With apologies to Garth Stein and his excellent novel by the same name, the art to racing in the rain isn't just an inherent skill that some drivers have and others do not. The truth is that learning to race in the rain - we're talking driving your car as fast as possible around a wet racing circuit - is part feel (or art, if you prefer) and part science. The feel part is something you have to develop from practice, practice and more practice. The science part, on the other hand, is something you can investigate intellectually, learn on paper and then go about applying in the real world. But to apply the science of racing in the rain skillfully and artfully, you've got to find yourself on a wet racetrack, which goes back to that whole practice, practice, practice thing.
A lot of racers in the United States, especially those of us who live in the dry southwestern part of the country, regard racing in the rain as something that should be avoided. I've seen some very reputable West Coast racing teams opt to park their car rather than risk a big off-track excursion in the rain. But in the Northeast, Northwest and throughout the UK and Europe, racing in the rain isn't only seen as a required part of a racer's skill set, it's often relished by drivers and seen as a way of separating the men from the boys. Racing in the rain is also an excellent way to develop smooth and precise inputs with the steering wheel, brakes and throttle because the repercussions of upsetting the car's tenuous grip are greatly magnified on a wet surface. The funny thing is, the smooth and precise driving technique required in the rain just so happens to translate to faster laps in the dry too, since the same principles of conservation of momentum and minimizing weight transfer apply just as much in the dry as they do in the rain.
A good place to start when thinking about how to go fast in the rain is to consider the one part of the car that's actually in contact with the wet tarmac: the tires. Before you even consider whether or not you're on a slick or semi-slick with little to no tread depth and ability to evacuate water from its contact patch, it's vital to realize that even the best rain tire reacts differently during use on a wet surface than it does on a dry surface.
A tire in the dry is able to build considerably more traction, and as it reaches its peak, the available traction levels off for some time before gradually falling off as you overdrive the tire and create excessive slip angle (experienced as understeer, oversteer or a four-wheel drift, depending on which tires lose traction). Since traction loss is fairly gradual in the dry, it means you'll be able to feel it happening soon enough to counteract the slip by reducing velocity and/or changing trajectory; in other words, you can catch the skid by braking a bit, easing off the gas and/or countersteering, depending on what the particular scenario calls for.
In the rain, however, a tire will reach its traction peak much earlier than in the dry and it will also level off more quickly and drop off more suddenly as slip angle increases. This means less time to react to a loss of traction, and if you don't react quickly enough things can go from bad to worse in a hurry. Whether you're conscious of it or not, you're probably using a slip angle of 6 to 10 degrees in the dry (generally considered ideal in terms of generating maximum grip from the tires), but in the wet the optimal slip angle is more in the range of 3 to 6 degrees. Understanding how your tires operate in the rain is the first step to maximizing speed while averting an unscheduled trip across the infield, but there are lots of other factors that will also help you find the fastest way around a wet circuit.
One of the first things you learn when racing in the rain is that the dry line is likely not the fastest/grippiest line in the wet. That's because the dry line tends to have a lot of rubber laid down on it, and this rubber layer can be slick when wet. The dry line can also be polished smooth over time, whereas the less used part of the pavement tends to be more abrasive and porous, making for better grip in the wet. It's also important to be aware of the fact that paint and concrete tend to have a lot less grip when wet (because these surfaces are less porous), so it's best to avoid concrete patches or painted lines or curbs.
Every track is different in the rain because the age and condition of its asphalt varies and drainage tends to differ too. So it always takes some experimentation to find the grippiest/fastest line around a circuit in the wet, but as a general rule it's best to shorten the turns and lengthen the straights by going in deeper, making a sharper but slower turn and then using a very late apex. And since water runs downhill, if a corner is banked you'll probably find more grip (less water) up high in the turn. Point being, use your eyes and your head and you'll find the driest and grippiest parts of the track to use when racing in the rain.
Now that you have an idea of how your tires will behave in the wet and how to find the best wet line, it's time to consider how your car behaves in the wet and how you as a driver can best control it. Traction changes in the wet, but not necessarily in a predictable way. Braking isn't nearly as compromised as acceleration. This means the driver will find himself struggling more with controlling wheelspin coming out of corners than fighting brake lock-up. It's also important to be conscious of the fact that lateral grip (cornering) is reduced more in the rain than longitudinal grip (braking and acceleration), so even though you may feel like there's great grip under braking don't be fooled into thinking the same amount of grip will be available in the turns or when jumping on the gas pedal.
Because lateral grip is so compromised in the wet, cornering speeds will be lower and weight transfer will be reduced, resulting in less body roll. This means that your car doesn't need as much roll resistance in the wet, allowing you to run your shock settings and sway bar settings at full soft (assuming your car is equipped with adjustable dampers and bars). Pro race teams even go so far as swapping softer springs onto the car if rain is a certainty. Softer suspension settings will allow the tires to stay in more constant and consistent contact with the road, and that's key to maximizing what little grip is available on a wet surface. Softer suspension settings will also make the car more forgiving to drive.
If you race a FWD or AWD vehicle that tends to understeer a fair bit in the dry, you may be accustomed to trail braking (braking while initiating turn-in for a corner) to induce some rotation. However, trail braking in the rain is a big no-no. In the rain, it's always best to brake, turn and accelerate in a deliberate and separate fashion - combine any of these and the result often includes a backward trip through the weeds.
Similarly, in the dry you may be accustomed to using short quick applications of the throttle to adjust the car's attitude in the turns and then jumping on the gas aggressively once you've got the car more or less pointed in the direction of the next straight. In the wet a subtler technique is required, where only smooth and gradual applications of the go pedal will avoid wheelspin and speed-robbing understeer.
Perhaps the biggest challenge when racing in the rain is visibility. Your helmet visor and windscreen can fog up and wheel spray from the cars in front of you can put up a wall of mist that is virtually impossible to see through. Hopefully, your car has a blower motor or windscreen defogger, but if it doesn't then applying some defogger solution (spraying on and wiping off foam shaving cream works remarkably well as a ghetto solution) to both sides of the windscreen and your helmet's visor is a good start. Rain-X is also a great product that helps speed rain drops off your windscreen, so much so that you won't even need to run your windshield wipers unless the rain is absolutely torrential.
As far as avoiding spray from other cars goes, the best solution is to be out front. If you follow the tips included in this story and get out there and practice, practice, practice in the rain, chances are you'll be out front kicking up lots of spray for the poor buggers behind you. Keeping your driving smooth, minimize tire slip angle, find the grippiest parts of the track and you'll also be the first to see checkered flag.
The Real Rainman
Jackie Stewart may not be able to count how many toothpicks hit the ground if you dropped a box of them on the floor, but he's undoubtedly one of the greatest rain racing drivers of all time. Under a thick fog and heavy rain, Sir Jackie dominated the '68 F1 German Grand Prix at the Nürburgring Nordschleife and a formidable lineup of racing legends including Jackie Ickx, Graham Hill and John Surtees. Starting sixth on the grid and nursing a broken wrist, Stewart knew he'd have no chance if he got caught in the blinding spray behind the lead cars, so on the first lap he charged to the front and never looked back, finishing the race some 4 minutes, 3 seconds ahead of the second-place finisher.