Raindrops form when water vapor in the air cools, sticks to tiny particles like dust or pollen, grows into cloud droplets, and then merges with other droplets until they are heavy enough to fall as rain.

From vapor to tiny droplets

Water from oceans, lakes, plants, and soil evaporates into the air as invisible water vapor.

High in the atmosphere, this vapor cools and condenses onto microscopic particles such as dust, sea salt, pollen, or pollution called cloud condensation nuclei , creating countless tiny liquid droplets.

  • These first droplets are extremely small, often much smaller than the period at the end of this sentence.
  • A cloud is essentially a huge collection of these tiny droplets suspended in the air.

Growing into raindrops inside clouds

On their own, those tiny droplets are too light to fall as rain and would evaporate before reaching the ground.

Inside a cloud, rising and turbulent air causes droplets of different sizes to move at different speeds, so they bump into each other and merge in a process called collision–coalescence.

  • As droplets collide and combine, one “lucky” droplet can grow by absorbing many smaller neighbors, becoming a full raindrop made from millions of original cloud droplets.
  • In colder clouds, ice crystals can also form, stick together (aggregation), and collect supercooled droplets (accretion); as these icy particles fall and melt, they turn into rain.

Falling to Earth as rain

When a droplet grows large enough that gravity overcomes the upward push of air in the cloud, it begins to fall.

As it falls, the drop keeps sweeping up smaller droplets along its path, which helps it grow further until it reaches a size where air resistance and breakup limit how big it can get.

  • Falling drops are not teardrop-shaped; small ones are nearly spherical, while larger ones flatten on the bottom and can look like a tiny hamburger bun due to air resistance.
  • Very large drops can become unstable, stretch, and split into smaller drops, which can then repeat parts of the growth process.

Why raindrop formation matters

Raindrop formation controls how quickly clouds can turn into rainstorms and how intense that rain can be.

Things that change the number and type of particles in the air—like pollution, sea spray, or dust storms—can influence how easily droplets form and how efficiently they grow into rain.

  • Understanding these microphysical processes helps improve weather and climate models that simulate rainfall patterns.
  • This same science underlies cloud seeding , where extra particles such as silver iodide are introduced to encourage droplet or ice formation in some experiments.

Information gathered from public forums or data available on the internet and portrayed here.