Yes, it is totally normal for myriad reasons. First of all, it is difficult (read: impossible) to maintain your peak fitness year round. So, if you were averaging 30km/hr at your best, then that will decrease in the winter when training volume is down. (and if training volume isn't down, then you are probably trying to do too much and just getting tired). Also, if it is cold out, it is harder to breathe, and, like you said, wind is also a big factor in the winter. Another factor, albeit small, is that in the winter your are wearing tons of clothing, adding weight and probably drag. So, take it easy(ish) in the winter, use it as a time to have some fun, recover from the season, regroup mentally, and don't worry so much about numbers. Start training again as Spring approaches and you'll see fast gains that should get you to last season's peak performance (or better) before you know it.