Movement of Raindrops on Car Windshield Has Scientific Explanation As Gravity Forces Cause Them to Move

Fluid mechanics researcher Sungyon Lee from the University of Minnesota in Minneapolis and Alireza Hooshanghinejad, a fluid mechanics researcher from Cornell University, applied mathematical equations to describe the forces on raindrops.

As specified in a ScienceNews report, staring at raindrops on a car windshield, is more than just a technique or strategy for one to get in touch with his "emotional side." Rather, he might want to discover some physics, as well.

Essentially, as a car speeds along in the rain, some water droplets coming from the rain slide up the windshield, others are sliding down, and some appear to get stuck in place. Commenting on what's being observed, Lee said, "It's very hypnotizing, isn't it?"

Forces from Gravity

The work entitled, Dynamics of a partially wetting droplet under wind and gravity, showed numerous factors that identify the behavior of a droplet, the two mechanics researchers reported earlier this month in the journal, Physical Review Fluids.

Lee and Hooshanginejad explained that raindrops on an angled windshield of a moving car simultaneously experience forces from gravity and the wind speeding over the vehicle. The direction in which a raindrop is moving is dependent on its size.

For larger raindrops, gravity wins, which is pulling the droplets down. Meanwhile, for tinier raindrops, the wind is prevailing, which makes it push them up the slope.

On the other hand, the forces are balancing out for medium-sized raindrops, and the droplets are sitting still. The smallest raindrops are staying put as well since the wind does not provide "adequate oomph to overcome" the inclination of the water to adhere to the glass.

Increase in Wind Speed

Other factors are impacting the behavior of raindrops, too. As the speed of the car, and therefore the speed of the wind increases, larger raindrops are getting pushed up the windshield.

Reducing the speed of the vehicle has the opposite impact. And if the car is moving slowly enough, there would not be an adequate wind to coax any raindrop to move upward.

On the other hand, the windshields are more steeply angled, enabling tinier raindrops to succumb to gravity than shallower windshields are doing, as suggested by the results. For the calculations to be simplified, the study investigators examined further an idealized version of raindrops, according to just two dimensions.

Meaning, the results will not be able to translate perfectly to real-world raindrops, explained Lee, although it can still provide a sense of the reason water droplets are doing what they are doing.

Speed of Falling Raindrops

According to The Weather Guy, as the raindrops fall, they are flattened with a shape like a hamburger bunny the drag forces of the air they fall through.

Raindrops are approximately 0.5 millimeters in diameter. One fill finds a raindrop any larger than roughly a quarter of an inch in diameter, bigger than that; the drop will break apart into smaller drops due to air resistance.

Essentially, raindrops are larger in size. More so, a large drop, roughly a quarter of an inch through, or roughly the size of a housefly, has terminal fall speeds of approximately 10 meters a second or roughly 20 mph. This kind of speed can lead to compaction and erosion, of the soil brought by force of impact.

Scientific explanation about the raindrops on the windshield is shown on the American News's YouTube video below:

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