A raindrop begins as an invisible speck of dust, salt, or even bacteria floating in the atmosphere, and it takes a surprisingly complex chain of events to turn that particle into water heavy enough to fall. The journey from microscopic cloud droplet to the drops that splatter on your windshield involves at least two distinct growth stages, and scientists still debate exactly how some of those stages work. What looks simple from under an umbrella is one of the harder problems in atmospheric physics.
Every Raindrop Starts With a Seed
Water vapor in the atmosphere does not spontaneously condense into droplets on its own under normal conditions. It needs a surface to condense onto, a tiny particle called a cloud condensation nucleus. These particles are everywhere: fine grains of mineral dust lofted from deserts, sea salt thrown up by breaking waves, soot from fires, sulfate particles from volcanic eruptions, and even organic material from plants. Without them, the air would need to reach extreme levels of humidity before any liquid water formed.
When moist air rises and cools, it eventually reaches a temperature where the water vapor it carries exceeds what the air can hold. At that point, vapor begins condensing onto whatever suitable particles are available. The number and type of particles matter. Research has identified two broad regimes for cloud droplet formation: one where particles are scarce relative to the strength of the updraft, meaning nearly all available particles activate into droplets, and another where particles are abundant and the updraft limits how many can activate.1Atmospheric Chemistry and Physics. Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN) In the first case, a cleaner atmosphere produces fewer but potentially larger droplets. In the second, a polluted sky can produce vast numbers of tiny droplets that struggle to grow large enough to rain.
Not all particles are equal. Giant sea salt aerosols kicked up by ocean waves can jump-start precipitation in marine clouds, particularly in low-lying sheet-like clouds called stratocumuli.2Journal of the Atmospheric Sciences. Impact of Giant Sea Salt Aerosol Particles on Precipitation in Marine Cumuli and Stratocumuli: Lagrangian Cloud Model Simulations These oversized particles give water vapor a bigger surface to condense on, producing droplets that have a head start in the race to become rain.
The Gap That Condensation Cannot Cross
Condensation alone builds cloud droplets to roughly 10 to 20 micrometers across, small enough that air resistance keeps them suspended. A typical raindrop, by contrast, is somewhere around one to two millimeters in diameter, roughly a hundred times wider and a million times heavier. Getting from one to the other by condensation alone would take far too long. Observations of deep, warm clouds that persist without producing rain confirm this gap: condensation gets droplets to a certain size, but something else has to take over before rain can form.3Australian Journal of Scientific Research Series A: Physical Sciences. The Growth of Cloud Drops by Condensation. II. The Formation of Large Cloud Drops
This “size gap” between what condensation produces and what gravity can pull down was a puzzle for decades. The resolution comes from collision and coalescence: once a few droplets manage to grow even slightly larger than their neighbors, they begin falling faster relative to the smaller ones around them, sweeping them up and merging. Each collision makes the collecting droplet bigger and faster, which leads to more collisions, creating a runaway growth process.
Turbulence as the Missing Ingredient
Even accounting for collision-coalescence, models that only consider droplets falling under gravity tend to predict rain forming more slowly than it actually does. Clouds in the tropics can start raining within 20 to 30 minutes of forming, faster than simple gravitational settling would allow. The missing ingredient, according to recent high-resolution observations and simulations, is turbulence.
Air inside a cloud is not calm. Updrafts, downdrafts, and chaotic eddies constantly jostle droplets around, and this turbulence does two things. It throws droplets together more often than gravity alone would, and it clusters droplets into pockets where local concentrations are higher, increasing the chance of collisions. A 2024 study comparing observations of tropical cumulus clouds with detailed simulations found that including turbulent effects on droplet collisions was necessary to reproduce the observed drop sizes and the timing of rain onset. Without turbulence, the models produced rain too slowly and too weakly. The researchers concluded that turbulent coalescence has a dominant influence on rain initiation in warm clouds.4PubMed Central. Are turbulence effects on droplet collision-coalescence a key to understanding observed rain formation in clouds?
