Why a Raindrop Has a Flattened Bottom
A drop’s shape is a tug-of-war between two forces. Surface tension—the pull among water molecules at the surface—acts like a thin elastic skin, resisting changes in shape. Left undisturbed, that pull favors a sphere, which encloses a given volume with the least surface area.
As a drop falls, it moves through air rather than empty space. Air resists that motion, and the faster the drop falls, the stronger the resistance becomes. The pressure is greatest where the air meets the drop, pushing upward against its lower face. The drop therefore flattens underneath while surface tension pulls the water back toward a rounder shape.
Size Changes the Balance
In a small drop, surface tension is strong compared with the air’s push, so it keeps the drop nearly spherical. As a drop grows, its weight and falling speed increase, and airflow can deform it more. Its underside becomes flatter; in larger drops, the middle may spread outward while the top remains curved.
The shape is not simply a tear pointed downward. A tear shape would require water to hold a narrow tip against the forces acting on a falling drop. Instead, the drop’s outline reflects the balance between the water’s surface pull and the air’s resistance.
Why Drops Do Not Grow Without Limit
Deformation also makes a large drop less stable. Air pushes on its broad front face, while surface tension tries to hold the whole mass together. If the airflow’s force overcomes that restoring pull, the drop can stretch, develop a hollowed or bag-like form, and break apart. The resulting smaller drops have less mass for the air to distort, so surface tension can round them up again.
That breakup puts a practical limit on how large a falling raindrop can remain intact: adding water eventually changes not just its size, but the balance of forces that holds it together.
