Rain is primarily water, but it’s far from pure. Every raindrop contains a mix of dissolved minerals, gases, microscopic particles, and trace organic material picked up during its journey from cloud to ground. The exact recipe depends on where the rain forms, what’s in the atmosphere, and what pollutants or natural aerosols it encounters along the way.
Water Plus Dissolved Ions
The bulk of any raindrop is H2O, but dissolved within it are varying amounts of sodium, potassium, magnesium, calcium, chloride, bicarbonate, and sulfate. These are the major dissolved components, and they come from natural sources: sea spray lofted into the atmosphere, windblown soil, volcanic emissions, and gases that dissolve into water vapor as it condenses. Nitrogen-containing compounds like ammonia and nitrate also show up consistently. In smaller quantities, rain carries iodine, bromine, boron, iron, aluminum, and silica.
The concentration of these minerals varies dramatically by location. Rain near coastlines tends to be saltier because ocean spray contributes sodium and chloride. Rain over continental interiors picks up more calcium and potassium from dust. In every case, though, rain is a dilute electrolyte solution, not distilled water.
The Tiny Particle at Every Raindrop’s Core
Every single cloud droplet forms around a microscopic solid or liquid particle called a condensation nucleus. These particles are everywhere in the atmosphere: smoke from fires or volcanoes, salt crystals from ocean spray, tiny specks of wind-blown soil. Without them, water vapor would have nothing to condense onto, and clouds wouldn’t form at all. So at the very center of every raindrop, there is literally a speck of dirt, dust, or salt.
These nuclei also influence the chemistry of the droplet from the moment it forms. Salt particles, for example, lower the vapor pressure around the droplet’s surface, which actually lets condensation begin before the air reaches 100% humidity. That’s why clouds can form in conditions that seem just short of fully saturated.
How Droplets Grow Into Rain
A condensation nucleus with a thin film of water on it is not rain. It’s a cloud droplet, roughly 10 to 20 micrometers across, far too small and light to fall. Getting from cloud droplet to raindrop requires one of two processes.
In cold clouds, which dominate storms across most of the mid-latitudes, ice crystals and supercooled liquid water coexist. Ice crystals grow at the expense of the surrounding liquid droplets because water molecules preferentially move toward the ice surface. The crystals keep growing until they’re heavy enough to fall. If the air near the ground is warm enough (above about 4°C or 39°F), those ice crystals melt on the way down and arrive as rain. This is called the Bergeron process, and it’s how most rain forms outside the tropics.
In warm clouds, especially in tropical regions, rain forms through collision and coalescence. A few slightly larger droplets fall faster than the tiny ones around them, colliding and merging along the way. As a growing drop reaches about 4 millimeters across, it becomes unstable and breaks apart into smaller droplets, which then start the process over. This cycle produces the heavy, warm rain typical of tropical downpours.
Why Rain Is Naturally Acidic
Even perfectly clean rain, with no pollution involved, is slightly acidic. Normal rain has a pH between 5.0 and 5.5, compared to pure water’s neutral pH of 7.0. The reason is carbon dioxide. The atmosphere contains CO2, and as water vapor condenses, it absorbs some of that CO2 and forms a weak carbonic acid. This has been true for as long as Earth has had an atmosphere with carbon dioxide in it.
Pollution pushes the pH lower. Sulfur dioxide from coal burning and nitrogen oxides from vehicle exhaust dissolve into rain and form sulfuric and nitric acids. This is what produces acid rain, which can drop below pH 4.0 in heavily industrialized areas. Even in regions where acid rain regulations have reduced emissions significantly, rain still carries measurable amounts of these compounds.
Pollutants and Microplastics
Modern rain contains a cocktail of substances that wouldn’t have been present a few centuries ago. Chloride and sulfate ions, nitrogen and phosphorus compounds, and combustion byproducts from heating, transportation, and industry all dissolve into falling rain. Polycyclic aromatic hydrocarbons, which come from burning fossil fuels and wood, are consistently detected in urban rainwater.
More recently, researchers have found microplastics in rain. A study sampling rainfall in two northern Chinese cities found concentrations of roughly 39 to 59 tiny plastic fragments per liter of rainwater. Snowfall contained even higher concentrations, likely because snowflakes have a larger surface area and fall more slowly, scavenging more particles on the way down. These microplastics are small enough to be carried into the upper atmosphere by wind, meaning rain deposits them even in remote areas far from cities.
Bacteria and Biological Material
Rain also carries living organisms. Bacteria, fungal spores, and pollen grains get lofted into the atmosphere by wind, and they can serve as condensation nuclei themselves or get swept up by falling droplets. Research published in Nature demonstrated one surprising pathway: when raindrops hit soil, they create tiny bubbles that burst and launch soil bacteria back into the air as fine aerosol droplets. A single raindrop can transfer about 0.01% of the bacteria on the soil surface into the atmosphere, and those bacteria can survive for more than an hour after being launched.
This means rain participates in a cycle of picking up and redistributing microorganisms. Bacteria that originated in one patch of soil can travel through the atmosphere and land somewhere else entirely, carried by the next storm. It’s one reason why rainwater, even collected far from obvious contamination, is not sterile.
How Location Changes the Recipe
The composition of rain shifts meaningfully depending on geography. Coastal rain is rich in sodium and chloride from ocean spray. Rain over deserts and arid plains picks up calcium and silica from mineral dust. Tropical rain in clean maritime air can be remarkably low in dissolved substances, while rain downwind of industrial centers or large cities carries elevated sulfate, nitrate, and organic pollutants. Even altitude matters: rain at higher elevations tends to be less contaminated simply because there’s less atmosphere below it to scavenge particles from.
Seasonal patterns play a role too. The first rain after a long dry spell, sometimes called a “first flush,” washes out accumulated dust, pollutants, and aerosols that have been building up in the atmosphere. That initial rain is typically dirtier, with higher concentrations of dissolved and suspended material, than the rain that follows in a sustained storm.