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 RAINDROPS AND STARS

METEOROLOGIST JEFF HABY

There are cases in which the name of an object does not reflect the shape that it is drawn. For example, a star in the sky is basically a round object. However, when a star is asked to be drawn typically the shape that will be drawn in the “star” figure shown in the diagram below. When viewing a star in the sky, the twinkling effect can make it look like a star is not perfectly round. The star drawn with the triangles coming off of it is more “aesthetically pleasing” than just drawing circles. If the American flag had circles for the 50 states instead of the 5-point star shapes it just would not look the same. The 5-point stars are the more attractive choice.

The case I want to focus the rest of this writing on is the shape of a raindrop as it falls. Just like with the sun, planets and stars, the shape of a falling raindrop is basically round (or is it?). When asked to draw a falling raindrop, a common drawing is the “tear drop” shape shown in the diagram below. For a variety of reasons, the teardrop shape seems like the logical choice for many people. For small size raindrops, a falling raindrop is basically a circular (spherical) shape as shown in the diagram below. Small raindrops fall slowly and the mass is within a smaller volume thus the raindrop can hold the basic spherical shape. As a raindrop grows larger, it becomes distorted due to the increase of falling velocity and the interaction of the air with the raindrop. As the raindrop distorts it begins to look more elliptical. Actually, a truer shape that a larger raindrop takes on looks more like the top of a hamburger bun shown in the diagram below. The interaction with the falling raindrop with the air flattens out the bottom of the raindrop.

As a hamburger bun shaped raindrop grows larger, it becomes even more distorted. As size increases, a concavity develops in the center portion of the raindrop. This concavity grows and can give the raindrop a “sun glasses” shape look such as in the diagram below. At this stage the raindrop is breaking into two. Raindrops have a limit to the size they can grow thus raindrops as big as golf balls and baseballs is not going to happen. Once a raindrop is several millimeters in diameter it will go through the process of breaking apart. Raindrop can break apart also by colliding with other raindrops. Thus in conclusion, the typically shape of a raindrop in reality is spherical for small drops and a hamburger bun shape for larger raindrops.