The River of Wind You Can't See
High above the Earth's surface — typically between 7 and 12 kilometers altitude — narrow bands of extremely fast-moving air circle the globe. These are the jet streams, and they are among the most powerful drivers of weather on Earth. While you can't see them from the ground, their position and strength directly influence whether your week will be sunny and mild or battered by storms.
What Causes Jet Streams?
Jet streams form at the boundaries between large air masses with dramatically different temperatures. The polar jet stream — the one most relevant to weather in the mid-latitudes — forms at the boundary between cold polar air and warmer subtropical air. This temperature contrast creates a pressure gradient that, combined with the Earth's rotation (the Coriolis effect), drives strong westerly winds in a narrow band at the tropopause (the boundary between the troposphere and stratosphere).
The stronger the temperature contrast between polar and tropical air masses, the faster and more focused the jet stream becomes. In winter, when the poles are much colder relative to the tropics, the polar jet strengthens significantly.
Polar vs. Subtropical Jet Stream
There are actually two main jet streams in each hemisphere:
- Polar Jet Stream: Found at roughly 30,000–39,000 feet altitude, this is the stronger of the two and the primary driver of storm systems across North America, Europe, and Asia. It typically sits between 40° and 60° latitude but meanders considerably.
- Subtropical Jet Stream: Found slightly lower and closer to the equator (around 20°–30° latitude), it is generally weaker and more consistent in position. It plays a key role in steering tropical moisture and precipitation patterns.
How the Jet Stream Steers Weather
The jet stream doesn't flow in a perfectly straight line. It meanders north and south in large waves called Rossby waves. These undulations are critically important for surface weather:
- When the jet dips south (a trough): Cold Arctic air is pulled down into mid-latitudes. Areas beneath a deep trough experience cold, stormy weather.
- When the jet bulges north (a ridge): Warm air flows northward, and the region under the ridge experiences mild, dry, and often sunny conditions.
- Storm steering: Low-pressure systems and frontal weather follow the jet stream's path. A forecaster watching where the jet is positioned can anticipate where storm systems are likely to track.
The Jet Stream and Extreme Weather Events
When the jet stream's waves become especially large and slow-moving — a pattern called a blocked pattern — weather anomalies can persist for days or even weeks. A blocking high can keep a heat dome in place over a region, causing prolonged heat waves. Conversely, a deeply amplified trough can lock cold air over an area for an extended period, producing prolonged winter storms or dangerous cold outbreaks.
Is Climate Change Affecting the Jet Stream?
This is an active area of scientific research. The Arctic is warming faster than the rest of the planet — a phenomenon called Arctic amplification. Because the polar jet stream is driven by the temperature contrast between polar and tropical regions, a warming Arctic reduces that contrast, which some scientists hypothesize may be weakening and destabilizing the jet stream. A weaker, more meandering jet could potentially contribute to more persistent extreme weather events. However, this remains an area of ongoing scientific debate.
Why Forecasters Watch the Jet Stream Closely
Modern weather forecasting relies heavily on tracking the jet stream's position and amplitude. Numerical weather prediction models simulate jet stream behavior out to 10–14 days, giving forecasters the ability to identify potential storm tracks, cold outbreaks, and warm spells well in advance. Next time you look at a long-range forecast, you're essentially seeing a prediction about where the jet stream will be — and what air masses it will steer toward you.