Clear-air turbulence. Jet streams are, at their most basic, high-altitude air currents caused by atmospheric heating and the inertia of the earth's rotation—and they're the reason why flights from west to east are faster than the same route traversed in the opposite direction.

Flying east is generally faster than flying west due to high-altitude, fast-moving wind currents known as Jet Streams. Because the Earth rotates from west to east, the atmosphere's movement—driven by the Coriolis effect and temperature differences between the equator and the poles—creates powerful "rivers" of air that flow toward the east. When a plane flies eastward (e.g., from New York to London), it "hitches a ride" on these tailwinds, which can reach speeds of over 200 mph (320 km/h), significantly increasing the aircraft's ground speed relative to the Earth's surface. Conversely, when flying westward, the plane must fight against these "headwinds," which slows it down and requires more fuel. This explains why a flight from NYC to London might take only 6 hours, while the return trip takes nearly 8 hours. While some people mistakenly believe the Earth's rotation "moves the destination closer" to the plane, this is a myth; the plane is part of the Earth's atmosphere and moves with it. The time difference is almost entirely a result of these atmospheric wind patterns.