On a single day in the Himalaya you can start in frost, walk in strong sun and finish in a cold wind. Rather than one heavy coat, you need layers you can add and remove as you walk.
Quick answer: Wear three layers: a wicking base layer (merino or synthetic), an insulating mid layer (fleece or light synthetic jacket) and a waterproof, windproof shell. Add a down jacket for evenings and high altitude. Avoid cotton, which stays wet and cold.
The layers
1. Base layer
Long or short-sleeved tops in merino wool or synthetic fabric move sweat away from your skin. Merino smells less after many days. Bring two tops and one pair of thermal leggings.
2. Mid layer
A fleece or light insulated jacket for cold mornings and rest stops. On cold passes you may wear it all day.
3. Outer shell
A breathable waterproof jacket and trousers for wind, snow and rain, essential in spring and the monsoon.
4. Down jacket
The warmest layer: for evenings in the teahouse, early starts and summit mornings such as Kala Patthar. Down jackets can be rented in Kathmandu.
Head, hands and legs
- Sun hat and a warm beanie; a buff for dust and wind.
- Thin liner gloves and warm insulated gloves or mittens.
- Two pairs of trekking trousers (zip-off legs are handy low down).
How to use the system
Start the day slightly cool: you warm up within ten minutes of walking. Take a layer off before you sweat, and put one on as soon as you stop. Wet base layers are a common cause of chills at altitude.
See also what to wear in Nepal and the packing list.