train1900's Blog

Raindrops are teardrop shaped...Right?

Actually, NO! Most illustrators and even science books will showraindrops as tear shaped. The idea has persisted in popular culture forunknown reasons. Perhaps the aesthetics of a tear-shaped rain dropcreate a joint level of understanding among people. We see the tearshape and we think...rain. Your local TV news channel will evenillustrate a coming rain storm with tear shapes!

Raindrops are Round...At First.
Raindrops start out as round high in the atmosphere as water collectson dust and smoke particles in clouds. But as raindrops fall, they losetheir rounded shape. Instead, a raindrop is more like the top half of ahamburger bun. Flattened on the bottom and with a curved dome top,raindrops are anything but the classic tear shape. The reason is due totheir 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 tensionof water. This surface tension is the "skin" of a body of water thatmakes the molecules stick together. The cause is the weak hydrogenbonds that occur between water molecules. On smaller raindrops, thesurface tension is stronger than in larger drops. The reason is theflow of air around the drop.

Air flow on the bottom of the water drop is greater than the airflow atthe top. At the top, small air circulation disturbances create less airpressure. Therefore, the surface tension at the top of the raindropwins this round! In other words, the surface tension at the top allowsthe raindrop to remain more spherical while the bottom gets moreflattened out.

Raindrops CAN Get Too Large
Even as a raindrop is falling, it will often collide with otherraindrops and increase in size. Once the size of a raindrop gets toolarge, it will eventually break apart in the atmosphere back intosmaller drops. This time, the surface tension loses and the largeraindrop ceases to exist. Instead it pulls apart when it grows toaround 4 millimeters or more.

How Do You See a Raindrop in Action?
Due to the high speeds at which water falls in the atmosphere andbecause downdrafted winds can actually increase the speed of a waterdroplet, it is very difficult to test this idea without some sort ofhigh speed photographic analysis. An experiment you can do at homerepresents an analysis of raindrop shape through experimentation.

References:
NOAA National Weather Center
USGS Water Science for Schools

http://weather.about.com/od/cloudsandprecipitation/a/rainburgers.htm