westerlies

The westerlies, or prevailing westerlies, are a belt of winds that blow from the west to the east in the mid-latitudes, roughly between 30° and 60° latitude in both hemispheres. They arise from the general circulation of the atmosphere, particularly the interaction of the Ferrel cell with the subtropical high-pressure systems and the Coriolis effect due to Earth's rotation.

In the subtropics, surface air tends to move poleward from the subtropical highs. As this air moves toward higher latitudes, the Coriolis force deflects it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, producing a predominant west-to-east flow near the surface. Upper-level westerlies are reinforced by the jet streams, which are fast-flowing ribbons of air that form at the boundaries between the major circulation cells and influence weather systems.

These winds are the primary latitudinal conveyor of weather in mid-latitudes. They influence the movement of extratropical cyclones and anticyclones, helping storms progress from west to east. Typical surface speeds average around 10 to 15 meters per second but can vary widely, with stronger episodes in winter and during active jet-stream phases.

Regional expressions of the westerlies include the Southern Hemisphere’s “roaring forties” and related zones, where persistent westerly flow is especially strong over the Southern Ocean. The westerlies interact with large-scale fronts and are modulated by phenomena such as the El Niño–Southern Oscillation. Climate variability and change can alter their strength, position, and the associated storm tracks, with implications for marine navigation, aviation, and regional climate.