Quick answer: Use layers year-round and always carry wind/rain protection; winter requires proper insulated clothing and footwear.
Dress for elevation, not just the town forecast
Temperatures, wind and precipitation can feel very different higher on the mountain. The forecast for your hotel area is not enough.
Summer still needs layers
Even in warm months, carry a light insulating layer and rain/wind protection for summit conditions.
Autumn changes quickly
Cool mornings and strong wind make flexible layers important. Gloves or a hat can be useful earlier than travelers expect.
Winter requires real cold-weather gear
Insulated outerwear, warm footwear, gloves, hat and face protection matter. Footwear should also handle snow and icy surfaces.
Editorial note: ticketing, scenic-area access and transport procedures can change. Verify time-sensitive details with current official Changbai Mountain tourism notices before travel.