Source: WeatherSTEM
As the Hadley Cells cycle air from the sub-tropical regions to the equator, this air is deflected West in both hemispheres by the Coriolis Effect. The trade winds are steady winds in the tropics both north and south of the equator. They get their name because their steadiness favored commerce during the era of trade via sailing routes. Conversely, as the Ferrel cells move surface air towards the poles, westerly winds are created. Westerly winds were used in conjunction with the trade winds to map the fastest sailing routes.