When a weather system moves over the ocean and picks up moisture, why does it not rain salt water?
Posted August 16, 2008
MIKE MOSS SAYS: Jason, The principle mechanism for weather systems to gain moisture from the ocean is evaporation, in which water vapor is lost from the sea surface but the salt that was dissolved in the water is left behind. If you've ever left some salt water in a cup or dish until the water has evaporated, leaving a crusty deposit of salt crystals. In an idealized sense, the water is "distilled" and becomes nearly pure water vapor, at least until it cools and condenses onto some particle floating in the air and becomes a cloud droplet, at which point how pure it remains will depend on the particulars of the material that made up the condensation nucleus.
In the case of some very strong storms such as tropical cyclones and nor'easters, there may be some slightly salty rain that develops because of small droplets of ocean water that are mechanically scoured from the ocean's surface (spindrift off of wavetops, for example) that can become caught in updrafts and incorporated into the cloud. Since the water in this case is physically lifted in the form of liquid droplets rather than evaporated as water vapor, it retains the salt content that it had in the ocean. This process accounts for a very small fraction of water that becomes rain, though, even from strong ocean-going storms.