Why do I have a hairy bonnet?

One of those cat hair / aerodynamics questions.

I washed my car with my new pressure washer toy the other day, and it blasted so much dirt out of the paintwork that it left the surface very low-friction. Schrodinger cat can’t resist sitting on a clean car, and I found him lounging about on the bonnet (hood) just asking to be tickled, which I did, causing him to slowly slide down the slope as he writhed happily. Leaving behind a shedload of cat hair as he went.

No problem, thought I, it’ll blow off when I drive away. Which I did a little later, a round trip of 250 miles (400km) to central London and back, cruising for about 3 hours at up to 100mph (160kph), and then lots of stop-starting when I hit London, so a constant cycling of speeds from 0 to 30mph (48kph). To my surprise, my car was still hairy when I stopped in London, and hairy still when I checked again upon my return home. The hairs weren’t stuck down with goo or static, and each and every one of them could be easily and demonstrably shifted by a gentle puff of air courtesy of my lungs. They were all, however, laid dead flat to the car surface. None had any gap between hair and car.

The question is: What bizarre effect causes easily dislodgable cat hairs to stick to my car for hours of driving at all sorts of speeds? I know there’s some low-pressure effects with car aerodynamics (which is why my rear windscreen never sees any raindrops above 20mph), but I’m damned if I can explain this. For the record, it’s Autumn (Fall) here, and at the time the air was about 10 to 15 degrees Celcius, not wet but with the usual Autumn relative humidity of 100%.

Since no one else has replied, I’ll take a stab in the dark.

The hairs are within the boundary layer?

A couple of weeks ago I saw a grasshopper cling to my windscreen at 50 mph. It was still there five miles later when I parked.

Thanks Johnny LA, boundary layers look like a good line of enquiry.

I may have to do further experiments - getting the cat to deposit fluff on areas like the roof or rear aerofoil, experimenting with cigarette papers etc. It’d be interesting to find how deep the boundary layer is, as I suppose it only needs to be a few cat-hair widths deep for my car to remain slightly fluffy. It can’t be much deeper than that, as I recall trying to use magnetic ‘L’ plates as a learner driver, and the bonnet one lifted off above 10mph.

Give that man a prize.

I have been doing further hairy research today, courtesy of Schrodinger cat. His shorter haired sister Amy doesn’t fluff up my car so much.

After about 1 hour of driving today (between 0 - 90mph (144kph) I have observed cat hairs on my bonnet, roof and rear windscreen, but none on the rear aerofoil. They’re all laying down flat, and point in the direction of the airflow over the car.

They’re not shifting with a puff of the lungs today, but it is a little drizzly, and I think they’re stuck to the traffic film a little. I shall polish my car again and repeat the experiment.

My inflation raised $.03 would be static electricity.

Strong enough attraction to avoid being blown away in the boundary layer slipstream that Johnny L.A. mentions, but not enough to withstand Fridgemagnet’s impressive strength.