Weather often moves from west to east across many regions. While this pattern is widespread, it does not hold true in every situation. This article explores the scientific principles behind the typical west-to-east movement and the circumstances where this pattern changes.
Global Weather Patterns
The Earth’s rotation and uneven heating create large-scale atmospheric circulation patterns that drive global weather. A significant force is the Coriolis effect, an apparent force that deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection influences global wind patterns, preventing air from flowing directly from high to low-pressure areas.
In the mid-latitudes, between approximately 30 and 60 degrees latitude, prevailing winds known as the westerlies dominate. These winds blow predominantly from west to east, steering large-scale weather systems across continents. The westerlies originate from high-pressure areas in the horse latitudes and trend towards the poles, playing a significant role in transporting moisture from oceans inland.
High in the atmosphere, narrow bands of strong winds called jet streams flow generally from west to east. These fast-moving currents, typically found between 30,000 to 39,000 feet above sea level, form due to temperature differences between air masses. Jet streams act as atmospheric conveyor belts, steering storms and influencing temperature patterns across vast distances.
Large-scale high and low-pressure systems are embedded within these global wind patterns. The jet stream’s position and strength determine the trajectory of these systems, which typically move along with the prevailing westerlies. This coordinated movement of global winds, the Coriolis effect, and pressure systems establishes the foundational west-to-east weather progression observed in many parts of the world.
Exceptions to the Rule
Despite the dominant west-to-east flow, certain weather phenomena and atmospheric conditions can cause significant deviations. Tropical cyclones, such as hurricanes and typhoons, often initially move westward in their early stages. This is due to the influence of easterly trade winds found in lower latitudes. As these storms move poleward, they may recurve and begin to move eastward under the influence of the middle-latitude westerlies.
Localized weather phenomena, driven by daily temperature differences, also exhibit unique movement patterns. Sea breezes, for instance, blow from the sea towards land during the day when land heats faster than water. Conversely, land breezes develop at night, flowing from land to sea as the land cools more rapidly. These localized circulations move perpendicular to coastlines, not necessarily west to east.
Large, nearly stationary atmospheric pressure patterns, known as blocking highs, can significantly disrupt the typical west-to-east flow. These systems can remain in place for several days or even weeks, effectively “blocking” or redirecting migratory cyclones. When a blocking high is present, weather systems may be forced to move around it, sometimes even moving in a retrograde (east-to-west) fashion.
Factors Influencing Local Weather
Geographical features and smaller-scale atmospheric dynamics frequently modify the general west-to-east weather patterns, creating localized movements that might seem contradictory. Topography, the arrangement of land features like mountains and valleys, plays a significant role. Mountain ranges can act as barriers, forcing moist air upwards, which leads to localized precipitation on one side and a rain shadow effect, or drier conditions, on the leeward side. This influences where weather appears and its intensity, even if the overall system moves east.
Large bodies of water, such as oceans and sizable lakes, also exert a moderating influence on nearby climates. Water heats and cools more slowly than land, leading to milder temperatures and often increased precipitation in coastal areas. This can result in phenomena like lake-effect snow, where cold air picks up moisture and warmth from warmer lake waters, creating heavy snowfall downwind, independent of broad west-to-east storm tracks.
Localized pressure systems, smaller and more transient than their global counterparts, can form and influence local wind and weather directions. These smaller cells can cause temporary shifts in wind patterns that deviate from the larger atmospheric currents.
Urban heat islands, where cities are significantly warmer than surrounding rural areas, also create their own microclimates. The dense concentration of buildings and impervious surfaces in urban areas absorb and re-emit heat, altering local wind patterns and potentially influencing localized precipitation.