Hill weather changes faster than any app can keep up with, and the classic beginner error is dressing for the car park. You leave warm, climb hard, sweat through everything, then freeze the moment you stop for lunch on an exposed ridge. Layering solves this not by keeping you warm but by keeping you dry and adjustable.
The base layer sits against the skin and has one job: moving sweat away from you. Merino wool or a synthetic does this; cotton does the opposite, holding water against your body and chilling you as it evaporates. This is the layer worth spending on, because everything else works better when it is dry.
The mid layer traps warm air. A fleece or a light insulated top is ideal precisely because it is easy to take off. The trick is to start slightly cold. If you feel cosy standing still at the trailhead, you are overdressed and will overheat within ten minutes of walking uphill.
The shell goes on top and keeps wind and rain out. A packable waterproof with pit zips lets you dump heat without stripping off. Wind, not cold, is what strips warmth from you on a ridge, so a shell earns its place in the pack even on a bright day.
The discipline is adjusting early and often. Layer down before you sweat, layer up before you shiver, and do it the moment you notice, not three switchbacks later. Teach children the same rhythm and you turn a miserable, whinging descent into a non-event.