What determines whether or not snow sticks to my windshield?

[Ori Vandewalle]

It snowed last week and I noticed that, when my car was stationary, snow built up on the windshield, but above about 10 mph (4.5 m/s, if SI units are required here), the snow didn't stick. I'm wondering what accounts for the difference.

My first thought was that the extra kinetic energy of my car's motion would be enough to sublimate falling snowflakes. After a few mathematical missteps, however, I determined that my car would have to be going about a thousand times faster than I was willing to drive it to get the required amount of energy. That is to say, this was too little energy by a factor of about a million.

Which leaves me at somewhat of a loss. I wouldn't be at all surprised if the temperature of my windshield plays a role, but I can't see how my car moving forward would affect that equation.

The only other thought that comes to mind is that the air my car pushes through might exert some pressure on an incoming snowflake, such that the snowflake isn't able to land. But the physics I've taken was a little light on fluid mechanics, so I'm not sure how to proceed from there.

Any help you folks can provide would be greatly appreciated. I don't even require anyone to spell out the answer for me; if I can be pointed in the right direction, I'll try to work out the problem myself.

Some relevant factoids: Wiki says a typical snowflake is 10¹⁹ water molecules, which would give it a mass of about 3x10^-7 kg, and I estimated that a snowflake's terminal velocity is somewhere around 1 m/s from this paper.

Thanks.

 

[I like Serena]

Welcome to PF, Ori Vandewalle!

Snow sticks due to friction.
When you drive at speed, the air exerts a force (drag).
When the force of the air is greater that the required friction to stick, the snow flakes will slide off.

 

[Ori Vandewalle]

Heh. The physics I've taken certainly wasn't light on objects sliding along inclined planes. I really should have guessed that myself. Thanks!

There is a wrinkle, though, which is that finding the coefficient of friction between snow and glass is not exactly easy. What I can do is accept your hypothesis as true, guess that my 10 mph estimate is accurate, and see if a reasonable μ pops out of the force equation. Doing that, I've got friction and weight pulling the snowflake down the windshield, and drag pushing it up.

With an esimate of 3x10^-7 kg for the snow, the drag coefficient of a sphere for the snowflake (.1), a circular snowflake with a diameter of 1 cm, and a 40° angle for my windshield, I get a μ_k of .53. Looking around at other coefficients of friction for snow, I bet this is a little high, which means the critical speed is lower (likely) and/or my rough estimate of the shape and size of a snowflake is wrong (very likely).

But hey, at least the results aren't entirely unreasonable.

 

[I like Serena]

I wouldn't know what the drag coefficient of a snowflake is.
It's not really spherical.

And I wouldn't know what the coefficient of friction is either.
It's not really a nicely defined mass with more or less smooth edges.

I guess the best way to find out is by trying it out.
And I suspect that different types of snow flakes exhibit significantly different behavior.

 

[LudusRex]

There are several factors that can determine whether or not snow sticks to your windshield. The first factor is the temperature of your windshield. If your windshield is below freezing, the snow will stick to it because the cold surface will cause the snowflakes to freeze upon contact. However, if your windshield is slightly above freezing, the snow may melt upon contact and not stick. The speed of your car can also play a role. As you mentioned, the kinetic energy of your car's motion may be enough to sublimate falling snowflakes. However, as you calculated, the speed required for this to happen is much higher than normal driving speeds.

Another factor to consider is the shape and size of the snowflakes. If the snowflakes are small and powdery, they may not stick to your windshield as easily as larger, wetter snowflakes. This is because smaller snowflakes have a lower surface area and less mass, making them more easily affected by air resistance and less likely to stick upon impact.

The airflow around your car can also play a role. As you mentioned, the air your car pushes through may exert pressure on the incoming snowflakes, making it more difficult for them to land and stick to your windshield. This is known as the Bernoulli effect, where the faster moving air creates a lower pressure zone, pushing the snowflakes away from your windshield.

In addition, the angle of your windshield can also affect whether or not snow sticks. A more angled windshield will create a smoother airflow, making it more difficult for snowflakes to stick. A more vertical windshield, on the other hand, may create turbulence and cause the snowflakes to stick more easily.

It's also worth noting that the type of snow can also affect whether or not it sticks to your windshield. Wet, heavy snow is more likely to stick than light, powdery snow. The moisture content and temperature of the snow can also play a role.

In conclusion, the temperature of your windshield, the speed and angle of your car, the shape and size of the snowflakes, and the airflow around your car are all factors that can determine whether or not snow sticks to your windshield. It's important to keep these factors in mind when trying to understand the physics behind this phenomenon.