A common misconception is that rain can only happen in the atmosphere, but the physical processes that drive precipitation—density differences and gravity—also occur beneath the waves. The question of whether it can “rain” underwater shifts from a simple meteorological inquiry to one of physical and chemical science. True rain, which is liquid water falling through gaseous water vapor, is unique to the atmosphere. However, the deep ocean hosts analogous phenomena involving the downward movement of denser liquids or the settling of solids from a fluid column, offering a deep-sea parallel to the familiar sight of rain falling from the sky.
Defining Precipitation in a Water Medium
Atmospheric precipitation occurs when water vapor condenses into liquid droplets or ice crystals that become heavy enough to fall through the less dense surrounding air. This process relies on a phase change from gas to liquid, which is not possible for liquid water in a liquid environment.
The deep ocean is a body of liquid water that is remarkably stable and homogeneous in its physical properties. Water in the deep sea is generally a single phase, meaning water droplets cannot form and fall through a medium of the same substance. Buoyancy, the upward force exerted by the fluid, acts against gravity. In a uniform water column, gravity and buoyancy are nearly balanced, preventing the continuous downward movement of liquid droplets that defines rain. Instead, underwater precipitation must rely on differences in density caused by variations in salinity or temperature, or the formation of solid particles.
The Phenomenon of Salt Rain in Brine Pools
One of the most direct analogs to underwater rain is the process that occurs around deep-sea brine pools. These pools are essentially lakes on the ocean floor, consisting of water that is three to eight times saltier and therefore significantly denser than the surrounding seawater.
This hypersaline water forms distinct, stratified bodies that do not readily mix with the less dense ocean above it. The brine often originates from seawater dissolving ancient salt deposits beneath the seafloor, making it extremely concentrated. This density difference is so pronounced that remotely operated vehicles can float on the surface of the brine.
When the dense brine contacts the less dense ocean water at the pool’s boundary, a distinct surface or “shoreline” forms. When the dense brine at the surface cools or is disturbed, its high density causes it to sink back down into the pool. This continuous downward flow of denser, saltier water, driven by gravity and density differences, is known as “salt rain.” This dynamic movement functionally mirrors the precipitation of a denser liquid through a less dense medium.
Mineral Fallout from Hydrothermal Vents
A distinct, secondary form of underwater precipitation occurs at hydrothermal vents, which are often called “black smokers.” These features are found along volcanically active zones where seawater seeps into the crust, becomes superheated to temperatures exceeding 350°C.
The fluid is enriched with dissolved metals and sulfur compounds. When this superheated, mineral-rich fluid is ejected back into the frigid deep ocean water, the rapid temperature drop causes an immediate chemical reaction, forming microscopic solid particles.
The dissolved minerals, such as iron, copper, and zinc sulfides, instantly precipitate out of the solution. These solid particles create a billowing, dark plume, which gives the vents their nickname. Gravity then causes the solid mineral particles to fall back down and settle onto the seafloor. This constant fallout of solid particulates, driven by gravity after a chemical phase change, represents a true precipitation of solids from a liquid solution.