In a band of latitudes roughly from 30 to 60 degrees in each hemisphere, the winds blow most of the time from the west toward the east. These are the westerlies — the prevailing winds of the mid-latitudes. Unlike the trade winds of the tropics, which are remarkably consistent in direction, the westerlies are more variable from day to day. But averaged over weeks and months, the underlying pattern is clear: winds at mid-latitudes come from the west far more often than from any other direction. The westerlies shape the climate of vast portions of the planet — the entire Pacific Northwest of North America, Western Europe, the Southern Ocean encircling Antarctica, southern South America, southern Australia, New Zealand. They drive storms across continents. They modulate ocean currents. They have shaped maritime trade and exploration for centuries. Understanding the westerlies is understanding how mid-latitude weather works and why certain regions have the climates they do.
Where They Come From
The westerlies are a consequence of the Ferrel cell — the mid-latitude atmospheric circulation between the Hadley cell's descending branch around 30 degrees and the polar cell's descending branch over the poles. The Ferrel cell's surface branch flows poleward (from the subtropical highs toward the polar front) in both hemispheres. As this air moves poleward, the Coriolis effect deflects it to the east — to the right in the Northern Hemisphere, to the left in the Southern Hemisphere. The combination of poleward motion plus eastward deflection produces winds that flow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. Averaged over time and corrected for the topographic and ocean-current effects that perturb individual storm tracks, the result is the prevailing westerly winds at mid-latitudes.
The Latitudinal Belt
The westerlies extend roughly from 30 to 60 degrees latitude in both hemispheres. The strongest westerlies are typically at around 40-50 degrees latitude, especially over the oceans where there are no continents to disrupt the flow. The "roaring forties" (40-50°S) and "furious fifties" (50-60°S) and "screaming sixties" (60-70°S) of the Southern Hemisphere are the most extreme expression of the westerlies — gale-force winds blowing essentially uninterrupted around the Southern Ocean. These are the strongest sustained surface winds on Earth, with average wind speeds in the 40-50 km/h range and frequent gusts much higher. The Northern Hemisphere westerlies are also strong but more disrupted by the presence of large continents (North America, Eurasia) that affect the flow.
The Westerlies and Weather Tracks
Mid-latitude storms move from west to east, riding along the westerlies. This is why weather systems in the United States generally move from west to east — the storms that hit the East Coast typically come from the Pacific Northwest, cross the Rockies and the Plains, and reach the East over the course of several days. The same pattern applies in Europe, where Atlantic storms move eastward across the continent. Forecasting mid-latitude weather is in large part a matter of tracking the eastward progression of storm systems through the prevailing westerly flow. The storms themselves are spawned by the temperature contrast across the polar front and propagate within the larger westerly current.
Why Western Coasts Have Mild Climates
The westerlies have a particular climatological consequence: they bring air from over the ocean onto the west coasts of continents. Maritime air masses are moister and have less seasonal temperature variability than continental air masses. The result is that western coasts at mid-latitudes typically have mild winters, cool summers, and substantial year-round precipitation. Compare Seattle (47°N on the west coast of North America) to Boston (42°N on the east coast): Seattle has milder winters, cooler summers, and more rain. Compare Bordeaux (44°N, west coast of Europe) to similar latitudes in eastern Asia (e.g., Hokkaido at 43°N): Bordeaux is much milder than the East Asian counterpart. The eastern coasts of continents at mid-latitudes get continental air brought to them by the westerlies — air that has dried out and become more thermally variable while crossing the continent. Western coasts get fresh maritime air.
The Pacific Northwest and Western Europe
Two regions stand out as poster children for westerly-dominated maritime climates: the Pacific Northwest of North America (coastal Oregon, Washington, British Columbia, Alaska) and Western Europe (coastal Ireland, the UK, France, the Netherlands, Norway). Both regions sit on the west coasts of continents at mid-latitudes. Both receive moist Atlantic or Pacific air brought by the westerlies. Both experience mild winters (well above what their latitudes would suggest, in the absence of westerly maritime influence) and substantial year-round precipitation. The lush temperate rainforests of the Pacific Northwest and the green countryside of Western Europe are both expressions of the westerly maritime climate. Without the westerlies, these regions would look very different — likely much colder in winter and drier overall.
