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April Flowers Bring May Showers?

May 04, 2015 05:05 PM EDT
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We all know the popular springtime saying, "April showers bring May flowers," but the opposite may also be true.

That's at least according to a new study from the University of Michigan (UM) and Texas A&M, which shows that pollen grains might seed clouds, not just trees and plants.

Until now, pollen has been largely ignored by atmospheric scientists who study aerosols, or particles suspended in the air that scatter light and heat and play a role in cloud formation.

"The grains were thought to be too large to be important in the climate system, too large to form clouds or interact with the sun's radiation," Allison Steiner, a UM associate professor of atmospheric, oceanic and space sciences, said in a statement. "And also the large particles don't last in the atmosphere. They tend to settle out relatively quickly."

But Steiner and her colleagues wanted to see if smaller grains might have an effect on the planet's climate. The research team set out to see if moisture could cause the pieces to break down.

"What we found is when pollen gets wet, it can rupture very easily in seconds or minutes and make lots of smaller particles that can act as cloud condensation nuclei, or collectors for water," Steiner explained.

During the study, researchers tested pollen from oak, pecan, birch, cedar and pine trees, as well as ragweed - the most common sources of wind-driven pollen in the United States. They soaked two grams from each source in pure water for an hour. They then used an atomizer to make a spray of the moist pollen fragments and sent the spray into a cloud-making chamber in a lab. It turns out that three different sizes - 50, 100 and 200 nanometers - of all six types began to pull in moisture and form clouds.

"Samples entering the cloud chamber are exposed to moist conditions representative of the relative humidity found in the atmosphere," said Sarah Brooks, a professor of atmospheric science at Texas A&M. "If a sample is an effective cloud activator, droplets will rapidly grow on the sample fragments, forming large cloud droplets."

To confirm their findings, researchers looked at the samples under a scanning electron microscope. They found that grains that had begun as around 20-50 micrometers in size had been reduced to the nanometer size range - well within the size that can lead to cloud formation.

"It's possible," Steiner concluded, "that when trees emit pollen, that makes clouds, which in turn makes rain and that feeds back into the trees and can influence the whole growth cycle of the plant."

The results are described further in the journal Geophysical Research Letters.

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