FlyerTalk Evangelist and Ambassador: The British Airways Club
Join Date: Jun 2008
Location: Somewhere between 0 and 13,000 metres high
Programs: AF/KL Life Plat, BA GGL+GfL, ALL Diam, Hilton Diam, Marriott Gold, blablablah, etc
Posts: 33,172
Primarily wind/weather conditions. The most extreme case I know are the East Asian routes to hel with current Russian airspace restrictions - depending on weather, some days would route via mid Asia and the Caucasus whilst the next day could route you via the (fantastic imho) polar route, Alaska, the North Pole and Lapland