The air temperature is unpredictable. We used to start hiking in the dark wearing t-shirts and shorts. Then the layers come up by halfway up. On some years, there was a stiff breeze at the summit, so I was wearing two layers on the bottom and four layers on top, with a warm hat and a shell hood. Thin gloves are not out of the question.
Often the wind is not blowing, so some of those layers are unused.
Basically, you have to protect yourself from bright sun and wind. It could be raining or snowing.