Your brakes are quiet all week, then it rains - or you park overnight in damp weather - and suddenly the first few stops squeak, scrape, or grind. This is one of the most common brake noises and, reassuringly, almost always harmless. Here is why wet weather makes brakes noisy.
The main cause: surface rust
Brake rotors are bare cast iron, and iron flash-rusts fast when wet. A thin film of surface rust forms on the rotor face during rain or an overnight damp spell - see why rotors rust overnight. The first few brake applications scrape that film off, producing a light squeal or grind that disappears within a mile. It is completely normal.
Water and dust between pad and rotor
Rain also washes grit onto the rotor and leaves a momentary film of water and dust between the pad and disc. Until the heat of braking dries and cleans the surface, that layer can chirp or squeak. Again it clears quickly once you have made a few stops and the brakes warm up - the same idea as cold vs hot brake noise.
When wet-weather noise is normal
If the squeak or light grind appears only when wet or first thing after rain and vanishes after a few stops, do not worry - it is flash rust, not a fault. This is true even on newer cars with quality pads. Ceramic and semi-metallic pads can differ slightly in how much they chirp when damp.
When to actually check
Be concerned only if the noise persists after the brakes are warm and dry, gets louder over time, or you feel grinding through the pedal - that points to worn-out pads, not weather; see why brakes squeak. If the noise never goes away, inspect the pads and fit fresh pads and hardware.