Normally we blame the movements of the jet stream for April showers, but new research suggests that moorlands, grasslands, meadows and woodlands could also be playing an important role in triggering these heavy bursts of rain.

As hay-fever sufferers can attest, many plants throw pollen into the air in the spring. But, other than causing sneezes, what impact do these particles have? Francis Pope, of the University of Cambridge, UK, has shown that pollen particles can affect cloud formation.

Studying pollen grains under a scanning electron microscope, and using an electrodynamic balance, Pope has shown that the large grain size and moderate water-attracting ability of pollen grains make them potent cloud-condensation nuclei under certain circumstances. “Large pollen particles can mop up smaller particles within a cloud, thus forming larger droplets and therefore possibly increasing a cloud’s precipitation probability,” Pope told environmentalresearchweb.

Essentially, the pollen acts as a point around which water droplets can grow. The faster a droplet grows and the larger it becomes, the quicker it will turn into a raindrop and fall out of the sky.

Pope’s results, published in Environmental Research Letters (ERL), showed that pollen grains can become active cloud condensation nuclei at supersaturations of 0.001% and lower – conditions that are not unusual in the atmosphere.

Although dust and soot are well recognised as cloud-condensation nuclei, little research had been done on pollen until now. However, Pope thinks pollen could have a significant effect on cloud formation. “Clearly, spring is the usual time for plant pollination, but different plants flower at different times of year,” he said.

In some regions and at certain times of year, pollen could be an important trigger of cloud formation indeed. For example, the clouds of pollen wafting up from northern European birch forests every spring could be contributing to April showers.

Right now no one knows how significant pollen is as a moderator of our weather, but Pope intends to investigate this further with colleagues at the University of Cambridge Environmental Processes group. The researchers plan to insert Pope’s pollen data into their high-resolution ATHAM model of cloud processes, and assess what kind of effect pollen has on cloud microphysics.

If it does turn out to be significant then pollen counts could help to improve weather forecasts. And looking to the future, the interaction between pollination and climate change may shift the timing of some of the classic weather patterns, such as April showers. “The changing of pollination seasons due to climate change could also effect precipitation,” speculates Pope. Next time you get caught in a downpour have a think about what lies upwind: it may be the pollen that is to blame.