Raindrops are teardrop shaped...Right?
Actually, NO! Most illustrators and even science books will show raindrops as tear shaped. The idea has persisted in popular culture for unknown reasons. Perhaps the aesthetics of a tear-shaped rain drop create a joint level of understanding among people. We see the tear shape and we think...rain. Your local TV news channel will even illustrate a coming rain storm with tear shapes!
Raindrops are Round...At First.
Raindrops start out as round high in the atmosphere as water collects on dust and smoke particles in clouds. But as raindrops fall, they lose their rounded shape. Instead, a raindrop is more like the top half of a hamburger bun. Flattened on the bottom and with a curved dome top, raindrops are anything but the classic tear shape. The reason is due to their speed falling through the atmosphere.
The Bonding Forces Holding Rain Drops Together
A raindrop falling through the atmosphere forms as a roughly spherical structure due to the surface tension of water. This surface tension is the "skin" of a body of water that makes the molecules stick together. The cause is the weak hydrogen bonds that occur between water molecules. On smaller raindrops, the surface tension is stronger than in larger drops. The reason is the flow of air around the drop.
Air flow on the bottom of the water drop is greater than the airflow at the top. At the top, small air circulation disturbances create less air pressure. Therefore, the surface tension at the top of the raindrop wins this round! In other words, the surface tension at the top allows the raindrop to remain more spherical while the bottom gets more flattened out.
Raindrops CAN Get Too Large
Even as a raindrop is falling, it will often collide with other raindrops and increase in size. Once the size of a raindrop gets too large, it will eventually break apart in the atmosphere back into smaller drops. This time, the surface tension loses and the large raindrop ceases to exist. Instead it pulls apart when it grows to around 4 millimeters or more.
How Do You See a Raindrop in Action?
Due to the high speeds at which water falls in the atmosphere and because downdrafted winds can actually increase the speed of a water droplet, it is very difficult to test this idea without some sort of high speed photographic analysis. An experiment you can do at home represents an analysis of raindrop shape through experimentation.
NOAA National Weather Center
USGS Water Science for Schools