The shape of a falling raindrop is often depicted as a perfect teardrop, round at the bottom and pointed at the top. This popular image is inaccurate. The actual form of water droplets changes dynamically based on their size and the forces acting upon them, ranging from a perfect sphere to a flattened shape resembling a burger bun. Understanding the true geometry of rain requires looking beyond the visual myth and into atmospheric physics.
The Physics of Drop Formation
The initial and most common shape for a raindrop is a nearly perfect sphere. This spherical shape is the direct result of surface tension, a force that pulls water molecules inward. Surface tension attempts to achieve the smallest possible surface area for a given volume, which the sphere naturally satisfies.
Tiny droplets, typically less than one millimeter in diameter, are dominated by surface tension. Since their mass is small, external forces like air pressure or gravity are negligible compared to the cohesive strength of the water molecules. These small, spherical drops represent the baseline form of precipitation and make up a large portion of rainfall.
Size Matters: How Air Resistance Distorts the Shape
As droplets collide while falling, they grow larger and their descent speed increases significantly. This increased velocity introduces aerodynamic drag, also known as air resistance. This upward force of air against the falling drop begins to overcome the cohesive pull of surface tension.
Drops greater than one millimeter in diameter start to flatten out due to air pressure from underneath. They no longer maintain a spherical form but become an oblate spheroid, which is round on top and flattened on the bottom. This shape is often described as resembling a small hamburger bun. The larger the drop grows, the faster it falls, and the more pronounced the flattening becomes.
If a drop continues to grow, approaching diameters of about 4.5 millimeters, the intense air pressure pushing against its base overcomes surface tension. The drop distorts further, forming a concave cavity underneath and inflating into a temporary, unstable parachute-like shape. This unstable form breaks apart into several smaller, more stable droplets. The maximum size of a stable raindrop is limited by this aerodynamic instability.
Explaining the Teardrop Myth
The persistent misconception of the teardrop shape comes not from observing rain in freefall, but from water detaching from a solid surface, such as a dripping faucet or the edge of a leaf. As a hanging drop grows, gravity stretches the water downward while the top remains attached, forming the familiar pointed neck. Once the drop detaches, surface tension immediately pulls it into a rounded shape as it falls. The teardrop image is reinforced by artists and animators who use the simple shape to represent rain in cartoons and weather icons, cementing the wrong shape in the public imagination.