The living world here on Earth is an extension of the physical world here on Earth. Living systems are intimately connected with their physical environment. Carbon, nitrogen, oxygen, and the other elements cycle back and forth into and out of living systems. Those organisms have come to change the physical landscape of this planet. They now exert their influence on many aspects of their surroundings — even going so far as to call down the rain.
The living carpet of green changes the reflectivity and the humidity of the land. The forests evaporate the ground water back into the skies, from which it rains back down — on places downwind. That improves conditions there for the growth of more forests.
The biomass generated by the forest burns in the oxygen generated by the forest. Forest fires change the environment. They change the reflectivity of the land, moderating the air temperature. And by injecting carbon dioxide and water vapor back into the sky, they change the climate.
The rain cannot fall from clean air. Water vapor cannot condense into raindrops by itself. The air can chill, but it will stay supersaturated with water vapor unless there are some solid, microscopic surfaces it can condense onto. The fire calls down the rain by providing the nuclei around which the rain drops form. They can condense onto soot particles or other bits of dust.
Over the shoreline, particles of sea salt provide those condensate surfaces. Those salt particles are the remains of levitated mists of sea water. The waves crashing against the rocks, and the wind whipping over the whitecaps put those mists in the sky. There, the sun evaporates them to dryness, and their salt crystallizes into solid, airborne, microscopic particles in the process.
Dry sea salt serves well as a nucleus around which rain drops can form and grow. Its surface is wetable – it pulls water vapor from the air. The earliest droplets that are formed this way are less than a micrometer in diameter. They form extremely bright clouds, because many tiny droplets reflect much more light than the same volume of water in fewer larger drops. Eventually, however, the small droplets combine into larger ones, and their clouds darken.
As the droplets enlarge, they dissolve the salt crystal nucleus around which they formed. Then, for a while, the droplets are as salty as the ocean. But the water droplets continue to pull vapor from the sky. They grow to dilute the salt within them down to fresh-water concentrations.
Water vapor releases heat when it condenses into liquid. This heat engulfs the new droplets and floats them higher through the surrounding cool stratosphere. Under chill conditions farther aloft, still more vapor dissolves into growing droplets. Soon they reach weights that are too heavy to stay airborne. Then it rains.
Inland rain forms in a similar fashion by condensation of water vapor onto the microscopic surfaces of dusts levitated by the wind. This is the most miraculous dust in the galaxy (1). Nothing on all the other barren, lifeless planets in the universe comes close to the complexity of dried strands of cellulose, chitin, and lignin that become airborne here – not to mention the even more complex components of dust like actin, keratin, DNA and RNA. Much of this dust is alive, as polyhedral virus, fungus, and pollen spores, and as even larger organisms.
Whole bacteria get swept up into the wind from dry soil. The wind can then spread this dust-up far and wide, dispersing the bacteria around the globe. But the high radiation environment aloft kills the living dust particles if they stay airborne too long. Most of the larger floating particles in the stratosphere are radiation-killed microscopic life forms.
One set of these wind-riding microbes has solved the challenge of getting back to Earth soon enough to stay alive. These bacteria carry a protein that specifically binds the vapor from the air. The protein imposes an order on the way the water molecules condense – they are formed into water crystals when they contact this protein – into ice.
Such a seed crystal of ice enlarges rapidly in the chill reaches between the clouds (2). It grows to become a snowflake with a living microbe embedded at its center. Soon it has grown heavy enough to descend. On the way down, it melts. Each snowflake serves to rain one bacterium back to Earth. On land, that microbe colonizes the soil or the leaf it dampens wherever it happens to find itself.
The seeding of the clouds by these bacteria hastens the rains. Where the wind has brushed over the lands the bacteria inhabit, downwind rainfall is increased by their presence in the sky.
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One expansive area — the becalmed ocean of the equatorial South Pacific — sees a shortage of airborne particles that would serve to initiate the rains. Those open reaches are far from land-borne sources of dust. The nighttime air there is sharply transparent. There are few solid particles to form a haze, or to nucleate raindrops. The southern stars sparkle at their brightest against that clean black backdrop.
This transparency can be a problem for some creatures, such as the atoll-building corals. They are injured by too much bright daylight and too high water temperatures. They can be bleached by continuous sea surface heat – it causes them to loose their symbiotic algae. But they have a solution to this problem – they simply bring in the shady clouds, and call down the rain to cool their shallow surrounds.
The corals call down the rain by altering the chemistry of the atmosphere above them. They produce a molecule with a methylated sulfur at one end – a molecule uniquely suited to changing the weather. The higher the water temperature, the more of this chemical the corals produce.
This chemical is quite water soluble. But after it is secreted, it is changed by the marine microbes in the water column – the methylated sulfur is cleaved away from the rest of the it. The freed methyl sulfur is a gas – it diffuses out of the water and into the sky.
