Temperatures can dip to single digits during the winter to about 7-8. Even this is considered tame to many including myself. Even though I am a native of the SAR, I have gotten to used to bitter cold Toronto that the term "cold" no longer creeps to my mind whenever I think of HK weather. Light jacket or sweather will do good for the OP. And for the OP, his definition of cold in HK will be ligher than mines considering that Montreal winters are somewhat colder than Toronto's.