Not only do pollen grains make us sneeze, they may also be reducing rainfall. In contrast to earlier findings, a new study shows that pollen grains suppress rainfall by an average of one-third in non-polluted continental areas, and in some areas they ensure that not a drop falls.

During pollen season every cubic metre of air contains somewhere between 10 and 1,000 grains of pollen. But when the weather is wet or humid these pollen grains can swell and burst, fragmenting into up to 100m tiny particles per cubic metre.

By modelling the impact that sub-pollen grains might be having, Matthew Wozniak, from the University of Michigan, and colleagues have shown that these minuscule particles are “constipating” clouds, encouraging the formation of tiny droplets, too small to fall as rain. Their findings, which are published in Geophysical Research Letters, show that in the US the effect is greatest in western continental areas, where as much as 100% of rainfall may be suppressed.

Potentially these grains are adding to hay-fever misery, too. “Sub-pollen particles are more likely to travel far from their source. This could extend allergy risk, and also explain risks related to thunderstorms or other precipitation events,” says Wozniak. “Including sub-pollen particles could improve allergy risk forecasts.”