The Cold Rain Pathway
Not all rain starts as liquid. In fact, most rainfall at mid and high latitudes begins as ice, even in summer. When cloud tops extend above the freezing level, ice crystals and supercooled liquid droplets coexist. At temperatures below zero, there is a physical asymmetry between ice and liquid water: ice has a lower saturation vapor pressure, which means ice crystals can pull water vapor away from nearby liquid droplets. The droplets shrink by evaporation while the ice crystals grow, a process that atmospheric scientists call the Wegener-Bergeron-Findeisen process.5Atmospheric Chemistry and Physics. Giant Cloud Condensation Nuclei enhanced Ice Sublimation Process: a potential mechanism in mixed-phase clouds
Ice crystals that grow this way can become snowflakes, which then aggregate with other crystals and collect additional supercooled droplets (a process called riming). Once these ice particles become heavy enough, they fall. If the air below the cloud is warm enough, the snowflakes melt on the way down and arrive at the surface as rain. The melting process itself is interesting: meltwater initially collects in concave pockets on the snowflake’s surface, then forms a liquid shell around an ice core, and eventually becomes a water drop. How heavily rimed a snowflake is affects whether it melts cleanly into a single drop or breaks apart into several smaller ones.6Journal of Geophysical Research: Atmospheres. Snowflake Melting Simulation Using Smoothed Particle Hydrodynamics
For ice to form in the first place, the cloud usually needs ice-nucleating particles, just as liquid water needs condensation nuclei. Mineral and soil dust are considered the most important ice nucleators on a global scale, though biological particles like bacteria and fungal spores also play a role.7Atmospheric Environment. Impact of bacterial ice nucleating particles on weather predicted by a numerical weather prediction model In regions far from desert dust sources, biological particles can become particularly relevant as ice triggers.8Atmospheric Chemistry and Physics. Biological and dust aerosols as sources of ice-nucleating particles in the eastern Mediterranean: source apportionment, atmospheric processing and parameterization
What Raindrops Actually Look Like
The classic teardrop shape you see in clip art is wrong. Small raindrops, under about two millimeters, are nearly perfect spheres. Surface tension holds them together tightly, and at that size, air resistance is not strong enough to deform them. As drops get larger, the air pushing up against them flattens the bottom while the top stays rounded, producing something closer to a hamburger bun shape. Polarimetric radar, which sends out horizontal and vertical pulses and measures the difference in how they scatter, can detect this flattening and use it to estimate drop sizes remotely.9Journal of Atmospheric and Oceanic Technology. Can a Unique Model Describe the Raindrop Shape–Size Relation? A Clue from Polarimetric Radar Measurements
There is an upper limit to how big a raindrop can get. As drops grow beyond about four to five millimeters, the aerodynamic forces pulling them apart overcome surface tension, and they break into smaller fragments. Laboratory experiments found that this breakup limit depends on altitude: at sea level, where air is denser, drops broke apart at roughly 4.5 mm, while at an elevation of about 3,300 meters, the thinner air allowed breakup at around 3.2 mm.10American Meteorological Society (Journal of the Atmospheric Sciences). Effects of Altitude on Maximum Raindrop Size and Fall Velocity as Limited by Collisional Breakup This means the largest raindrops at high-altitude cities are smaller than the largest ones at the coast.
Not Every Drop Reaches the Ground
Rain falling from a cloud does not always make it to the surface. As drops descend through unsaturated air below the cloud base, they evaporate. How much evaporation occurs depends mainly on the size of the drops and on how dry the air is. Modeling work shows that drops smaller than about 0.5 mm can evaporate completely in less than 700 meters of fall through typical subtropical marine boundary layer conditions.11PubMed Central. A Simple Model for the Evaporation of Hydrometeors and Their Isotopes You may have seen this as virga, those curtains of rain hanging beneath a cloud that never touch the ground.
Radar observations over Barbados confirmed that the average drop size is the key variable controlling how much rain evaporates below the cloud. When the typical drop diameter is small, the fraction of rain that evaporates climbs steeply. When drops are larger, evaporation is slower and more gradual.12Geophysical Research Letters. Microphysics Dominates Sub‐Cloud Rain Evaporation in Trade Cumuli Over Barbados This is one reason drizzle from thin, low clouds often evaporates before reaching you, while heavy convective downpours tend to make it all the way to the surface with drops largely intact.
Convective Rain Versus Stratiform Rain
Rain does not come in one flavor. The two main types differ in how their drops are distributed. Stratiform rain, the steady, widespread kind associated with large weather systems, tends to have lots of small drops. Convective rain, the heavy bursts from towering thunderstorm clouds, produces fewer drops overall but shifts the distribution toward larger sizes. Measurements have shown that average drop diameters for convective rain (around 1.0 mm) exceed those for stratiform rain (around 0.8 mm), and convective rain has a greater number of drops at every size, which is why it delivers much higher rainfall rates.13Scientific Reports. Geographical characteristics of raindrop size distribution for rainy season in Eastern China
These differences are not just academic. The way drop sizes are distributed affects everything from how radar estimates rainfall, to how quickly rain erodes soil, to how efficiently a storm cleans the air. Convective rain with large drops, for instance, has higher kinetic energy per drop, which translates to more erosion on bare soil and stronger splash effects when it lands.
