Pollen May Help Clouds Form, Increase Rainfall

Some scientists think trees could be influence the amount of rain in a particular area. Joseph Xu, Michigan Engineering Communications & Marketing

It's common knowledge that rain helps flowers bloom. But new research suggests the opposite may also be true: Pollen from flowers could help form clouds and bring about rain.

In a study published this week in the journal Geophysical Research Letters, atmospheric scientists from the University of Michigan and Texas A&M found that pollen from six different plants and trees effectively formed clouds in a laboratory setting.

For rain to fall, you first need clouds. These are created when gaseous water, or steam, condenses into tiny droplets. Small particles known as cloud condensation nuclei start this process, giving the water something to condense upon. But water won't glom on to just any particle; it must be the right size—minuscule—and the right material, specifically something that can absorb moisture.

Scientists used to think that pollen grains were too big to function in this way, says atmospheric scientist and study co-author Allison Steiner. But her work and that of others has shown how pollen grains fracture when they encounter moisture, producing particles that can be thousands of times smaller. These little bloom-bits are both the appropriate size and made of the right stuff—hygroscopic carbohydrates and proteins—to form clouds.

The study shows that "pollen can help clouds form, and you might effect the cloud properties enough that you could change the rain," says Athanasios Nenes, an atmospheric chemist and climate scientist at the Georgia Institute of Technology who wasn't involved in the research.

Steiner says she's already working toward the next step: testing the quantity and distribution of these particles in the atmosphere, to quantify

what impact pollen might have on clouds and weather patterns.

Her study took place in a laboratory, so it's still unclear to what extent pollen affects clouds in nature, she says. Nevertheless, she thinks "the results translate to the real world. Certainly [pollen] is involved in cloud formation in specific areas at specific times of the year…but how much of the time, that's something we're still trying to understand."

One theory she finds particularly intriguing is the idea that "trees themselves could be influencing the amount of rain they get."

When a cloud becomes thicker, it eventually produces more rain, Nenes says. But by enabling cloud formation, pollen might actually postpone the point at which rain falls. Generally clouds don't precipitate while they are still building and becoming thicker, he adds. But thicker clouds do produce more rain (and more intense downpours) than thinner clouds, he says.

If pollen can help make clouds thicker and whiter, this could also cause local cooling by reflecting more sunlight, Nenes says. It will be interesting to see how significant this effect is in the real world; factoring it into climate models could help improve them, he notes.

But Dan Cziczo, an atmospheric chemist at MIT, says that while the study is well-done, it is rather limited since nobody knows the exact quantity of pollen particles in the atmosphere. For that reason, pollen may not have any effect on cloud formation outside of a lab. "It's putting the cart before the horse, so to speak," he says.