The Arc'teryx Cerium SL is a super light (185g), compressible and highly insulating down filled mid-layer, check out our review of it here.
Arc’teryx Cerium SL Mid-Layer Jacket
The Cerium SL is very versatile – you could use it just as well in summer as an insulating outer layer as in winter when it will keep you incredibly warm on colder days as a mid-layer.
The Cerium SL has been designed to cut down weight through clever construction, top range materials and fit, whilst the high loft 850 fill power European grey goose down delivers maximum insulation for minimal weight.
It also looks really good, and for me it will be travelling in my pack year-round for those times when I need extra warmth without weight or bulk.