The Physics
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Speed of a Falling Raindrop

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Corbert, John H. Physical Geography Manual. 1974. 5th ed. N.p.: Kendall/Hunt, 2003. 127. "A large drop of about 5 mm (3/16 in.) diameter reaches a maximum speed of about 9 m/sec." 9 m/s
"Climate." Encyclopedia Britannica.2007, Encyclopedia Britannica Online. 25 May 2007. "Large raindrops, up to six millimeters in diameter, have terminal velocities of about 10 metres per second and so may cause considerable compaction and erosion of the soil by their force of impact." 10 m/s
Beard, K.V. Terminal Velocity and Shape of Cloud and Precipitation Drops [pdf]. Journal of the Atmospheric Sciences (May 1976): 851-864. [see graph 1 below] 9–13 m/s
Spilhaus, A.F. Raindrop Size, Shape, and Falling Speed [pdf]. Journal of Meteorology. 5 (June 1948): 108-110. [see graph 2 below] 9.3 m/s
Holladay, April. Falling raindrops hit 5 to 20 mph speeds. Wonderquest. Albuquerque: 19 Dec 2001. "At sea level, a large raindrop about 5 millimeters across (house-fly size) falls at the rate of 9 meters per second (20 miles per hour). Drizzle drops (less than 0.5 mm across, i.e., salt-grain size) fall at 2 meters per second (4.5 mph)." 9 m/s

Rain is the liquid form of precipitation on Earth. It is part of the hydrologic cycle that begins when water evaporates and forms clouds in the atmosphere. The water that forms these clouds is frozen and vaporized. Once enough water has evaporated, it is then released in the form of droplets of rain back to the surface of the Earth.

The average size of a raindrop is 6 millimeters in diameter, about the size of a housefly. Of course all raindrops vary in size due to the strength of a specific rainstorm, but this is considered a reasonable value of a typical raindrop. When a raindrop falls to the surface of the Earth, it is acted on by two main forces, gravity and drag. A stationary raindrop initially experiences an acceleration due to gravity of 9.8 m/s2, as would any falling body. As gravity increases the speed of the raindrop in its descent, drag retards the downward acceleration of the raindrop. Usually, air resistance that comes in contact with the water molecules as they fall causes the drag. The combination of these two forces causes a raindrop to reach a terminal velocity when the drag force is approximately equal to the weight of the raindrop. At this point, a raindrop experiences no further acceleration and therefore falls at a constant velocity.

The magnitude of the terminal velocity of an object is also affected by its orientation. A common misconception is the shape of the raindrop. It is often depicted as pointy and lopsided. However, research has found the shape of a raindrop to be rather spherical or slightly flattened on the bottom by airflow like a hamburger bun.

The terminal velocity of a 6-millimeter raindrop was found to be approximately 10 m/s. This value has been found to vary between 9 m/s and 13 m/s when measurements were taken on different days. The variance has been contributed to different air temperatures and pressures. In comparison, a human being falling to the surface of the Earth experiences a drastically larger terminal velocity of approximately 56 m/s.

Evan Kaplan -- 2007

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