here were about 725 airlines in the world in 2012 providing different range of services. Theoretically, air transport enjoys greater freedom of route choice than most other modes. Yet while it is true that the mode is less restricted than land transport to specific rights of way, it is nevertheless much more constrained than what might be supposed. Early in the history of aviation, physical obstacles such as the Rocky Mountains and the great gap of the North Atlantic limited the articulation of air transport networks. While those limitations have fallen, physical geography still affects the geography of intercity air transportation. Weather events such as snowstorms and thunderstorms can temporarily create major disruptions. Aircraft seek, for instance, to exploit (or avoid) upper atmospheric winds, in particular the jet stream, to enhance speed and reduce fuel consumption. Volcanic eruptions may also impede air travel by releasing ash in the atmosphere, which can damage and even shot down turbofan engines. Such occurrences are however rare and punctual, with the exception of April 2010 when a volcanic eruption in Iceland forced the closing down of airports in most of Europe as well as several North Atlantic routes. This represented the largest natural disruption of air travel in history.