Is rain dependent on soil moisture?

It rains in summer most frequently when the ground holds a lot of moisture. However, precipitation is most likely to fall in regions where the soil is comparatively dry. This is the conclusion reached by researchers at ETH Zurich following an analysis of worldwide data. Their study contributes to a better understanding of soil moisture, a little explored climatic factor.
The water content of soil has a great impact on the regional climate, but many of the connections are still not clear. Researchers at ETH Zurich’s Institute for Atmospheric and Climate Science, together with colleagues from Belgium and the Netherlands, examined when and where it rains most frequently on summer afternoons. They wanted to clarify whether more rain fell on days when the soil was dry or moist. And where exactly it was most likely to rain on these days. The contradictory findings of other scientists was the reason for their study. Some researchers observed afternoon precipitation in particular on days with high soil moisture, while others seemingly came to the opposite conclusion – the rain fell in places where the soil, compared with surrounding areas, was driest.
The new study now provides some clarity. “On average, it rains most on days with high soil moisture,” explains Benoit Guillod, the first author, who led the study as part of his doctoral thesis in the group of Sonia Seneviratne, Professor for Land-Climate Dynamics, and who is now working at the University of Oxford. “Most precipitation falls, however, over the driest sub-region.” The phenomena can be explained in the following way: over the course of a day, the sun warms the earth’s surface, causing the water in lakes, rivers, oceans and the ground to evaporate. This water vapour rises throughout the day, where it meets colder layers of air and condenses. It then starts to rain. The soil’s moisture content plays a decisive role, particularly in areas far from the coast: The more moisture in the soil, the more water can evaporate, which increases the likelihood of precipitation.
But where exactly does it rain? Within a humid area, the areas with lower soil moisture produce the warmest air, permitting the water vapour to rise the highest and thus meet the colder air layers the soonest. As a result, it rains most frequently at these locations.