Falling raindrops hit 5 to 20 mph speeds
 What is the speed of a falling raindrop?
Ed, Las Vegas Nevada
Umbrella with raindrops. Photo courtesy of Fir0002 and Wikipedia
It depends on the size and weight of the raindrop how fast it falls: the heavier, the faster. 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).
A raindrop starts falling and then picks up speed because of gravity. Simultaneously, the drag of the surrounding air slows the drop's
fall. The two forces balance when the air resistance just equals the weight of the raindrop. Then the drop reaches its terminal velocity
and falls at that speed until it hits the ground. This simple view neglects updrafts, downdrafts, and other complications.
The air resistance depends on the shape of the raindrop, the cross-sectional area presented to the airflow, and the raindrop's speed.
Most drops are fairly round — the small ones spherical, larger ones flattened on the bottom by the airflow. At high speeds, the air
resistance increases with the square of the velocity.
By the way, a falling human hurtles to the ground at a terminal velocity of about 125 miles per hour.
Further Reading:
Alistair Fraser, Bad science: Bad raindrop shape
Weather Doctor: Early measurements of raindrops
(Answered December 19, 2001) Comment |
