It really depends on how far south you go and how high up. We went first week of Jan a couple of years back and there was much snow about; The "Chicken-Tika" pass was closed for a day but opened about midday the next. It was bloody cold in the early morning on a bike, OK in the middle of the day then very noticable temperature drop. The days are quite short and because the sun is low, in the mountains you can be in shade for a long time. We had some proper rain and a bit more snow. Some river crossings were fine, some were a bit scarey.
Once a bit further "south and sandy", about the same as a good summer's day in the UK.
Remember the "short days" bit though.