The Earth’s atmosphere is constantly in motion, driven by the sun’s uneven heating and the planet’s rotation. Among the most influential wind patterns are the westerly winds, also known as the prevailing westerlies. These global, surface-level winds move predominantly from west to east, steering weather systems as they circle the globe. They are a major element of the Earth’s general atmospheric circulation, distributing heat and moisture around the world.
Defining the Mid-Latitude Prevailing Winds
The prevailing westerlies occupy the middle latitudes of both the Northern and Southern Hemispheres, existing between approximately 30 and 60 degrees latitude. These winds originate from the subtropical high-pressure zones and trend poleward toward the subpolar low-pressure belts. Due to physical forces, the winds generally blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.
This mid-latitude wind belt is part of the three-cell model of global atmospheric circulation, which includes the Hadley, Ferrel, and Polar cells. The westerlies are the surface manifestation of the Ferrel Cell. This cell acts as a transitional “gear” between the warmer Hadley Cell near the equator and the colder Polar Cell. Air in the Ferrel Cell sinks at about 30 degrees latitude and rises around 60 degrees latitude, transferring energy between the other two cells.
How Earth’s Rotation Creates the Westerlies
The eastward movement of the westerlies results from two primary mechanisms: pressure differentials and the Coriolis effect. The pressure differential is established as air sinks at the high-pressure subtropical ridge (around 30° latitude) and flows toward the low-pressure subpolar regions (around 60° latitude). If the Earth did not rotate, this air would simply flow straight from south to north in the Northern Hemisphere and north to south in the Southern Hemisphere.
The planet’s rotation introduces the Coriolis effect, an apparent force that deflects the path of moving air masses. In the Northern Hemisphere, this deflection is to the right, and in the Southern Hemisphere, it is to the left. Air moving poleward from the 30-degree high-pressure belt is turned progressively eastward, creating the prevailing flow from the west.
This deflected poleward flow constitutes the surface winds of the Ferrel Cell. The air mass retains angular momentum from its origin near the equator, a region of faster rotation. Consequently, it moves faster eastward than the land beneath it at higher latitudes. This speed mismatch, combined with the Coriolis deflection, results in the consistent west-to-east path of the westerlies. The strength of this flow is influenced by seasonal changes, becoming stronger in the winter hemisphere when the temperature difference between the equator and poles is greater.
Influence on Global Climate and Ocean Circulation
The west-to-east trajectory of the westerlies makes them the primary steering mechanism for mid-latitude weather systems. Extratropical cyclones, or storms that form outside the tropics, are carried along by these winds, moving across continents like North America and Europe. This movement dictates the general pattern of weather changes, bringing alternating high and low-pressure systems and influencing precipitation distribution.
Beyond the atmosphere, the westerlies exert a powerful frictional drag on the ocean surface, driving major surface currents. This wind stress helps maintain the large, rotating currents known as ocean gyres in both hemispheres. Examples include the North Atlantic Drift and the North Pacific Current, which transport warm, equatorial water toward the poles. This heat transfer significantly moderates the climate of western continental coasts, such as Western Europe, making these regions warmer than other locations at similar latitudes.
The Roaring Forties and Furious Fifties
The Southern Hemisphere provides the most intense manifestation of the westerly winds, giving rise to the terms “Roaring Forties” and “Furious Fifties.” These names describe the high-speed, turbulent westerlies found between 40 and 50 degrees south latitude and 50 and 60 degrees south latitude, respectively. Sailors coined the names after experiencing the ferocious conditions while sailing further south.
The exceptional strength of these winds is due to the lack of significant landmasses to obstruct their flow across the Southern Ocean. This allows the wind to build up speed unimpeded as it circles the globe. In the Northern Hemisphere, large continents break up the wind pattern, but south of the equator, the westerlies develop into powerful, consistent gales. Historically, sailors utilized the relentless eastbound flow of the Roaring Forties to speed up voyages, such as those along the Clipper Route, demonstrating their influence on global navigation and commerce.