Rain is the process of condensed water vapor falling back to Earth. The popular image of a falling raindrop as a perfect, tapered teardrop is not an accurate representation of its true form. The actual shape a raindrop takes depends entirely on its size, as the forces acting on it change significantly as it falls.
The Spherical Reality of Small Raindrops
The smallest raindrops, typically those less than one millimeter in diameter, maintain a nearly perfect spherical shape. This rounded form is dictated by surface tension, a physical property of water that pulls molecules inward. Surface tension minimizes the drop’s surface area, and a sphere is the shape with the least possible surface area for a given volume. Because these tiny drops possess very little mass, air resistance (drag) is negligible, allowing surface tension to dominate and pull the droplet into a stable globe.
Air Resistance and the Flattened Shape
As a raindrop falls and collides with other droplets, it grows larger, and its shape changes dramatically when it exceeds about one millimeter in diameter. At this size, the drop’s falling velocity increases, making air resistance a significant factor.
The air rushing past the drop exerts greater pressure on its underside, pushing upward and flattening the bottom surface. This pressure distorts the drop from a sphere into a shape resembling a flattened dome or the top of a hamburger bun, with a rounded top and a nearly flat base.
This flattened form continues to widen as the drop grows. Once a drop reaches approximately five millimeters in diameter, the aerodynamic forces overwhelm the surface tension, causing the center of the base to become concave. This instability leads to the drop breaking apart into several smaller droplets, which limits the maximum size of raindrops observed in nature.
Why Falling Rain Looks Like Streaks
The visual experience of falling rain as a series of long, vertical streaks is not a representation of the drop’s static shape but rather an optical illusion. This streaking effect is caused by a phenomenon known as motion blur, which relates to the speed of the drops relative to the limitations of human vision.
Raindrops, especially larger ones, move quickly enough that the human eye cannot resolve them as distinct, stationary shapes against a distant background. Instead, our visual system registers the path of the drop over a brief period of time, smearing the image into a line. The appearance of a tail or streak is similar to what happens when a fast-moving object is captured by a camera with a slow shutter speed. Viewing rain at night, particularly with a strong backlight like a streetlamp, exaggerates this streaking by highlighting the path of the illuminated drop.