If the Sahara gets any drier, it could make California wetter. That’s because the dust and microbes that help form clouds can travel around the world on narrow air streams called “atmospheric rivers”, causing rain.

The particles, or aerosols, help clouds form by acting as seeds for water vapour to condense around. Atmospheric rivers carry this dust-laden water until they hit mountains, such as California’s Sierra Nevada, where their cargo turns to precipitation.

To see how these rivers affect weather, Kimberly Prather and colleagues at the University of California, San Diego, flew planes through six storms over the Sierra Nevada in 2011, gathering particles from the air. They also collected samples of rain and snow.

The team was able to identify the type and origin of the particles by analysing their chemistry and using satellite data to model where the storms’ air currents began. The storms picked up the vast majority of particles over Asia. In some cases, dust had been picked up earlier, over the Sahara and the Middle East.

In two storms with otherwise identical conditions, the one containing more dust was much wetter, suggesting that in future, extra dust from desertification and activities such as agriculture could make far-flung places wetter.

Unexpectedly, microbes made up 10 per cent of the particles collected. “The atmosphere is a global conveyor belt for microorganisms,” and they may be far more diverse than we previously thought, says David J. Smith of NASA’s Kennedy Space Center in Florida.

The role of aerosols in clouds is one of the largest uncertainties in climate modelling, says Anna Gannet Hallar of the Desert Research Institute in Steamboat Springs, Colorado. Mapping the geographical sources of these aerosols could allow researchers to better understand the global impacts of the dust stirred up by human land use, biomass burning and desertification.