Sprinklings of life appear key to the recipe for rain.
Lofted flecks of organic material like bacteria, pollen and fungal spores play a profound role in regulating rainfall patterns, a new study suggests. These bioparticles can make up a major portion of all the particles that can seed rain in the sky, and their levels fluctuate in a daily cycle, researchers report May 5 in npj Climate and Atmospheric Science.
The study is the first to clearly show that the movements of bioparticles drive daily fluctuations of rainmaking particles more broadly. “This really has not been included in any [weather] models before,” says atmospheric scientist Athanasios Nenes of EPFL, the Swiss Federal Institute of Technology in Lausanne. “It’s something we need to start thinking about.”
For many raindrops, life begins as an ice grain in a cloud. These grains ultimately melt as they plummet to Earth. But for them to form in the first place, there needs to be something for cloud vapor to freeze around — a nucleus. And bioparticles are “the best ice-nucleating particles that you have in the atmosphere,” Nenes says.
It’s not entirely clear why bioparticles possess this frigid ability. Regardless, researchers have shown that various bacteria, fungal spores and pollen possess molecules that promote ice growth. “We wanted to see how those things interact with the clouds,” Nenes says.
So, he and colleagues used sensors on Greece’s Mount Helmos to track rainmaking particles in the lower atmosphere during October and November 2021. One instrument measured the fluorescence of proteins and amino acids present in biological material, allowing the team to distinguish bioparticles from the rest of the dust.
Bioparticle levels were low at night but typically climbed 4 to 5 times higher by midday, they found. Moreover, during normal midday weather conditions bioparticles made up most if not all of the rainmaking particles detected, and roughly 10 to 30 percent when winds freighting Saharan dust coursed over the mountain. And when the researchers incorporated their results into a weather simulation, precipitation levels increased up to ten times.
Updating weather models will become more crucial as climate change compels ecosystems to release more bioparticles, Nenes says. “With warming everywhere across the globe,” he says, “this may lead to even more extreme events.”