One of Tibet’s quirks is that even in winter, due to the high altitude, it usually feels a lot warmer than it actually is. Those pesky UV rays easily penetrate the thin air though, so hats and sunglasses are essential. It can be a good idea to visit Tibet in December, if you can cope with temperatures that average -9°C to 9°C, and much lower after nightfall.
Many parts of the country will be out of reach due to heavy snow closing off the roads. Lake Namtso will be covered in ice, while trekking around Mount Kailash would be hazardous. Some intrepid souls do make it out to see Mount Everest at this time of year, but it can get as cold as -20°C in that region, so most won’t stay for long.
In the capital, Lhasa, it is quite pleasant (positively mild), and of course most religious sites remain open during December. That’s also the case in Shigatse, which is slightly colder but still perfectly manageable. Later in the month, monasteries across Tibet mark the death of Tsongkhapa, founder of the Gelugpa order, with dancing and processions.