Dear Mr. McQuaid,
Long time reader, first time questioner (is that even a word). I was recently informed that if you are driving fast enough in a convertible with the top down while itâ€™s raining then you, and the interior of the car itself, will not get wet. Is this true? And, if it is true, how fast must you be going in order to stay dry?
Curious in Worcester,
It’s doable…but extremely unlikely in a convertible. The problem is that you are unlikely to achieve the required speeds in any car. Sprinkling and misting are another story….
In order for the steady rain to not fall in the car, you will need to provide some force that pushes it away (up or to the side). Generally speaking, the rain will fall downwards and into the car when it is moving at normal speeds – aerodynamic flow is not enough. There’s always going to be some rain ready to fall into the car no matter how fast you are going. If I could draw in MSPaint, I’d supply you a picture, but for our purposes, imagine raindrops falling across the screen, and your car under them.
If you wanted to go fast enough to repel the rain, you’d need to create a shock wave powerful enough to shift the rain away from the vehicle. This requires some major speed, and the only way to do it reliably is to approach and break the sound barrier. The compressed air from your vehicle traveling at above the speed of sound should be enough to shift the rain away from car and keep you dry. Of course, you are dealing with other risks, such as the air damaging your head…
Now, if it’s just sprinkling or misting, that’s another story. The aerodynamic flow of air over the roof of the car is probably enough to divert most of the rain in that case. The speed required is proportional to the size of the raindrops. But a hard rain won’t be shifted by anything less than a shock wave…