A raindrop is a small mass of liquid water that falls to the Earth’s surface from a cloud. These droplets play a fundamental role in the planet’s water cycle, transferring water from the atmosphere back to land and oceans. Rain provides freshwater for most ecosystems and human activities. Formation of individual drops involves a complex series of atmospheric processes.
From Vapor to Cloud Droplet
The journey of a raindrop begins with water evaporating from bodies of water, soil, and plants, transforming into an invisible gas called water vapor. This water vapor rises into the atmosphere, carried upward by air currents. As moist air ascends, it gradually cools because atmospheric pressure decreases with altitude.
When the air cools to its dew point, water vapor begins to condense. It needs tiny airborne particles, known as cloud condensation nuclei, to condense upon. These microscopic particles, such as dust, pollen, and sea salt, provide surfaces for water molecules to attach.
Upon these nuclei, billions of minuscule cloud droplets form, typically ranging from 0.005 to 0.02 millimeters in diameter. These droplets are so small and light that air currents keep them suspended. They are too tiny to fall to Earth as precipitation.
Growing a Raindrop
For these microscopic cloud droplets to become raindrops large enough to fall, they must grow significantly, sometimes to a diameter of 2 millimeters or more. This growth occurs through two main mechanisms, depending on cloud temperature: the collision-coalescence process and the ice crystal process.
In warmer clouds, where temperatures are above freezing, the collision-coalescence process dominates. Larger cloud droplets, which may have formed on larger condensation nuclei, fall slightly faster than smaller droplets. As they descend, these larger droplets collide with and absorb numerous smaller droplets. This merging, known as coalescence, allows them to grow larger.
The ice crystal process, also known as the Bergeron process, is prevalent in colder clouds, where temperatures are below freezing. These clouds often contain a mix of supercooled water droplets (liquid water below 0°C) and ice crystals. Ice crystals have a lower saturation vapor pressure than supercooled water droplets at the same temperature. Water vapor molecules are more likely to deposit onto ice crystals than to remain as supercooled liquid.
Water vapor molecules migrate from the supercooled water droplets to the ice crystals, causing the ice crystals to grow rapidly at the expense of the liquid droplets. As these ice crystals grow large enough, they begin to fall. If they fall through a layer of air that is above freezing, they melt and become raindrops.
The Fall to Earth
Once a cloud droplet has grown sufficiently large, either through collision-coalescence or by melting from an ice crystal, it becomes heavy enough to overcome upward air currents. Gravity then pulls the raindrop downward toward the Earth’s surface. As the raindrop falls, it encounters air resistance.
The speed of a falling raindrop increases until the force of air resistance balances the force of gravity. The raindrop reaches its terminal velocity, a constant speed. This velocity depends on its size, with larger drops falling faster than smaller ones. For example, a typical raindrop with a diameter of 2 millimeters might fall at about 6.5 meters per second.
Contrary to popular depictions, falling raindrops do not maintain a teardrop shape. As they fall, they tend to flatten on the bottom and become more spherical or even take on a slightly hamburger bun shape, due to pressure exerted by the air underneath. This shape is maintained until the drop becomes too large and eventually breaks apart into smaller droplets before reaching the ground.