ASK THE WEATHER GUYS | PRECIPITATION
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A large raindrop, about one-quarter of an inch across, or about the size of ahouse fly, has terminal fall speeds of about 20 mph. The smallest raindrops fall at about 2 mph. |
Question: What is the shape of raindrops?
Answer: While cartoonists typically draw raindrops like a teardrop or a pear shape, raindrops are not shaped like that.
They are drawn as teardrops to give the image of falling through the atmosphere, which they do. But as they fall, raindrops are flattened and shaped like a hamburger bun by the drag forces of the air they are falling through.
Raindrops are at least 0.5 millimeters or 0.02 inches in diameter. You will not find a raindrop bigger than about one-quarter of an inch in diameter. Larger than that, the drop will break apart into smaller drops because of the air resistance. Precipitation drops smaller than 0.02 inches in diameter are collectively called drizzle, which is often associated with stratus clouds.
The typical speed of a falling raindrop depends on the size of the drop. Gravity pulls everything downward. As an object falls it experiences a frictional drag that counters the downward force of gravity. When the gravity and frictional drag are balanced, we have an equilibrium fall speed that is known as the terminal velocity of the object. The terminal velocity depends on the size, shape and mass of the raindrop and the density of the air. Thus, it is worth talking a bit about the shape and size of raindrops.
A large raindrop, about one-quarter of an inch across or about the size of a house fly, has terminal fall speeds of about 10 meters per second or about 20 mph. That kind of speed can cause compaction and erosion of the soil by the force of impact. Since raindrops come in a variety of sizes, they fall with different speeds. The smallest raindrops fall at about 2 mph. Water droplets smaller than these smallest raindrops (known as cloud liquid water droplets) can resist falling in the atmosphere because there is upward moving air that overcomes the force of gravity and keeps them suspended in the cloud.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
The terminal velocity depends on the size, shape and mass of the raindrop and the density of the air.
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