Here’s how to stay dry in the rain with the top down

TL;DR – You have to be driving fast enough. How fast? That depends on what car you have.

Aerodynamics is a different type of animal when it comes to cars. Imagine that you can have the most powerful engine in the world, but if you have the aerodynamics of a box, your car might struggle to make ends meet because air carries weight as an object moves through it. In a Youtube Video by Drive Tribe, Mike Fernie explains the science behind keeping dry in the unfortunate event that it starts to pour. 

So let’s say you’re in a convertible car like the Mazda MX-5. Great car in the dry, since you can opt to have wind in your hair and the sun unimpeded as it beats down on your skin. It’s something else with the top down, we’d like to call it an experience because there is nothing more liberating than being in a convertible with the top down, except maybe for a motorcycle, now that’s a truly raw experience. 

It’s all well and good when the weather is cooperative, but this rainy season maybe keeping the top up is a wiser choice. Without a roof over your head, the occupants of your posh car will be left exposed and battered by the potentially torrential rains that make landfall onto Philippine soil. 

So you’re left with two options when it starts raining, either you pull over to the side of the road and deploy the roof as fast as you can, or you can drive it out. Let’s say you go with the first option, you’d be lucky to find yourself a nice little overpass to rest under and put up your roof, but if your luck isn’t that good well you’d get wet for sure.

But, let’s say you opt for the second option and refuse to stop. In theory you can stay dry in the rain without the top, that is if you can find the space to drive fast enough. Let’s explain how aerodynamics can get you out of this sticky situation. Remember that when air hits an object at speed, it follows the body lines and creates a boundary layer between the solid object and the air itself. 

So as air moves over your car’s body, it tends to stick close to the boundary layer of whatever it hits. Just like water flowing underneath the hull of a boat, air does the same thing. However if there is no more surface for the air to stick on it becomes turbulent and goes in different directions. In a convertible with the top down when the windscreen ends it creates a separation point that divides the air into two types of flow, high speed and low speed. High speed air goes up, while low speed air goes into the cabin. If you watch some videos of people driving fast in convertibles, you notice that air tends to blow people’s hair in the opposite direction of travel.

If you move fast enough, the previously mentioned boundary layer acts like a pseudo raincoat, keeping the rain from falling on you. This is because the air flow is enough to push the falling water molecules away from the occupants of the cabin provided that the car is moving fast enough. 

So the question is, how fast do you have to go? The answer is, it depends on the rake angle of your windshield. What’s a rake angle? Well in simpler terms it is how close the end of your windscreen is to the body. The Ford Mustang Convertible has a lower rake angle, and the MX-5 has a higher rake angle because the Mazda’s windshield is much more upright. So what does this mean? Essentially, the more upright your windshield is, the lower your speed has to be. At fairly medium speeds, the air passing through the body of your car will be accelerated upwards much faster than a car with a lower slung windshield. The dramatic change in angle lets you drive slower, yet still remain as dry as our other example the Ford Mustang Convertible. 

The answer is that it depends, but provided that the rain is falling perpendicular to the ground, that means that you need to achieve a speed of more than 72km/h – provided your windscreen is steep. In cars with windscreens that are less upright, expect to drive faster, unless it comes with a wind deflector on the windscreen and behind your head. These deflectors dramatically increase the amount of air being deflected up and away significantly giving you the same effect, but with a lower slung roof line. 

So there you have it, drive at around 70km/h in a light to medium drizzle and you’ll be fine. Though I guess option A is more feasible since traffic in the Metro moves slow. But hey, the more you know. 

Source: Drive Tribe