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Raindrops and snowflakes generally start to form around something else in the air, like a speck of dust, but sometimes that something else is bacteria.

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Clouds capture our imagination when we’re kids.

They look like fluffy cotton balls, and any Mario game would lead you to believe that you could jump around on them. Eventually you learn that you would fall right through them though, because they’re made of tiny ice crystals or water droplets.

Not only that, but it turns out that clouds are also chock full of… bacteria. And those bacteria may actually be pretty important, and play a role in precipitation patterns around the globe. For raindrops or snowflakes to form and grow big enough to fall to Earth, there generally needs to be something for the water molecules to start collecting onto, called a condensation nucleus.

This can be a speck of dust or other inorganic stuff drifting around in the air, but sometimes a floating microbe will do instead. Some microbes can actually cause ice to form at warmer temperatures than inorganic particles can, thanks to special proteins that encourage the orderly arrangement of water molecules. We think these proteins on the surface of their cells can arrange water into strips of loosely and tightly packed molecules.

This mimics the boundary between liquid water and air, where molecules are more likely to have space to form ice crystals in nature. When microbes act as condensation nuclei, scientists have a special word for it: bioprecipitation. And even though it’s hard to study what’s going on at the microscopic level inside a cloud, we do have some solid evidence that bioprecipitation happens.

In 2008, researchers collected freshly fallen snow from a bunch of locations in Europe, North America, and Antarctica. They melted the snow samples and filtered out any tiny particles inside, and then mixed the particles into different samples of pure water to control as many variables as possible. Then, they slowly re-froze each mixture, reasoning that a higher freezing point probably means that there were more nuclei in the sample.

Basically, with more molecular help from the ice nuclei, water doesn’t need to be quite as cold to form ice crystals. Then, they treated the samples to kill any microbes that were hanging around and repeated the experiment to see if the freezing points changed. This gave them an estimate of how many of the nuclei were biological.

And their math showed that there were around 4 to 120 nuclei in each liter of melted snow, most of which were probably microbes. Plus, microbes were in every sample, even the ones from Antarctica. And this suggests that they can travel /really/ long distances in clouds, even between continents.

And in 2010 and 2011, a researcher collected hailstones after three storms on or around the campus of Montana State University and analyzed their structures with more melting and refreezing experiments. He found lots of bacteria in the hailstones’ cores, thousands per milliliter of meltwater, but almost none in the outer layers. In other words, it looked like bacteria were the seeds that got the hailstones forming.

So clouds might be a helpful way to travel around, while acting as condensation nuclei gives bacteria a way to get back out of the atmosphere and to the ground. Scientists think that a lot of the bacteria that can do this are probably plant pathogens, microbes that cause diseases in plants. Their ability to get water to start freezing at higher-than-normal temperatures lets them make ice that ruptures plant cell walls, so they can feast on goopy nutrients.

Plants, in turn, release bacteria into the air along with the water vapor they produce, so microbes can get back into the atmosphere and keep the cycle going. Precipitation can also spur plant growth, which provides more food for the bacteria. One specific bacteria species found to do this is a plant pathogen called Pseudomonas syringae.

Its specialized ice crystallization protein, InaZ, is actually used in machines that make artificial snow for places like ski resorts! But bioprecipitation isn’t limited to bacteria. Scientists think that things like fungi, diatoms, and algae could all act as nuclei too.

And researchers are beginning to think that this microscopic life could have a bigger influence on global weather patterns than we expected. This is difficult to study directly, but more studies have been done in recent years that suggest that microbial condensation nuclei do have important effects on clouds and precipitation patterns in different regions. Turns out that these organisms that are too small for us to see are hanging out in rain and snow, too.

Whether it’s natural, or in our man-made winter wonderlands. Thank you for watching this episode of SciShow! If you want to learn more about ice, check out our list show where Olivia talks about a bunch of weird and sometimes beautiful things that ice can do.