iPhone as RainVision Goggle

I was driving through a rain storm so severe that more than half the interstate traffic had pulled over to wait it out on the shoulder. I'd have done likewise, but, late for an appointment, I drove on, at about 30 m.p.h., with my wipers going full tilt.

For a while, it felt like driving underwater; a solid wash, the rain equivalent of a white-out. And for some reason I recalled that cameras don't register rain (filmed rain scenes are done via special methods). So I picked up my iPhone, which was already opened to the "Camera" app, and held it in front of me. Sure enough, visibility was near-perfect. I could even see cars ahead of me which I couldn't spot unassisted. It was amazing; akin to driving through pitch blackness with infrared goggles.

So I asked Pierre Jelenc, my longtime Technical Advisor (and proprietor of the legendary Gigometer) to explain why this works. Pierre's never come up short before, and this was no exception:

Jim,

The terminal velocity of large raindrops is about 9 m/s from what I can see (range 5-19 m/s). That translates to 37 cm in 1/24th of a second or 15 cm in 1/60th. A raindrop smeared into a streak of tens of centimeters will be invisible, though it will blur slightly each single frame.

The eye does not see individual frames, however: instead it averages out the blurs, blotches, visual noise, etc, which is why a running movie looks so much sharper than any of its individual frames. To film rain scenes, they use a showerhead just above the actors, so that the drops fall very slowly and can be captured in each frame without much blurring.

Cheers,
Pierre

It really works! Try it the next time it rains hard!

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