Winter is definitely a good time to go but for those who have not been to high altitude before, I suggest u go somewhere lower first or at least prepare before u go. If your health is not so good, u will feel miserable and end up not enjoying a thing. My mum's friend went to JZG just after the winter season and one of her tour mate die up there. U can check with the CN agency when u r there how many people fall sick and pass way especially during the absence of the plane.
Every day there are tour buses going down the slope while going JZG. My winter tour coming back to Chengdu is via bus because the snow was too thick and slippery to land and take off. Somemore the buses tyres are chained with spikes to move in the snow and not normal tyres anymore. When we return, our Chengdu tour guide has been waiting for many hours in our hotel and once he saw us, he came out and tell us how glad he is and he even went to temple to pray for us.
Just think about it, if the airport cannot even allow planes to land imagine the condition of the roads down the slope back to Chengdu.
As a precaution, because Singaporean are full of low altitude, wet and warm related illness, once u go up to low O2, dry, cold and windy area, many of the symptons start crawling out. My FIL is a chinese physician and he mention usually when there are hidden illness in the body, once your body goes through deprivation of the 3 essentials i.e food, h20 & O2, and in a foreign environment, many will surface or aggrevate.
But if u have Rhumatism like me, then u don't have to worry except in the summer months which more steamboat and spicy food will help. Once u r up in JZG, you suddenly realised your bones become very strong, humidity does more harm in this case.:nono: