Foul weather, including storms and low-pressure systems, is governed by large-scale, predictable atmospheric principles. The movement of air masses and storms is a consequence of the planet’s rotation and the uneven heating of its surface.
Prevailing Movement in Mid-Latitudes
For the majority of the world’s populated areas in the mid-latitudes (between 30 and 60 degrees north and south of the equator), foul weather typically approaches from the west and moves toward the east. This includes most of North America, Europe, Asia, and South America. Mid-latitude cyclones, the dominant cause of stormy weather, generally travel eastward across continents and oceans.
These systems, which often bring cold fronts, warm fronts, and precipitation, follow a general west-to-east path due to the influence of the prevailing winds in this region. The speed of these weather systems can vary, but they often cover hundreds of miles in a single day as they traverse the globe. The eastward movement is the rule of thumb for most extra-tropical storms and low-pressure centers outside of the tropical zones.
Atmospheric Forces Guiding Storm Paths
The primary driver for this consistent west-to-east movement is the prevailing Westerlies, a global wind belt in the middle latitudes. This immense flow of air is the direct result of the Earth’s rotation and global circulation patterns, steering weather systems across the hemisphere in an easterly direction.
Embedded within this broad wind belt is the Jet Stream, a high-altitude, fast-flowing ribbon of air that acts as a highway for weather systems. It is concentrated near the boundary between cold polar air and warmer air masses. Its powerful currents direct weather systems, making the Jet Stream’s path a key factor in determining a storm’s precise track and speed.
The underlying cause of these global wind patterns traces back to the Coriolis Effect, an apparent force resulting from the Earth’s rotation. As air moves from high to low pressure, the Coriolis Effect deflects the moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection establishes large-scale circulation cells, including the Ferrel cell, which generates the Westerlies wind belt. The combination of temperature differences, pressure gradients, and this rotational force maintains the upper-level flow that guides foul weather eastward.
Significant Deviations from Typical Movement
While the west-to-east track dominates the mid-latitudes, significant exceptions exist, particularly in the tropics and during atmospheric blocking patterns. Tropical cyclones, such as hurricanes and typhoons, exhibit a different initial movement due to a separate wind belt. These storms, which form near the equator, are initially steered by the Trade Winds, which blow from the east to the west.
A developing hurricane in the Atlantic Ocean, for example, will typically move westward across the ocean basin. Only later, as the storm moves poleward and encounters the influence of the mid-latitude Westerlies, does its path curve toward the north and eventually the northeast. The initial track is governed by the Trade Winds, which are a strong easterly flow in the lower latitudes.
Other major deviations occur when a low-pressure system becomes detached from the main flow of the Jet Stream, forming what is known as a “cut-off low.” These systems are isolated from the fast-moving atmospheric river that normally pushes them eastward. A cut-off low can become nearly stationary for several days or begin to move erratically, sometimes even reversing course and moving westward—a rare phenomenon known as retrograding. Since they are no longer being steered by the prevailing flow, these low-pressure centers can bring prolonged periods of heavy rainfall and unsettled weather to the same localized region.