Is Beijing covered in smog most of the time ?

Yes

Whilst it's both technically and otherwise true that Beijing is smog-filled "almost all of the time" -- the degree varies considerably.

Those not living in China or other fast developing economies only hear about BJ smog when it hits AQI 500+ and is in the international news. This winter has been exceptionally bad, I agree. But actually, until a month ago, 2015 was pretty decent overall, with a large number (20%?) of decent days, and the majority of days being bad but not too bad (around 120-150).

The 12 month average AQI for BJ is actually ~120, which is roughly twice as bad as LA and a bit more than twice as bad as Paris (can't get accurate long-term stats for Paris). Of course, this is not great. But it's far from the media-hyped smogapocalypse.

Having said all that, psychologically, this winter has been the worst for smog since we moved here 4 years ago. I've started wearing a mask regularly when going to/ from work, which I never did before -- given that we're indoors most of the time and both work and home have decent air purifier solutions.

tb

Is flight cancellation due to smog a frequent thing in Beijing ? May be I should consider going by train from Shanghai.

You may want to take the train anyway for other reasons, but smog cancellation of flights is extremely uncommon. I don't know what happened during the recent red alerts, but I've certainly flown out with really bad pollution without issues.

tb

Considering how often domestic flights are delayed, I'd take the train. If you can, book business class. The seats are great and the whole experience is very pleasant.

I do sh-,bj at least once per month, and maintain meticulous stats on door-to-door times. During this time of year, flying wins.

I flew CTU-PEK during the middle of Beijing's red flag alert last December. No delays, nor did I see any on CA's other near-hourly flights between the two cities. The pollution was breathtaking (pun) to behold on landing in PEK, but still, fog is a bigger issue than smog.

Funnily enough, I just did the opposite a week ago, looking forward to not be bothered by as much smog in CTU compared to Beijing. Turned out to be a fresh-air day (AQI <150) in Beijing while my suburb of choice in Chengdu boasted an AQI of 540 for a while before returning to "healthy" 250-ish :/

CTU was very bad last week, 02 january. A lot planes was delayed for hours, some also canceled. Our international flight was delayed for 3 hours. A friend who live around 200km away from CTU said the highway was closed for many hours because of the "bad weather". And if I'm right it was just "orange alert", not red.