staff Writer

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