do bacteria make it rain?
MICROBES ARE EVERYWHERE—IN YOUR GUT, ON YOUR SKIN, EVEN IN THE sky. Along with water vapor and particles, clouds also contain living bacteria. And certain airborne species have a superpower: They can cause water to freeze into ice, which in turn prompts the heavens to open. Once frozen granules form around bits of dust, minerals, or tiny organisms, they fall, melting on the way down to become raindrops or forming flakes to make snow. This means the little life-forms might contribute to rainfall—but how much? “If you loaded up a cloud with these microbes, it would create freezing and the processes that lead to precipitation,” confirms University of Florida’s Brent Christner, who studies atmospheric bacteria. “The unknown factor is, are there enough of them up there to matter?” Christner and other researchers are sending weather balloons and planes on sampling missions to try to answer the question—and shed light on what these citizens of the sky get out of the deal. They might use clouds to travel long distances before raining down on fresh new habitats.
1. Cloud Storage
The frigid vapor a couple of miles above Earth’s surface seems pristine, but it’s filthy with particles, ranging from clumps of sulfates and mineral dust to bacteria. Each cubic foot of air can contain from 300 to 30,000 microbes.
4. In Formation
Scientists think that as water molecules in the cloud get near the proteins, chemical forces cause the H2O to line up along the bacterium’s surface in an orderly lattice. That positioning encourages the molecules to form ice.
2. Zoom In
At least one of those microbe species, Pseudomonas syringae, can spur ice formation at relatively warm temperatures. First identified decades ago on plants, the easy-togrow bacteria now helps ski resorts make snow for their slopes.
3. Ice Shaping
P. syringae produces proteins that arrange themselves on the bacterium’s surface. Scientists know that the pattern matters: When a microbe dies, or scientists deliberately modify its protein arrangement, it doesn’t form ice as quickly or easily.
5. Cold Fusion
The frozen kernel attracts more and more H2O, gradually growing larger and heavier. When it gets too heavy to remain airborne, it plummets to Earth as snow or rain, depending on the temperatures it encounters on the way down.