Hydroplaning (Aquaplaning)

Hydroplaning (called aquaplaning in Europe and Asia) occurs when water on the roadway accumulates in front of your vehicle's tires faster that the weight of your vehicle can push it out of the way. The water pressure can cause your car to rise up and slide on top of a thin layer of water between your tires and the road. While hydroplaning your vehicle rides on top of the water, like a water skier on a lake. In less than a second, your car can completely lose contact with the road, putting you in immediate danger of sliding out of your lane. This usually happens at higher speeds, over 40 miles per hour. Try to imagine your vehicle traveling at a high rate of speed on a sheet of ice: that image approximates what will happen if you try to brake or steer while hydroplaning.

The 3 main factors that contribute to hydroplaning:

Vehicle speed. As speed increases, wet traction is considerably reduced. Since hydroplaning can result in a complete loss of traction and vehicle control, you should always reduce speed, paying attention to the traffic around you.
Tire treads depth. As your tires become worn, their ability to resist hydroplaning is reduced.
Water depth. The deeper the water, the sooner you will lose traction, although even thin water layers can cause a loss of traction, including at low speeds.
Let's examine what happens to a tire in the midst of a hydroplane. When entering a puddle, the surface of the tire must moves the water out of the way in order for the tire to stay in contact with the pavement. The tire compresses some of the water to the sides, and forces the remaining water through the tire treads. With good tires, a moderate rate of precipitation, and a well-drained roadway surface, hydroplaning rarely occurs below 55 mph. However, if any of those conditions are not met, it can happen at speeds as low as 35 mph.

On a smooth polished road in moderate rain at 60 mph, each tire has to displace about a gallon of water every second from beneath a contact patch no bigger than a size nine shoe. Each gripping element of the tread is on the ground for only 1/150th of a second; during this time it must displace the bulk of the water, press through the remaining thin film, and then begin to grip the road surface. Although bald tires give better grip on dry roads than treaded tires, they are unsafe in rain because water is a lubricant on rubber. (Also, punctures are more common in the rain.)

Hydroplaning science - Hydroplaning is the result of your tires moving quickly across a wet surface - so fast that they do not have sufficient time to channel that moisture away from the center of the tire. The result is that the tire is lifted by the water away from the road, losing all traction.

Of course the word 'quickly' is a relative term. Tread design, tread depth, weight of your vehicle, tire pressure, depth of water and even the consistency of that water - (whether it is highly aerated or not, for example) - all play a part in determining at what speed the tire will begin to hydroplane. It is a pretty safe bet to assume that any speed in excess of 60 MPH is fast enough to support hydroplaning regardless of the other variables. This is not to say that at 55 MPH you are safe, however.

The exact point at which your vehicle becomes waterborne (transition point in the above diagram) is complicated and depends on all of the following variables:

Tire size - the size and shape of a tire's contact patch has a direct influence on the probability of a hydroplane. The wider the contact patch is relative to its length, the higher the speed required to support hydroplaning.
Tire tread pattern - certain tread patterns channel water more effectively, reducing the risk of hydroplaning.
Tire tread depth - as your tires become worn, their ability to resist hydroplaning is reduced.
Tire pressure - keep your tire pressure within the manufacturers recommended pressures.
Water depth - the deeper the water, the sooner you will lose traction, although at higher speeds even thin water layers can cause a loss of traction.
Water composition (oil, temperature, dirt, & salt can change its properties and density)
Vehicle drive-train: because of their computer-assisted differentials, all-wheel-drive vehicles are more likely to hydroplane than two wheel drive vehicles in certain situations. A sudden uncontrolled transfer of power from the front tires to the rear tires can put a hydroplaning AWD vehicle out of control.
Vehicle speed - as speed increases, wet traction is considerably reduced. Since hydroplaning can result in a complete loss of traction and vehicle control, you should always reduce speed.
Vehicle weight - the lighter the vehicle, the more likely it is to hydroplane.
Road surface type - non-grooved asphalt is considerably more hydroplane-prone than ribbed or grooved concrete surfaces.
How can you tell that you're hydroplaning? It is often hard to tell when you are hydroplaning. The rear end of your vehicle may feel a little squirrelly (loose, giving you the sensation that it has moved to one side or the other), especially in a high crosswind. The steering may also suddenly feel loose or little too easy. Watch the road ahead for standing or running water. You can also pay attention to the spray being kicked up by the cars in front. If it suddenly increases it's possible that the driver has hit a patch of water that could cause you to hydroplane.

What to do if you start to hydroplane - There are two absolutely essential no-no's to remember should you experience the beginning of hydroplaning:

Do not apply your brakes
Do not turn your steering wheel
If you find yourself hydroplaning, do not brake or turn suddenly. This could throw your car into a skid. Think of your steering wheel as the rudder of a boat (your vehicle is a boat when in the middle of a hydroplane). Hold the wheel firmly and don't steer in any other direction but straight ahead. Ease your foot off the gas until the car slows and your steering returns to normal. If you need to brake, do it gently with light pumping actions. If your car has anti-lock brakes, then brake normally; the car's computer will mimic a pumping action, when necessary. If your vehicle's tires are still in partial contact with the road surface, you should be able to regain control of the vehicle in the same way that you would on snow or ice.