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Soil moisture alters next-day rainfall in the United States

Will it rain tomorrow? Don’t look to the skies, because the answer depends partly on the dampness of the ground beneath your feet. Although the seasons and long-term weather patterns like El Niño matter more, a new study finds that soil moisture also plays a role in influencing next-day rain in the United States. For more than a third of the country, out-of-the-ordinary soil moisture can change the likelihood of next-day rain by a median factor of 13%. The effect depends on where you live. In the West, the feedback is positive: Wet soils increase the chance of a next-day downpour, and dry soils diminish that chance. But east of the Mississippi River, the feedback flips: Wet soils lower the likelihood of rainfall, and dry soils raise it. Why? Rainfall, in general, depends on two things: moisture and daytime heat that create rising, raincloud-producing updrafts. In the sunny, arid West, there is plenty of heat but limited moisture, and so a process called moisture recycling is at work—today’s storms supply the water that evaporates into tomorrow’s rainclouds. But in the east, moisture abounds, and the sun’s energy often goes into evaporating it. This keeps damper regions cool. Rare dry patches are the ones that can heat up enough to form rainclouds. The study, published today in Science, used 9 years of soil moisture data from NASA’s venerable Aqua satellite along with data from a network of rain gauges. Scientists are hoping that newer satellites, like Europe’s Soil Moisture and Ocean Salinity or NASA’s Soil Moisture Active Passive, can start to accumulate long-time series of soil moisture data. Once that happens—and its predictive power is validated—the approach could be folded into weather forecast models, researchers say.