Raindrops

Raindrops


Raindrops are not tear-shaped (they more resemble the shape of a tiny hamburger bun).


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Much like the snowflake symbolizes all things winter, a teardrop is a symbol of water and rain. We see them in illustrations and even on weather maps on TV. The truth is, a raindrop assumes several shapes as it falls from a cloud—none of which resemble teardrops.

Raindrops, which are collections of millions of tiny cloud droplets, start out as small and round spheres. But as raindrops fall, they lose their rounded shape thanks to the tug-of-war between two forces: surface tension (the water's outer surface film which acts to hold the drop together) and the airflow which pushes up against the raindrop's bottom as it falls.

When the drop is small (under 1 mm across), surface tension wins out and pulls it into a spherical shape. But as the drop falls, colliding with other drops as it does so, it grows in size and it falls faster which increases the pressure on its bottom. This added pressure causes the raindrop to flatten on the bottom. Since the airflow on the bottom of the water drop is greater than the airflow at its top, the raindrop remains curved on top, the raindrop resembles a hamburger bun. That's right, raindrops have more in common with hamburger buns than falling on them and ruining your cookout—they are shaped like them!

As the raindrop grows even bigger, the pressure along its bottom increases further and presses a dimple into it, making the raindrop look jelly-bean-shaped.

When the raindrop grows to a large size (around 4 mm across or larger) the airflow has pressed so deeply into the water drop that it now resembles a parachute or an umbrella. Soon after, the airflow presses through the raindrop's top and breaks it apart into smaller drops.

To help visualize this process, watch the video, "Anatomy of a Raindrop," courtesy of NASA.

Due to the high speeds at which water droplets fall through the atmosphere, it is very difficult to see the variety of shapes it takes in nature without the use of high-speed photography. However, there is a way to model this in the lab, the classroom, or at home. An experiment you can do at home represents an analysis of raindrop shape through experimentation.


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