There is a specific science to how the brakes on your vehicle work and how different factors can affect braking performance. When you apply the brakes hydraulic pressure squeezes the brake pads against the rotor, slowing the vehicle. As simple as that seems, there is a lot more to what is actually occurring because energy isn't dissipated, its transferred to another medium. Your vehicle slows down because the energy from your vehicles momentum is converted to heat when the brakes are applied, which is then dissipated through the vents in the middle of the rotor. With this in mind, you would think that cold weather or cold air would increase braking efficiency, but that isn't necessarily true.
A Deeper Look
The braking system used by most modern vehiclesare completely sealed hydraulic systems. As the brake pedal is applied, a plunger in the master cylinder applies pressure equally and in all directions through the brake fluid. The pressurized brake fluid applies pressure against the piston in the brake calipers, which then presses against the inside brake pad causing the caliper to slide a bit and apply pressure to the outside brake pad. It is common belief that the brake pads do not touch the brake rotor until the brakes are applied, but this is inaccurate; the brake pads maintain a slight connection with the brake rotor at all times, causing a pre-heating condition in the brake pads and rotors.
The Effect of Temperature
Heat is generated very quickly due to the friction between the brake pads and the brake rotors, so the brake system will remain warm long after the vehicle has been parked. Heavy braking can overheat the brakes, leading you to experience brake fade that reduces the efficiency of the brakes. Unlike heat, cold weather doesn't affect the braking performance much, once the pads and rotors have been warmed from the first time you stop on a warm morning. The main thought is that cold weather would allow less heat to be created slowing the vehicle faster, but this couldn't be farther from the truth.
The Cold Truth
Truth be told, the brakes are actually hotter on a cold or rainy day because the water and snow can act as a lubricant, forcing you to apply more pressure to the brakes to effectively stop in the same distance. In essence, the temperature of the brake pads and rotors are actually 20 to 30 percent higher on a cold and rainy day as opposed to a dry day because of the extra friction created by the additional pressure to overcome the lubrication effects of water and snow. If you have moisture in your brake fluid, the cold can cause the water to freeze, leaving the possibility for the pressure in the brake system to remain applied even after the pedal has released; this will cause the brakes to drag. The moisture in the fluid could also begin to boil from the excessive heat created from the extra friction, causing the brakes to fade and lengthen the distance it takes for your vehicle to come to a stop.
Ensuring that your brakes are as efficient as possible requires routine inspection and regular maintenance of the brake system. Change your brake fluid and bleed the system once a year to keep the fluid in optimal condition and to prevent any moisture build up. Frequently inspect the brake fluid cap gasket and replace it as necessary; this keeps moisture out of the system. Inspect your brake pads and rotors every six months and replace as necessary with an OE style pad and rotor. Pay special attention to the brake fluid you use, if you live or travel through areas with steep grades, such as San Francisco or the mountainous regions, as you will need brake fluid with a higher boiling point to compensate for the extra heat produced in these areas compared to flatter areas.