Geography
How the Jet Stream Steers Storm Tracks Across Mid-Latitudes
Quick fact
The jet stream can reach speeds of 250 miles per hour (400 km/h), making it one of the fastest winds on Earth.
Why this is interesting
Have you ever wondered why a storm seems to follow a path, crossing entire countries? Or why a weather forecast changes so quickly? The answer lies in a ribbon of wind high above our heads.
Read the full explanation
Understanding How the Jet Stream Steers Storm Tracks Across Mid-Latitudes
Imagine a fast-moving river of air, hundreds of miles wide, flowing from west to east in the upper atmosphere. This is the jet stream. It exists because of the temperature contrast between the cold, dense air near the poles and the warm, lighter air near the equator. This temperature difference creates a pressure gradient that strengthens with altitude. The stronger pressure gradient, combined with the Earth's rotation, produces strong westerly winds. But the jet stream isn't a straight, steady wind. It meanders in waves (Rossby waves). These waves are crucial because they act as steering currents for weather systems at the surface. A storm is like a boat on this river: it is carried along by the current. Similarly, low-pressure systems (mid-latitude cyclones) are guided by the jet stream. The jet stream also provides the energy these storms need to grow. When the jet stream dips southward (a trough), it brings cold air and creates conditions favorable for storm development. When it bends northward (a ridge), it brings warm air and clear skies.
A deeper explanation
The jet stream steers storms because of its connection to the polar front, the boundary between cold polar air and warm subtropical air. This front is where contrast in temperature is strongest, and it provides the energy that fuels mid-latitude cyclones. At the surface, when a disturbance occurs along the polar front, it can form a wave-like pattern. The jet stream above helps this wave develop into a full-fledged storm system. The jet stream's core is a region of strong pressure gradients, which results in a narrow band of very fast winds. This band, through the conservation of potential vorticity, influences the vertical motion of air. When the jet stream curves, it creates areas of divergence (upper-level spreading) and convergence (upper-level piling up). Divergence aloft lowers surface pressure, promoting upward motion, cloud formation, and storm development. Convergence aloft raises surface pressure, leading to calm, clear conditions. As the jet stream's waves move and change, they steer the paths of surface low-pressure systems. A storm tends to travel along the jet stream's path, toward the northeast in the Northern Hemisphere, until the jet stream's steering current weakens or the temperature contrast diminishes. This is why weather forecasts must keep a close eye on the jet stream: its position and shape determine when and where storms will form and travel.