Raindrops break the speed limit

Some of those tiny raindrops that keep falling on your head may be outlaws, of a sort. They have been caught breaking the speed limit.

A falling object reaches what’s known as its terminal velocity when friction — the slowing force of air — cancels the downward pull of gravity.  That means the drop stops speeding up and keeps falling at a steady rate. This should be the top speed at which a droplet can move. Yet scientists have observed raindrops plummeting faster than their terminal velocity.

Michael Larsen is an atmospheric physicist at the College of Charleston in South Carolina. Bigger raindrops have a faster maximum speed than smaller ones. That’s why meteorologists often use terminal velocity to estimate the size of raindrops, he says. These estimates help determine how much rain a storm deposits over an area. So the existence of fast-fallers suggests that rainfall estimates could be distorted, Larsen told Science News.

“If you’re going to understand rain, you need to make guesses,” he says. However, he adds, “If our guesses are wrong as to how fast these drops are falling, that could ultimately affect a whole bunch of other work.”

The puzzle

A raindrop's size grows inside a cloud. A drop’s one-way ride begins when it becomes heavy enough that gravity pulls it toward the ground. But air friction slows it down. Eventually, these upward and downward forces cancel out, and the drop should maintain a constant speed: its terminal velocity. (Velocity is a measurement of how fast and in which direction an object moves.) Every object falling through the atmosphere, from skydivers to hailstones, has a terminal velocity.

Raindrops larger across than 0.5 millimeter (0.02 inch) fall with a terminal velocity of several meters (feet) per second. Smaller drops fall more slowly — less than 1 meter (3.3 feet) per second. Several years ago, scientists reported seeing small drops falling faster than their predicted terminal velocity. Those researchers suspected these drops might have broken off of larger ones as they splashed against the sensor that was used to measure drop speeds.

Larsen wanted to know if such fast drops really exist. So he and his team used a rain monitor that each second took more than 55,000 pictures of falling rain. Those images helped the researchers measure the size, speed and direction of the falling drops. The researchers collected data on 23 million individual drops that fell during six major storms.

Among the smaller drops, 3 out of every 10 fell faster than their terminal velocities, Larsen’s team reported online October 1 inGeophysical Research Letters.

“We don’t know exactly what the cause is, but we’re very confident it’s not just hitting the edge of the instrument,” Larsen toldScience News. The small drops might have broken off larger drops in-flight. These may then have continued to fall at the faster speed, he says. If they had kept falling long enough, they might eventually have slowed down to their predicted terminal velocity.

Francisco Tapiador is a climate scientist. He works at the University of Castilla-La Mancha in Toledo, Spain. The smaller drops aren't true “rain,” he argues. They're just drizzle, he told Science News. So scientists may need to find a different way to calculate the terminal velocity of these mini drops, he says. Then, the data may show that the problem is not with the drops, but with how their top speed is calculated.

Power Words

climate   The weather conditions prevailing in an area in general or over a long period.

drizzle    Light mist-like precipitation caused by water droplets smaller than those due to rain, meaning usually much smaller than 1 millimeter (0.04 inch) in diameter.

estimate  To calculate approximately (the amount, extent, magnitude, position, or value of something).

force  Some outside influence that can change the motion of a body or produce motion or stress in a stationary body.

friction  The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage the surface of the materials rubbing against one another.

gravity  The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

terminal velocity  The fastest speed at which something should be able to fall.

velocity The speed of something in a given direction.

weather  Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.