The westerlies are prevailing winds that blow from west to east in Earth’s middle latitudes. These atmospheric currents are a component of the planet’s atmospheric circulation system. They influence global weather patterns and play a role in shaping climates and impacting oceanic processes.
How Westerlies Form and Where They are Found
The formation of the westerlies is rooted in the Earth’s atmospheric pressure systems and rotation. In the subtropics, around 30 degrees latitude in both hemispheres, descending air creates high-pressure zones known as the subtropical high-pressure belts, also referred to as the horse latitudes. Around 60 degrees latitude, the meeting of warm and cold air masses leads to subpolar low-pressure belts.
Air moving from these subtropical high-pressure areas towards the subpolar low-pressure belts is influenced by the Coriolis effect. This force, resulting from Earth’s rotation, deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection causes the poleward-moving air to turn eastward, establishing the west-to-east flow of the westerlies. These winds are found between approximately 30 and 60 degrees latitude in both the Northern and Southern Hemispheres.
Key Characteristics of Westerlies
The westerlies blow consistently from west to east, though their exact path can vary. In the Northern Hemisphere, they typically blow from the southwest, while in the Southern Hemisphere, they originate from the northwest. Their strength and position show seasonal variability. These winds are generally stronger during the winter months in each hemisphere, a time when the temperature difference between the equator and the poles is more pronounced.
Within the westerly flow are concentrated, fast-moving ribbons of air known as jet streams. The polar jet stream is found closer to the poles (typically 30-60 degrees latitude), while the subtropical jet stream is around 30 degrees latitude. These jet streams, flowing from west to east, play a role in steering weather systems. The westerlies can also exhibit turbulence, especially at higher altitudes.
Westerlies Influence on Global Weather and Climate
Westerlies influence daily weather patterns by steering major atmospheric systems across the mid-latitudes. They guide the movement of low-pressure systems, high-pressure systems, and frontal systems, leading to changes in temperature, precipitation, and wind. These winds direct large-scale storm tracks across continents and oceans, particularly in the Northern Hemisphere over regions like North America and Europe.
The westerlies shape regional climates. They transport moisture from oceans inland, impacting the western coastlines of continents. This brings consistent rainfall and contributes to the mild and variable climates observed in many mid-latitude areas. Changes in the westerlies’ strength or position can influence the frequency and intensity of storms and precipitation.
Broader Impacts of the Westerlies
Beyond their influence on daily weather, the westerlies have had significant long-term and historical implications. During the Age of Sail, these winds were crucial for maritime trade and exploration. Sailors utilized the strong westerlies, particularly the “Roaring Forties” (40-50 degrees south latitude) and “Furious Fifties” (50-60 degrees south latitude) in the Southern Hemisphere, for faster eastward voyages. The relative absence of landmasses in the Southern Hemisphere allows the westerlies there to be stronger and more consistent.
The westerlies also drive significant ocean currents. For instance, they propel the North Atlantic Drift, which moderates Western Europe’s climate by transporting warm water poleward. Similarly, in the Southern Hemisphere, they drive the powerful Antarctic Circumpolar Current, which flows eastward around Antarctica. These wind-driven currents are important for global heat and nutrient distribution, influencing marine and terrestrial ecosystems, including nutrient upwelling.