There this gas is oxidized by oxygen that was produced by the algae living in the corals and by the free-living plankton as well. This oxidized form of the sulfide molecule is not a gas, it is a liquid. As its concentration increases above the sea surface, it condenses into tiny droplets dissolved in the air – “aerosols.” These suspended droplets form a haze that reflects sunlight away from the surface below.
The droplets are readily wetable; they attract water. Water vapor rises along with the temperature over the tropical ocean in the morning. With few suspended solid particles to catalyze cloud formation, the warming air becomes more humid. But the vapor quickly dissolves in the methyl sulfate aerosols as they appear. These growing aerosol droplets of water are swept along with the up-drafts that form around them. They rise and condense into cumulus clouds. The clouds shade, and cool the coralline creatures that produced the methyl sulfate.
By afternoon these cumulus columns crest above the ocean and burst forth with rain. The rain partially evaporates in its own slipstream as it falls toward the sea, cooling itself. It cools the corals when it touches down on the sea and lowers the surface temperature.
By late afternoon, the south seas mirror parallel ranks of cumulus ramparts that trail away to the far horizon, burning in the glow of sunset. These armadas of clouds trail skirts of rain, which clean the air for another crystal-clear night.
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Some of the water vapor that rises over the tropics rides away from the equator in the stratosphere to the north and south. This engine drives the world’s atmospheric circulation. Those rivers of air spread the rains across the continents. The biosphere provides seed nuclei that induce the growth of the raindrops over both the land and the sea. The tradewinds whip the dusts up from North Africa and carry it out into the North Atlantic – where it nucleates the precipitation that spawns hurricanes later in the summer. Soot from fires in Asia descends within the rains and snows falling in the mountains of the Americas. We are still learning how much these weather patterns are shaped by the living world.
Calling down the rain: notes. (1) See: “Dust to Dust”: Chapter 21 in “Between the Rocks and the Stars.” (2) See: “Window on the Sky”: Chapter 24 in “The Shark and the Jellyfish.”
At the cirrus cloud level, where the temperature is –50°, water vapor will spontaneously condense into ice. But the formation of liquid water in the sky happens at lower levels. At the higher temperatures of these lower altitudes, gas-phase water does not condense spontaneously. But water will condense from the vapor-phase onto solid surfaces. Close to sea level, the sky is full of such microscopic surfaces. Vapor-saturated air condenses contrails of fog along the tracks of ships — droplets condensed around the soot produced by ship engines. Airborne bacteria along continental margins carry ice-nucleating proteins in their cells. They have the cloud-seeding potential to initiate the rainfall (Morris et al, 2004).
When the warmth in the air becomes too much for the corals in the shallow tropical seas, they send a chemical signal into the sky that calls down the rain (Charlson et al, 1987). They produce more of this chemical the hotter they are (Raina et al, 2013). The chemical compound goes through a series of changes, which carry it from the water into the air, and then convert it back into a hygroscopic, air-borne liquid. This aerosol droplet form of that airborne liquid dissolves airborne water molecules into its own surface, converting the vapor form of water into liquid water, which swells the droplets into raindrops. 
Both the corals (such as this purple staghorn Acropora coral at right [note the filefish]) and their symbiotic algae excrete this chemical (DMSP; structure shown as (a) below) when they are stressed by high water temperature. That compound is highly water-soluble. It is metabolized by a second set of organisms, the free-swimming microbes in the water column, leaving just the doubly methylated sulfide (DMS, the second compound shown (b) below). DMS is poorly dissolved in water, and evaporartes into the air as a gas. There it reacts with oxygen to become metane sulfonic acid (the third compound (c) below). That compound is a liquid, and it condenses out of the air into aerosol droplets. They adsorb and dissolve water vapor into themselves, swelling into raindrops, which form clouds that cool the coral with their shade and their rain. Through that thermostatic process, the living creatures interact with the sea and sky to stabilize the ambient temperature (Raina et al (2013) and citations there in; for a dissenting interpretation on the process, see Quinn & Bates (2011)). The extent to which incidences of bioprecipitation impact overall global rainfall is the subject of an on-going assessment.
(a)
CH3
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S–C–C–COOH dimethylsulphoniopropionate
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CH3
(b)
CH3–S–CH3 dimethyl-sulfide
(c)
OH
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O=S-CH3 methane sulfonic acid
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OH
Charlson, R. J. et al (1987) Oceanic phytoplankton, atmospheric sulfur, cloud albido and climate. Nature 326, 655-72
Morris, C. E. et al (2004) Ice nucleation active bacteria and their potential role in precipitation. Journal de Physique IV 121, 87-103
Quinn, P. K. & Bates, T. S. (2011) The case against climate regulation via oceanic phytoplankton sulphur emissions. Nature 480, 51-6
Raina, J-B et al (2013) DMSP biosynthesis by an animal and its role in the coral thermal stress response. Nature 502, 677-680
Steve Daubert speaker nature bay area