Originally Posted by
Gantless
I've often wondered what it means to be certified for instrument flight rules if the planes can't land in fog! I once flew PUB-DEN and the plane wouldn't take off because of fog in Denver. This was over the holidays and caused massive IRROPS. Isn't the point of IFR to be able to land WITHOUT visibility and rely only on the instruments?
Of course the planes (commercial transport, at least) are still capable of landing under reduced viz, but separation requirements are increased, which in turn reduces the number of aircraft landing per hour.
I didn't check the data, but if there was very low viz, then FAA likely instituted a GDP... and UA saw an opportunity to cancel your flight and move a couple of their other ones up in the queue (intra airline substitution) and reduce the delay on some of their flights coming from somewhere else.