A stable high sits over the western Alps for the next three days. In Zermatt that means near 6°C at dawn and 23°C by afternoon, a 17-degree swing before lunch. Chamonix climbs even warmer, up to 27°C on Friday. No rain in the forecast. Start cold, finish hot: a merino base layer plus a light windproof shell covers both ends, without the bulky midlayer you would carry unused all afternoon.
Merino base layers ↗