The Westerlies and Ocean Currents
The westerlies drive major ocean currents through their friction on the sea surface. The Antarctic Circumpolar Current — the largest ocean current on Earth by far — flows eastward around Antarctica, driven by the westerlies of the Southern Ocean. It transports about 130 million cubic metres of water per second eastward — far more than any river system on Earth. In the Northern Hemisphere, the westerlies drive the eastward-flowing currents that link the Gulf Stream to the North Atlantic Drift and that bring warm Atlantic water to the coast of Norway, keeping that high-latitude region habitable. The North Pacific Current similarly carries warm water from East Asia toward the coast of North America, moderating the climate of the Pacific Northwest. Without the westerlies driving these currents, the heat distribution across the world's oceans would be very different.
The Sailing Routes
Sailing ships throughout history have exploited the westerlies for long-distance voyages. The clipper route — used by tea, wool, and grain clippers in the 19th century — ran eastward through the Southern Ocean from Britain to Australia, taking advantage of the consistent westerly winds. Ships could circumnavigate the globe in this latitude band without ever having to fight strong contrary winds. The downside was that the westerlies in the Southern Ocean were often too strong — gales were frequent and ship losses were common. The Cape Horn passage at the southern tip of South America, where the westerlies funnel through the relatively narrow gap between Cape Horn and Antarctica, was particularly dangerous and had a high attrition rate. Modern shipping is mostly motorised, but yacht racing (the Vendée Globe, the Clipper Round The World Race, etc.) still uses the Southern Ocean westerlies for fast circumnavigations.
The Jet Streams
The polar jet stream — the fast-moving river of air at the tropopause in each hemisphere — is the upper-air manifestation of the westerlies. The jet stream flows from west to east at speeds of 100-300 km/h, with cores typically at 9-12 kilometres altitude. The jet stream's position is variable; it meanders north and south in patterns called Rossby waves, with wavelengths of thousands of kilometres. The waviness of the jet stream is what determines whether mid-latitude weather is "zonal" (lots of westerly flow, weather systems moving quickly eastward) or "meridional" (lots of north-south air motion, weather systems being more persistent and producing extreme heat/cold and droughts/floods). Climate change appears to be making the jet stream wavier on average, partly because Arctic warming is reducing the temperature contrast that drives the jet stream.
Disruptions and Variability
The westerlies are not constant. They have substantial variability on multiple timescales. On weekly timescales, individual storm systems move through and produce day-to-day weather variations. On monthly to seasonal timescales, the jet stream's mean position can shift by hundreds of kilometres, producing extended periods of wet or dry weather, warm or cold weather. The North Atlantic Oscillation and the Arctic Oscillation are large-scale patterns of jet-stream variability that have substantial effects on Northern Hemisphere mid-latitude winters. La Niña events shift the Pacific westerlies; El Niño events shift them the other way. The complex variability of the westerlies is one of the major reasons mid-latitude weather is hard to forecast more than a week or two in advance.
The Westerlies Under Climate Change
Climate change is affecting the westerlies in several ways. The Hadley cell's poleward expansion is pushing the boundary between the Hadley cell and the Ferrel cell poleward, which shifts the storm tracks poleward and broadens the westerly belt. The polar jet stream is becoming wavier due to differential Arctic warming. The Southern Ocean westerlies have been observed to strengthen and shift poleward over the past several decades, partly in response to the Antarctic ozone hole. These changes in the westerlies have substantial implications for regional weather patterns, ocean circulation, and ecosystem dynamics. Continued observation and modelling of how the westerlies will evolve under continued warming is one of the active areas of climate research, with practical implications for water resources, agriculture, and storm risk in many of the world's major populated regions.