Rain as an Atmospheric Scrubber
Raindrops do double duty as air purifiers. As they fall through polluted air, they collide with and dissolve aerosol particles and gases, a process called wet scavenging. This is why the air often smells and feels cleaner after a storm. Measurements in Beijing during summer rainfall events found that rain scavenged between 62% and 100% of aerosol particles within the first hour of heavy precipitation, though the researchers noted that light rain had a negligible cleaning effect and could even increase certain pollutant concentrations by promoting chemical reactions in humid air.14Journal of Geophysical Research: Atmospheres. New Insights Into Scavenging Effect of Aerosol Species During Summer Rainfall Process in Beijing
Below-cloud scavenging accounts for a meaningful share of total wet deposition. A separate study in Beijing estimated that below-cloud processes were responsible for over half the sulfate and nitrate found in collected rainwater.15PubMed. Below-cloud wet scavenging of soluble inorganic ions by rain in Beijing during the summer of 2014 Rain intensity matters: heavier rain scavenges particles more efficiently because bigger, faster drops sweep through more air volume per unit time. This is partly why air quality tends to improve dramatically after a thunderstorm but barely changes after a brief drizzle.
What Happens When Rain Hits the Ground
The story of a raindrop does not end when it lands. High-speed imaging has revealed that when a raindrop strikes porous soil, tiny air bubbles get trapped inside the drop. When those bubbles rise to the surface and burst, they launch microscopic jets of liquid into the air, carrying soil particles, microorganisms, and organic compounds with them. Experiments using fluorescent dye confirmed that material from within the soil’s pores gets lofted into the atmosphere this way.16Nature Communications. Aerosol generation by raindrop impact on soil This is one mechanism behind petrichor, that distinctive earthy smell during and after rain, as soil-derived compounds are launched into the air you breathe.
That splash effect has broader atmospheric consequences. Field measurements have found that ice-nucleating particle concentrations in the air increase during rainfall, correlating with the cumulative kinetic energy of the falling rain. In other words, rain hitting the ground kicks up particles that can then seed ice formation in future clouds.17Journal of Geophysical Research: Atmospheres. Ice‐Nucleating Particles Are Emitted by Raindrop Impact Rain generates part of the raw material for more rain, a feedback loop that connects surface conditions to cloud formation in ways that are still being quantified.
Rain on Other Worlds
Earth is not the only place where it rains. Titan, Saturn’s largest moon, has a methane cycle that produces liquid methane rain falling through a thick nitrogen atmosphere. Mars likely had rain in its distant past. Venus could theoretically have sulfuric acid rain, though it evaporates long before reaching the scorching surface. Researchers have developed frameworks for calculating raindrop shape, terminal velocity, and evaporation rate in any planetary atmosphere, showing that these basic physics tightly constrain the possible size range of drops regardless of the specific liquid involved.18Journal of Geophysical Research: Planets. The Physics of Falling Raindrops in Diverse Planetary Atmospheres
The result is that across very different worlds, raindrops tend to converge on a surprisingly narrow range of sizes. Whether the drops are made of water, methane, or liquid iron (as may occur on some exoplanets), the interplay between surface tension, gravity, and air resistance keeps drops within roughly the same order of magnitude. The physics of rain formation turns out to be more universal than you might expect.
Cloud Seeding and Artificial Rain
Humans have tried to speed up rain formation since the 1940s, most commonly by introducing silver iodide into clouds. Silver iodide crystals have a structure similar to natural ice, so they can serve as artificial ice-nucleating particles, encouraging the Bergeron process to kick in. Simulations of convective storms on the U.S. High Plains found that silver iodide seeding at the right moment, when the cloud top passed through the minus-10°C level in the zone of strongest updraft, increased accumulated surface precipitation by 20 to 30 percent.19Atmospheric Research. Silver iodide seeding impact on the microphysics and dynamics of convective clouds in the high plains The seeding did not just add ice; it triggered a chain reaction where enhanced precipitation created stronger downdrafts, which in turn generated new convective cells that produced additional rain.
Liquid carbon dioxide is another seeding agent. Modeling comparisons have shown that when injected at very cold cloud levels (minus-15 to minus-20°C), liquid CO₂ and silver iodide produce similar effects. But when CO₂ is injected lower in the cloud, near the 0 to minus-5°C zone where supercooled water is most abundant, it can produce a stronger dynamic response and more precipitation by spawning new convective cells at lower altitudes.20Atmospheric Research. A numerical comparison study of cloud seeding by silver iodide and liquid carbon dioxide These results come from model simulations, and the real-world effectiveness of cloud seeding remains hard to verify statistically, since you can never know for certain what an unseeded cloud would have done. Countries from China to the United Arab Emirates invest heavily in operational seeding programs, but the scientific community generally treats the technique as modestly effective at best and difficult to evaluate outside controlled experiments.