It's been a while since I took a January vacation and I've never tried to limit one to a carry on back back. But we will be week spanning the last week of January and the first week of February in Barcelona. I gather I should expect highs in the 50s to 60s and lows in th 40s. I also expect at least one rainy day.
Other than one day hiking around Montserrat, this will be a city trip. I expect to wear two layers most on the time with three and four layers doable if if gets below 30f.
I'm a fan of merino wool as it is soft, dries quick, and can look like silk.
Here's my plan:
On the plane:
Jeans, compression wool blend socks, mid-weight dressy wool shirt, light weight wool cardigan, cotton scarf to use as blanket during flight, dressy Zaire shoes (cork soled flats dressy enough for concerts).
In my carry on backpack:
1 mid-weight and two light weight long sleeved wool shirts (can be layered for warmth)
1 light weight short sleeved wool shirt (as a cold weather extra layer or if we get a warm day)
1 wool zip up hoodie mid to heavy weight, and one light weight wool jacket each of which go with all shirts.
Small dressy silk scarf that goes with all shirts.
Extremely light weight breathable rain jacket.
Dressy black wool blend pants.
Silk-wool blend leggings which can be worn under pants or jeans or by themselves.
Light weight wool hat and gloves.
3 pairs wool socks of various weights.
3 each cotton panties and bras.
Walking shoes.
Beyond clothing, I'll have, camera, smart phone, tablet, very small purse, small daypack, sketchbook and pencils, mp3 player, toiletries, small spice kit for apartment cooking, guidebook sections, prepurchased tickets. Since we are only staying in one apartment I'll carry everything on the plane, and to and from the airports. Otherwise it will all live in the apartment.
All together it weights less than thirty pounds and fits in the overhead bin.
Will I be warm enough? Or too warm? I'm 55 and warm in two wool layers plus rain coat for 40 degree hikes in Oregon's mild winters. For weather in the 30s I add leggings under my paints and a third layer up top.