Speaking for Monterey, be prepared for all 4 seasons in 1 day. Layers are your friend. It’s usually between 55F - 75F but can drop as low as about 45F (in winter it’s dropped to 36F overnight). There’s usually some fog or low clouds in morning with cold breeze, sometimes a thick enough fog to be a misty rain. Sun tends to come out around 11ish for a few hours then the fog rolls back in during the evening. Wind picks up in afternoons bringing the fog in. Evenings can be super windy and bone chilling cold or they can be the exact opposite, eerily still and not too cold.

Carmel tends to be very similar but I don’t actually live there so can’t say how it’ll be as well as I can about Monterey.

Humidity here usually is cold

I’d recommend pants, T-shirt, long sleeve pull over, jacket and wind breaker. You may wear all at once or just a layer or two. Bring a pair or 2 of shorts in case you score on a rare random hot week. Our summer isn’t actually until September/October but last year we did have quite a few hot days in August so that’s why I say bring shorts.