At end December or beginning of January, there's the problem of flights being canceled due to fog. Even if a flight's departure is in the afternoon, would it be safer for hikers to travel from the hilly areas to Delhi airport by bus and take a flight from Delhi or would it be pretty much the same risk of flight cancellation even if they didn't have to travel so far to Delhi, and instead got a flight from Dehradun or Chandigarh?
I don't have hard numbers w.r.t cancellations for the smaller airports, but I'd like to answer this based on my (limited) experience.
Given the situation that you've mentioned, I would always book flights in winter out of
Delhi and never
Chandigarh, the Chandigarh airport is relatively small and notorious in terms of flight cancellations even on days when the visibility is reasonably good. What's worse is that there are very few alternatives that you can rebook on (This of course depends on your destination). I have never flown in and out of
Dehradun so I can't comment about that airport but I suspect that the situation would be similar.
Delhi airport is severely impacted by fog in the winters as you know but unless the situation is exceptionally bad, it results in mostly delays and not cancellations. I would of course recommend that you book and airline that flies in and out of
T3 to minimize the chances of cancellations.
As for getting to Delhi from Chandigarh, I'd recommend taking one of the early morning
Shatabdi trains, which typically experience the minimum delays and getting to the airport from
NDLS is quite easy with the