Since the jet streams flow from west to east, they make one leg of the journey much faster (when flying with the stream) and one slower (against the stream). Imagine going downstream or upstream a river, or how it feels when you are cycling against the wind, as opposed to when you have it at your back.

Flying from west to east is faster primarily due to the Jet Stream, a high-altitude "river of air" that flows from west to east at speeds often exceeding 200 mph. These winds are created by the Earth's rotation and the temperature difference between polar and tropical air. When an aircraft flies eastward, it hitches a ride on these powerful tailwinds, significantly boosting its ground speed without burning extra fuel. For example, a flight from New York to London might take only 6 hours, while the return trip takes nearly 8 hours fighting against the "headwinds." The Coriolis Effect—a result of the Earth spinning on its axis—deflects moving air to the right in the Northern Hemisphere, which helps sustain these westerly winds. In 2026, flight planners use advanced AI overlays like "Windy" to "surf" the strongest parts of the jet stream. Occasionally, strong winds allow a plane to fly a longer "physical" distance around the globe while arriving faster because the speed boost from the wind is so significant.