Snakes don't hibernate. They do not wear plaid either. What they do is called "Brumation", our word for today. And here is a gratuitous shot of a Bamboo Pit Viper that I found in the Lung Fu Shan Country Park.
While endotherms and other heterotherms are described scientifically as hibernating, (things like bears and hedgehogs and previously my teenage daughter, when it was time for school), the way ectotherms - like snakes and lizards - become dormant in cold is very different, and a separate name was invented for it in the 1920s, brumation. So what does it mean? well wait for it....
Brumation differs from hibernation in the metabolic processes involved.
Reptiles generally begin brumation in the later months of the year, which in Hong Kong is around November, depending on the weather and more specific times may depend on the species - as some like it colder. These snakes can wake up to drink water and return to "sleep", and, being snakes, they can go for months without food. A good time to look for snakes is before the cold weather, as these guys may eat more than usual before the brumation time. But, as mentioned, they do need to drink water. The brumation period varies, and normally in hong kong is only a few months. Brumation is triggered by lack of heat and the decrease in the hours of daylight in winter, similar to hibernation.
So, yes, it its sunny in mid "winter" you can still see snakes out sunning themselves and getting that much needed drink.