Raindrops are a common atmospheric phenomenon, but the typical size of a falling water droplet is surprisingly small, usually measuring less than two millimeters in diameter. Even the heaviest downpour is composed of drops that rarely exceed three or four millimeters. The extreme rarity of raindrops exceeding five millimeters represents a strict physical ceiling imposed by atmospheric forces.
How Raindrops Grow
The journey from a microscopic cloud particle to a fully formed raindrop requires a significant increase in volume. Initial cloud droplets form through condensation, where water vapor collects around tiny airborne particles called condensation nuclei. This initial growth is slow and produces droplets too light to fall as rain.
The primary process responsible for creating the larger drops is called collision-coalescence. In this mechanism, larger, faster-falling droplets, known as collectors, collide and merge with smaller, slower droplets in their path. It can take approximately one million tiny cloud droplets to combine into a single one-millimeter raindrop. This continuous merging allows the drop to gain enough mass to overcome atmospheric updrafts and begin its descent.
The Critical Limit: Speed and Shape Deformation
As a raindrop grows, the force of gravity pulling it downward increases, causing acceleration. This acceleration is quickly countered by air resistance, or drag, which pushes upward against the falling drop. The speed at which these opposing forces balance is called the terminal velocity, where the drop falls at a constant speed.
For a small drop, the terminal velocity is low, and the drop maintains an almost spherical shape due to the cohesive forces of surface tension. However, as the drop’s diameter increases, its mass grows much faster than its surface area, leading to a higher terminal velocity. This high speed generates significant aerodynamic pressure on the drop’s underside, overwhelming the surface tension.
The intense air pressure causes the drop to flatten dramatically from a sphere into a shape resembling an oblate spheroid. As the drop approaches the critical size limit, this flattening becomes more extreme, taking on a highly unstable, parachute-like shape. This distortion signals that the drop is nearing its maximum sustainable size.
Aerodynamic Breakup: Why Drops Fragment
The rarity of raindrops larger than five millimeters lies in the inevitable consequence of severe shape deformation: aerodynamic breakup. Once a drop reaches a diameter of about 4.5 to 5.0 millimeters, the forces acting on it exceed the liquid’s ability to hold itself together. The aerodynamic stress against the flattened base is too great for the water’s surface tension to counteract.
This process results in catastrophic fragmentation, often described as a “bag breakup.” The thin, stretched membrane of the unstable drop bursts open, and the rim disintegrates. This single large drop instantly shatters into a multitude of smaller, more stable droplets, typically under two millimeters in size.
Fragmentation occurs quickly, preventing drops from sustaining sizes beyond the four-to-five-millimeter range. This aerodynamic instability places a firm ceiling on the maximum size of a sustainable raindrop. This constant cycle of growth and fragmentation ensures that most raindrops observed remain well below the five-millimeter limit.