The strong, persistent winds that circle the globe in the mid-latitudes, between 30° and 60° north and south, follow a curved path that moves them strongly toward the east. This predictable atmospheric pattern is a direct consequence of two major physical principles working in concert: the global movement of air masses driven by temperature and pressure differences, and the momentum effects caused by the planet’s rotation. Understanding how air first moves poleward and is then bent eastward explains this powerful and influential wind system.
Global Air Movement: The Engine of the Winds
Atmospheric movement in the 30° to 60° latitude band is initially driven by a large-scale circulation pattern known as the Ferrel Cell. This circulation is sandwiched between the warm-air-driven Hadley Cell to the south and the cold-air-driven Polar Cell to the north. The air in this mid-latitude region begins its journey at a belt of high pressure situated around 30° latitude in both hemispheres, where air descending from the Hadley cell creates the Subtropical Highs.
Air naturally flows outward from these high-pressure zones toward areas of lower pressure. In the case of the Ferrel Cell, the air moves poleward from the 30° high-pressure belt toward a persistent low-pressure zone located around 60° latitude, known as the Polar Front. This movement from high to low pressure establishes the initial north-south component of the wind.
This poleward flow provides the necessary momentum for the next physical process to take effect. If the Earth did not rotate, this air would simply flow straight from 30° to 60° latitude. However, the movement of air across the surface of a spinning planet introduces a powerful deflection that changes the wind’s direction from poleward to eastward.
The Coriolis Effect: Why Movement Appears to Deflect
The planetary rotation introduces an apparent force called the Coriolis Effect, which is responsible for the eastward curve of the winds. This effect is an apparent deflection observed when an object moves over a rotating surface, causing its path to bend to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The magnitude of this effect increases with latitude, becoming strongest near the poles and negligible at the equator.
The key to the eastward curve lies in the conservation of angular momentum, which is the tendency of a moving object to maintain its initial rotational speed. The Earth rotates fastest at the equator and progressively slower toward the poles; the rotational speed at 30° latitude is significantly faster than the speed at 60° latitude.
Air moving poleward from the 30° Subtropical Highs carries with it the high eastward velocity it possessed at that lower latitude. As this air mass travels toward the poles, it moves over ground that is spinning progressively slower. Since the air is not firmly attached to the ground, it maintains its initial, faster eastward momentum relative to the slower-moving surface beneath it. This difference in speed causes the air to effectively outrun the ground, resulting in a strong apparent deflection to the east. This deflection transforms the initial north-south push from the pressure gradient into a powerful, pronounced curve toward the east.
The Result: Defining the Prevailing Westerlies
The resulting wind system, created by the poleward flow being strongly deflected eastward, is known globally as the Prevailing Westerlies. These winds dominate the weather patterns across the mid-latitudes, blowing predominantly from the west between approximately 35° and 65° latitude. They are responsible for steering large-scale weather features, such as cyclonic storm systems, across continents in an eastward direction.
The Westerlies are closely linked to the powerful, high-altitude wind currents known as the jet streams. Specifically, the polar jet stream forms near the boundary between the Westerlies and the cold air of the Polar Cell, often situated near the 60° low-pressure zone. This narrow river of air flows from west to east and is driven by the sharp temperature contrast between the two air masses.
The path and speed of the jet stream influence the day-to-day weather across the mid-latitudes. When the Westerlies are particularly strong, especially in the Southern Hemisphere where there are fewer landmasses to slow them down, they are known by names like the Roaring Forties. This entire system illustrates how fundamental physical laws govern the planet’s atmospheric circulation.