Windshield Raindrops

At what speed would you have to drive for rain to shatter your windshield?

Daniel Butler

Fast.

Raindrops are tiny. Even in the heaviest rainstorms, the water in the air weighs less than the air itself (which is one of several reasons you can't swim upward in a rainstorm). Even at very high speeds, they can't break a windshield via their momentum alone.

Under ordinary circumstances, raindrops don't damage car windshields at all. However, they can destroy the windows of supersonic aircraft.

Here's what happens when a raindrop hits a glass surface at high speed:

When the droplet makes contact with the surface, a shockwave travels back up through the droplet.

Normally, this shockwave would move at the speed of sound within the liquid—about 1300 m/s, four times faster than in air. However, at high impact speeds, this shockwave actually moves substantially faster than the speed of sound in water.

The water is squeezed between the incoming drop and the glass surface, which makes it squirt sideways in all directions. These jets of water can move even faster than the original (already supersonic) droplet, and even faster than the shockwaves we mentioned.

One paper ran a simulation of water droplets hitting a surface at 500 m/s (about Mach 1.5), and found that the water sprayed out from the point of contact at over 6000 m/s—Mach 18.[1]That's a pretty simple way to expel material at 6 km/s. I wonder if anyone's ever tried to come up with a spacecraft propulsion system using it ...

The sharp pulse from the shockwave can crack glass. The highest pressures are found in the ring around the edge of the droplet, and only exist for a tiny fraction of the impact.

In addition to the direct downward pressure, the water jetting sideways can cause damage, too. If the material has any microscopic holes, cracks, or bumps, those jets can strike them and create new cracks or widen existing ones.

Even at high speeds, a raindrop won't create a bullet hole on its own—but a long series of supersonic droplets would start to eat away at the glass, cracking and pitting it like sand.[2]This type of "erosion" can also cause damage to steam turbine blades. Eventually, the windshield could fail catastrophically.

Luckily, cars can't drive at Mach 1 without lifting off, so your windshield is safe from ordinary rain. On the other hand, if you're driving under a thunderstorm with strong updrafts ...

... the precipitation can smash your windshield at any speed.