The terms westerlies easterlies describe the dominant wind patterns that circle the planet, defining the climate and weather for entire continents. While the name suggests a simple left-to-right flow, the reality involves a complex dance between temperature, pressure, and the rotation of the Earth. Understanding these air currents is essential for grasping why deserts form, why storms track specific paths, and how energy moves around the globe.
Mechanics of Global Wind Belts
At the heart of the atmospheric circulation are three distinct cells per hemisphere: the Hadley, Ferrel, and Polar cells. Near the equator, warm air rises, creating low pressure and leading to the doldrums. This air eventually cools and sinks around 30 degrees latitude, forming the high-pressure zones where the trade winds end. The region where the sinking air begins to move poleward sets the stage for the interactions between the westerlies easterlies.
The Birth of the Westerlies
Between approximately 30 and 60 degrees latitude, the winds primarily blow from the west toward the east. These are the westerlies, driven by the temperature contrast between the warm equatorial air and the cold polar air. The Coriolis effect, caused by the Earth’s rotation, deflects these winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, creating a predominantly southwest flow in the north and northwest flow in the south.
The Persistence of the Easterlies
Closer to the poles, the pattern flips. Cold air sinking at the poles creates high pressure, which pushes air toward lower latitudes. Because of the Coriolis effect, these winds—the polar easterlies—are deflected to create an eastward flow just south of the Arctic and Antarctic Circles. These easterlies clash with the mid-latitude westerlies, creating a dynamic buffer zone where weather systems are born and intensified.
Impacts on Climate and Geography
The interaction between the westerlies easterlies is a primary driver of regional climate. The prevailing westerlies are responsible for the mild, maritime climates found in Western Europe and the Pacific Northwest of North America. These winds pull moisture off the oceans, delivering rain and sustaining ecosystems far inland. Conversely, the descent of dry air at the boundary of the easterlies creates the world’s major deserts, such as the Sahara and the Australian Outback.
Storm Tracks and Jet Streams
High in the atmosphere, the temperature gradient between the poles and the equator creates the jet streams, fast-flowing rivers of air that steer weather systems. The polar jet stream, located near the boundary of the Ferrel and Polar cells, acts as a guide for mid-latitude cyclones. These storms follow the path of least resistance, which is often the corridor carved by the jet stream, making the westerlies the main conveyor of weather for populated mid-latitude regions.
Seasonal Variations and Modern Observations
These wind belts are not static; they shift with the seasons. During summer, the zone of maximum solar heating moves poleward, causing the wind belts to follow. In winter, the contraction of the polar vortex can cause the easterlies to push further south, leading to sudden cold snaps in normally temperate zones. Monitoring the strength and position of these currents is critical for long-range weather forecasting and understanding climate change.
Conclusion for the Curious Mind
The balance between the westerlies easterlies is a fundamental component of a stable climate system. These invisible rivers of air connect the tropics to the poles, distributing heat and moisture across the planet. For anyone seeking to understand the weather out the door or the climate of the distant past, looking at these grand-scale wind patterns provides the ultimate context.
