Why are raindrops round?


Ah, an excellent question. So simple and graceful.  During his Miracle Year (Annus Mirabilis) of 1905, Albert Einstein  wrote four major papers: the photoelectric effect, Brownian motion, special relativity, and mass-energy equivalence.

His 1921 Nobel Prize was for his explanation of the photoelectric effect.

Decades later, Einstein was asked, why he, rather than other scientists, was successful in uncovering the secrets of the universe. He gave two reasons. First, “I never gave up, I was as stubborn as a mule” and secondly, “I always asked the simplest questions, the kind that children ask. I ask them still”. It is these “children’s questions” that underlie some the most deep and profound mysteries of the world around us.

In the case of raindrops, the secret is surface tension, the cohesion of water. Molecules that are alike tend to stick together. The cause is the weak hydrogen bonds that occur between water molecules. The surface tension of water acts as a thin film-like membrane. This allows a person to place a needle or paper clip on water and not have it fall through, even though steel is eight times more dense than water. Surface tension also allows those water strider or water skeeter bugs to stay atop the water surface.

Surface tension always wants to make the smallest area possible. There’s some neat math here. A sphere is the geometric figure that has the least surface area for any given volume.  Surface tension, same as cohesion, will pull any liquid into a round or spherical shape.

Raindrops start out high in the sky as water vapor condenses and collects on dust, smoke, or oxygen molecules. Tiny falling raindrops are round. But raindrops  collide  on the way down, and become bigger. Falling through the air and encountering air resistance, raindrops lose their round shape. They become more like the top half of a hamburger bun, flattened on the bottom, with a curved dome top.

That teardrop shape, popular with artists, weather forecast maps, and television weather reports, is clearly wrong.

The Earth and other planets have a round spherical shape for the same reason that raindrops are round. The Earth and the other solid planets were liquid when they formed. Surface tension and gravity force fluids, including planets and stars into round spheres. Stars, like the Sun, are round for the same reason. A sphere, or ball, is the figure that has the least surface area.

Some of the smaller moons of Jupiter and Saturn, and the rocks that make up the rings of planets, plus asteroids, are not spherical. They are misshapen and deformed chunks of material. They were never liquid when they formed.

The beauty and elegance of science is using the same physical principles to explain a variety of phenomena that are seemingly unrelated.


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