I recently received a question from a reader about humidity, asking if
it has an effect on brake pads. The mechanic informed the car owner that
the reason her front pads were squealing was due to humidity. Still not
convinced that the cause of the car’s brakes sounding like a cat
squealing at a high pitch was humidity, she reached out to me for
There have been studies done on this topic with results that are quite interesting. Passenger cars with NAO (Non-Asbestos Organic) disc pads were subjected to low and high humidity conditions. Humidity was found to measurably affect pad dimensions, pad hardness, compressibility, friction, pad wear, disc wear, disc roughness, DTV (Disc Thickness Variation) and brake noise. Also, the friction film is found to absorb a significant amount of moisture.
In order to clarify
the influence of humidity on the coefficient of friction and brake
squeal generation, a series of brake rig tests have been performed. The
influence of both air humidity and pad humidity
was evaluated. The results show that, between 20 and 80 percent relative
humidity, air humidity has a limited influence on the coefficient of
friction. Nevertheless, in a humid atmo- sphere, the friction
coefficient was considerably lower than in a wet environment for two of
the pads and higher for one pad. This is believed to be an effect of
different mechanisms of tribofilm formation on the pad surfaces.
The tribofilm observed on the pads mainly consists of iron oxide originating from the disc. The film is more easily formed in dry environments and usually by the pads with low often seen during each individual metal content. The friction increase stop is slightly lower at high air humidity. This effect is more pronounced for the pads with high metal content. Generally, the pads with smooth surfaces were less sensitive to changes in humidity. Only a limited correlation was found between relative humidity and brake squeal generation. One of the pads generated considerably more squeal in low humidity. None of the pads showed the opposite behavior.
If you’re lucky, the squealing or squeaking noise that your brakes make when you first drive your car in the morning, particularly after rain or snow, is just surface rust being scraped off the rotors by the brake pads the first few times you apply the brake pedal. It could also be the result of moisture and dirt that collects on the rotors, including from condensation caused by high humidity. If the brake squeal goes away after a few brake applications, no worries.
If the noise persists most times or every time you apply the brakes, or you hear squeals continuously while you’re driving, the cause is more serious – and the brake job will be more expensive. Because there are several possibilities causing squealing brakes – and because brakes are a crucial safety feature – it’s best to have a pro inspect and diagnose your vehicle’s brake noise.
A grinding sound usually means that the brake pads have worn away, and now the backing plates on which they were mounted are being squeezed against the rotors. This metal-to-metal contact means that you will need to replace the rotors, as well – and that you probably ignored some earlier warning signs of brake wear. This can be avoided by reducing the noise within the vehicle while driving so that you are able to hear what your car is trying to tell you. In this case, if your brakes are metal to metal, not only did you not hear the brake wear indicators screeching, the cost of a brake job will be
The advice to my reader is, get a second opinion and be sure that your vehicle is using the type of brake pads recommended by the manufacturer. If you find that aftermarket parts (not made by the manufacturer) are repeating the same issue, then replace them with an OEM (original equipment manufacturer) set of pads to see if this eliminates the problem. After the repair is performed, request a test drive with the service writer or mechanic