# DEFY S. McQUAID! #74: Rain and the speeding automobile

Posted by smcquaid

The Question

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,
Bisol

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…

Filed in Features, Smcquaid
Jul 18, 2006
6:55 am
#1 Kurt from Work :

hhhhmmmm….Sonic Boom Acclaim…..I see a science experiment when MikeD gets home.

Jul 18, 2006
12:24 pm
#2 Aaron :

Here’s my thought: let’s assume it’s a short-bodied convertible, like a Miata or a TT. Maybe a 2 foot drop fromthe top of the windshield to the top of the door, and thus, the interior of the car (probably giving myself too much room here, but stay with me). Say from the end of the windshield to the end of the passenger compartment is 4 feet. This means that in order to keep from getting hit by the water, you have to travel twice as fast as the terminal velocity of the rain. Right? So, if you can just get some really thick air to drive through, and slow down the rain, you should be able to do it without a shock wave.

Jul 19, 2006
8:18 am
#3 ben :

The Mini Convertible has mentioned this endlessly, you can outrun rain at about 50+…. there was even a flash animation. While I’ve not tested it myself, it’s hard to argue with a convinving flash animation.

Jul 19, 2006
10:22 am
#4 smcquaid :

Showers or misting, fine – the aerodynamics (of a Mini, especially) can do it. But a hard rain? Nope.

Jul 19, 2006
12:12 pm

In a hard rain, would it be possible to drive a Mini fast enough so that your head doesn’t get rained on (but maybe the back seats got wet?)?

I think this is possible on a motorcycle with a big CHIPS-style windshield. Then again, you are much closer to the windshield on a motorcycle.

Dec 4, 2006
10:13 am
#6 Elby :

This can probably be doable but very unlikely to be accomplish. You must realize that you will be driving between raindrops. Not likely.

Dec 5, 2006
1:06 pm
#7 Jes Saint :

That depends on how hard it’s raining.

Aug 17, 2007
11:31 am
#8 KRistoph :

UMMMM YOU GUYS ARE ALL WRONG, I had a MASZDA MX-5 I WITH REAR WIND BLOCKER AND I DROVE THAT CAR HUNDREDS OF TIME IN THE RAIN WEATHER IT BE MISTING OR A RAIN STORM!!!! I ONLY GOT WET FROM THE WATER ATTACHING TO SIDE WINDOWS THEN SWINGING INTO THE SIDE OF THE CAR.

Aug 21, 2007
9:53 am
#9 smcquaid :

The fact that this response is in all caps means it HAS to be true. Right?

No, wait, not at all.

I’ve been thinking about this one, and if the windshield is high enough, Old Man Schenk may be right in the possibility that the front seats stay dry, while the rear gets wet. The MX-5, of course, is so small that it doesn’t quite have rear seats, so Mister Capital Letters may be correct.

Aug 22, 2007
11:49 am
#10 Theresa :

Mr McQuaid is awesome. I love this column.

Mar 11, 2008
10:37 pm
#11 spiro :

i have a mercedes, smart car, 2 seater , was going 80-100 klm per hour and i wasnt getting wet! pouring rain , the back part of the car got wet , but me , the front part of the car and the passenger seat.made it clear! my roof broke this afternoon so got trapped out when it started rainning! didnt want to test the theory! , just wanted to share this.

Mar 12, 2008
8:41 am
#12 mike d. :

So I decided to do some math to try and put this to rest. I’m not sure Mr. McQuaid is completely accurate in his breakdown. First lets look at logic with the MSPaint that SMcQuaid alludes to.

MSPaint

In order for Joe Convertible to stay dry, he has to be going fast enough so that the first drop of water that’s not blocked by his windshield won’t hit him in the head. I took a basic guess with car dimensions and said that he’d have to cover 2 feet in the time it took a drop of water to fall 6 inches.

Some additional research leads us to this website which tells us that raindrops hit the ground between 5 and 20 mph. This converts to about 87 – 351 inches/second.

6/351 = 0.017
6/87 = 0.069

This means it takes between 0.017 – 0.069 seconds for a raindrop to fall 6 inches.

So we have to cover 24 inches in that same time frame in order for Joe to stay dry. What’s that come out to?

24″/6″ = 4

This means we have to be driving four times faster than the raindrop.

With our example, this means that on a typical rainy day Joe Convertible can drive 20-80 mph (32-128 km/h) in order to stay dry.

We can expand this to include the whole car by switching to variable analysis.

‘y’ is the distance the drop must fall
‘x’ is the full length of the cabin
‘v’ is the velocity of your raindrop

(v*x)/y will give you the speed you need to travel in order to out run the water drops.

If your windshield is 24 inches tall, and your cabin is 8 feet long then the math works out the same way.

Now, rain never falls straight down. There’s an obvious wind factor. But to take this into consideration, just take the speed of the wind in the x direction and add (or subtract) it to the speed of your car. That should do it!

So yes, I think it is possible to outrun rain. But you’ll be out of luck at your first stop light.