This is a water world. Life here depends on water both in the sea and above the shoreline. Organisms on the land depend on the rain. Over the eons, the things living here have grown connected with their water world – so much so that they have come to manage the way the rain falls, both on the sea and above the shore.
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, improving conditions there for the growth of more forests.
An equilibrium has been established between the carbon and oxygen in the bodies of living organisms, and the carbon dioxide in the atmosphere. This balance controls the air temperature, which controls the climate. The oxygen that has been injected into the air by the biosphere fuels wildfires – another climate-altering phenomenon, which may be unique to our little place in the galaxy.
The rain cannot fall from clean, empty air. Water vapor cannot condense into raindrops by itself. The air will chill but stay supersaturated with water vapor unless there are some solid surfaces it can condense onto. It can condense onto soot particles, or other forms of dust. Over the shoreline, particles of sea salt fill the air to provide those 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 into the sky. There, the sun evaporates them to dryness, and their salt crystallizes into solid, airborne, microscopic particles in the process. Dusts generated by the biosphere provide the same function over dry land.
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Dry sea salt serves well as a nucleus for rain drops because its surface is wetable – it pulls water vapor from the air. The earliest droplets 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 ones combine into larger ones, and their clouds darken.
As the droplets enlarge, they dissolve the salt crystal nucleus around which they formed. For a while, they are sea water – as salty as the ocean. But after they are established, they continue to pull vapor from the sky, diluting the salt within them down to fresh-water concentrations.
Water releases heat when it condenses from vapor into liquid. This heat engulfs the new droplets and floats them higher through the surrounding stratosphere. Under those higher chill conditions, 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. 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 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. It grows to become a snowflake with a living microbe embedded deep in its center. Soon it has grown heavy enough that it starts to descend. On the way down, it melts. Each snowflake serves to rain one bacterium back to Earth – to colonize the soil or the leaf it dampens wherever it happens to land.
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 air.
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One expansive area, the calm, open ocean of the southern summer, sees a shortage of airborne particles to initiate the rains. 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, over the middle of the South Pacific.
This transparency can be a problem for creatures that are injured by too much bright daylight and too high water temperatures – such as the atoll-building corals. 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 let the rains cool the their shallow surrounds.
The corals call down the rain by altering the chemistry of the atmosphere above them. They produce a special 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 gas 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. And, the droplets are readily wetable.
The humidity 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 sticky. But the vapor quickly dissolves in the methyl sulfate aerosols as they appear. These growing aerosol droplets of water and are swept along with the up-drafts that form around them. They rise and condense into cumulus clouds.
By afternoon these cumulus columns crest above the ocean and burst forth with rain. The rain evaporates in its own slipstream as it falls toward the sea, cooling itself, and also cooling the corals when it touches down and lowers the sea 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. They 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 those 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. At the cirrus cloud level, where the temperature is –50°, water vapor will spontaneously condense into ice, for example, into contrails behind jet aircraft. 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. Closer sea level, the sky is full of such solid surfaces, such as the various salt crystals and dusts. Airborne bacteria in that dust that carry ice-nucleating proteins in their cells have the cloud-seeding potential to initiate the rainfall (Morris et al, 2004).
The vapor-saturated air low over the warm ocean condenses into ship-tracks – contrails of fog condensed around the soot produced by ship engines. When the warmth in the air becomes too much for the corals in the shallow tropical seas, they call down the rain (Charlson et al, 1987) by sending a chemical messenger into the sky. 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 pulls 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 [note the filefish])and their symbiotic algae excrete this chemical (structure shown as (a) below) when they are stressed by high water temperature. That compound is highly water-soluble. It is found by a second set of organisms, the free-swimming microbes in the water column, and they metabolize most of it, leaving just the doubly methylated sulfide (DMS, the second compound shown (b) below).
DMS is a gas. It diffuses from the water and into the sky. There it reacts with oxygen to become methyl sulfonic acid (the third compound (c) below). That compound is a liquid, and it condenses out of the air into aerosol droplets, which adsorb and dissolve water vapor into themselves, swelling into raindrops, which form clouds that cool the coral with their shade and their rain.
The first compound in this series is thought to be able to protect the coral and algal cells from the molecular consequences of overheating. Its diffusion from those cells into the surrounding waters initiates a thermostatic process through which 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 incidents of bioprecipitation described here are evident in their respective settings; the extent to which they impact overall global rainfall is still being assessed.
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 on line 23 Oct