Airborne Bacteria Make It Rain, Snow, Researchers Find

By | February 29, 2008

The sky is not an ethereal, sterile realm. It’s teeming with bacteria, and scientists say that the microbes play a powerful role in producing rain and snow.While the idea that bacteria could prompt precipitation was previously known, a paper published this week in Science shows that they’re more important than anyone expected.


Researchers led by Louisiana State University microbiologist Brent Christner analyzed snow samples from around the world, categorizing the content of their “nucleators” — tiny particles that help water vapor coalesce and freeze.

All snow and most rain begins as ice. Though water is widely thought to have a freezing point of zero degrees Celsius, it’s not so simple in the clouds, where pristine vapors only bind to form ice crystals at exceedingly cold temperatures. Nucleators let crystallization happen in the less extreme conditions that prevail in much of Earth’s troposphere.

Christner found bacteria, technically known as “biological ice nucleators,” in an atmospheric context. High levels of bacteria were present in nearly every sample.

“Atmospheric scientists haven’t previously recognized that these particles are so widely distributed,” he said.

The findings raise the question of how climate change and human activities will affect bacterial balances in the sky. More immediately, they’re a starting point for research on bacterial contributions to cloud formation and precipitation.

In its latest report, the International Panel on Climate Change said that the impact of feedback loops involving clouds on global weather patterns are the “largest source of uncertainty” in current predictions of climate change.

Christner’s findings won’t overturn the IPCC’s fundamental conclusions — a high probability of dramatically rising global temperatures — but they should spur research that will help scientists predict the changes in greater detail, said Princeton University climate scientist Leo Donner, who was not involved in the study. …

The fact that bacteria could cause snow and rain was discovered almost by accident in the 1970s by study co-author David Sands, a Montana State University plant pathologist, during his research on Pseudomonas syringae, a microbe that causes ice to form on leaves.

Unable to discover the source of repeatedly infected fields, Sands exasperatedly took to the skies. He did the scientific equivalent of dragging a cup through the clouds — and lo and behold, there was P. syringae.

P. syringae is not the only biological ice nucleator, but it is the most common, and all varieties share a protein structure that provides a scaffold for free-floating water molecules. Once bound to the bacteria and to each other, the water vapors are able to freeze, and eventually fall back to Earth.

In a pure state, water vapors freeze at temperatures below -35 degrees Celsius. Nucleators allow this to happen in warmer conditions, and Christner’s study found that bacteria are the most common warm-temperature nucleators of all.

Researchers never realized bacteria could be so widespread in the clouds, said Christner, because the technologies used to measure fine dust — traditionally seen as the most important nucleator — ignore microbe-sized particles.

“It’s not that these atmospheric scientists are idiots — they’re not,” he said. “But biological nucleators were not previously recognized as being that abundant or important. They’re going to have to revise that.” – wired

Still tempted to eat fresh snow? Don’t worry. Go ahead. Few people know that there are more bacterial cells than human cells in our bodies. And guess what? You know that fresh smell after it rains? You guessed it… bacteria.

Most people notice a distinctive smell in the air after it rains. It’s frequently linked with spring, as the smell of fresh cut grass is associated with summer. You’ll find it in a lot of poetry and also on many inspirational lists of things to be happy about. But what causes it?

As it turns out, the smells people associate with rainstorms can be caused by a number of things. One of the more pleasant rain smells, the one we often notice in the woods, is actually caused by bacteria! Actinomycetes, a type of filamentous bacteria, grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produces spores in the soil. … The moist air easily carries the spores to us so we breathe them in. These spores have a distinctive, earthy smell we often associate with rainfall. – howstuffworks

2 thoughts on “Airborne Bacteria Make It Rain, Snow, Researchers Find

  1. Patrick

    Xeno, it seems to me that this article is illustrative of all these articles that are coming out about bacteria and microorganisms playing a much larger–and crucial role in life than we ever knew before. from micorobes in our stomaches, to microbes in our brains, from panspermia, to autism, to schizophrenia and even down to DNA… I’m not a scientist but it seems to me as a writer who looks for patterns that a major paradigm shift is occurring in science. Am I incorrect? And just as Newtonian mechanics (am I using that correctly?) and then relativity eventually trickled down changing the public consciousness, might we not then be on the verge of a major change happening in our perception as all this info and proof comes in showing us that far from being autonomous–even our personalities are reliant upon–possibly–billions of little microbes?

    Maybe this comment is too long for this forum, but I’ve been reading your site for some time, and would love to email you at length about this as I know very few scientists in the L.A. area to talk about this sort of thing with–and I find it fascinating and cant help bu think that we’re seeing the beginnings of a major change in scientific and therefore public thought.


  2. Xeno Post author

    It really is amazing, isn’t it? Most of us ask “What I am?” and the paradigm shift may be that, “I am a ship, a vessel filled with non-sentient life forms, some of which control where I go and what I do.”

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