I was checking the flight duration between two places over a 2-3 month period. The flight duration for the same flight is different for different months. For example, some days the flight takes 13 hrs 10 minutes and some other days it's 12 hrs over the same distance. How does this happen? Moreover, this data is available long before the departure of the flight; I am curious to know the reason. What are the factors that decide this?
I'll expand on the other answers here.
Wind: The biggest factor in this is wind. Winds at common cruising altitude can sometimes reach well into the 100 Knot spectrum. Keep in mind that this can be in any direction. So lets say a long haul flight has a 100 Knot headwind half the year and a 100 Knot tail wind for the other half. That's a 200 Knot ground speed difference which can really make an impact on flight time.
ATC Holds Vary: ATC for various reasons may put a plane into a holding pattern on arrival. This may add to the time spent in the air for a given flight depending on the hold.
Landing Slots: There are 5 high density airports here in the US (and presumably others elsewhere) that require planed ahead landing slots. If the slot time changes but the departure time remains the same an airline may chose to slow the plane a bit (or speed it up a bit) to compensate.
Aircraft Loading: Aircraft efficiency (and subsequently speed) will vary a bit with the loading of the aircraft. Since not every flight on a given route is loaded equally there may be efficiency gained or lost as a result of loading.
Several possible reasons:
- different equipment (not every aircraft flies at the same speed)
- different routing
- known seasonal changes in high altitude winds (which can prompt routing changes)
- extra time built in in some periods for historically experienced frequent delays during those periods (e.g. in summer when a lot of inexperienced travelers with small children are expected, boarding and deplaning tends to take longer than in spring and autumn when there's more business travel).
I'm sure there are other reasons as well.
I was hoping for a specific example, but based on your statement that
some days the flight takes 13 hrs 10 minutes and some other days it's 12 hrs over the same distance
I suspect that you are determining the flight duration from the published schedules available to the public, that use local times for arrivals and departures, thus I would like to add a possible alternative to the ones proposed in the other answers:
Daylight Saving Time: different countries enter/leave (if they use it) DST in different dates. Example: the US and the UK change to DST 2 weeks apart, having a difference of only 4 hours between New York and London in that period, while normally they have 5.