Fog can roll in very suddenly (so, they don't always know before they take off), but the pilots get automatic weather updates every 15-30 minutes. Sometimes if the destination airport has bad weather at before takeoff from the departure airport, they will still takeoff because the weather patterns tell them that it will clear up by the time they arrive there. Before flight departs, part of the flight plan issued to the FAA (Federal Aviation Administration) requires that you list an alternate airport (before you depart). A flight will be diverted to whatever alternate airport was listed on the flight plan unless that airport also has problems (maybe runway lights are not operational, or weather related). Yes, certain airlines do get priority at certain airports. Whose ever hub (main base of operation) that particular airport is, they get priority. US Airways biggest hub is in Philadelphia, so they get priority over other airlines because each one of their planes is needed very quickly for another flight, (because it is their hub, they have a lot of flights out of that airport). Air India has priority at Mumbai and New Delhi. British Airways at London Heathrow and London Gatwick. Olymic Airways at Athens, Turkish Airlines at Bulgaria. Air France at Paris CDG and Paris Orly. Etc. You get the picture.
Once I was diverted when I was on a flight from Mumbai to New Delhi (1.75 hour flight). New Delhi is notoriously know for the thick and heavy amount of fog it has year-round. New Delhi was fine when we took off, but by the time we were half-way there, New Delhi was fogged in below the minimums (minimum levels of visibility that planes can land), so we turned back to Mumbai.
Source(s):
My experiences, and the fact that I love the aviation industry so much, that I have researched it so much now that I now know why airlines do what they do.
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