Description

A barrier jet is a narrow band of strong winds which occurs when an air mass encounters an orographic obstruction such as that presented by a range of mountains. Unlike a jet stream, a barrier jet is located near the surface, not near the tropopause.

Key characteristics 

Barrier jets occur on the windward side of steep mountain ranges, between 1,000ft and 8,000ft above the surface, and run parallel to to the barrier. Wind speeds are typically 20 to 60 kts and the jet is narrow; often less than 50 nm in width.

The strength of the barrier jet depends on the stability of the upstream airflow, the speed of the wind approaching the terrain, and the size of the mountain range.

Barrier jets are primarily a winter phenomenon although they can occur during stable conditions all year round.

Flight safety considerations

Strong low level winds and the associated turbulence can pose a hazard to aircraft. Heavy snow or rainfall can be expected along the mountain slopes affecting operations from airports close to the mountain range. Barrier jets can fuel the development of overnight thunderstorms.  

Examples of barrier jets

Appalachian Mountains (North America)

• Context: A prime example of cold air damming.
• Mechanism: When a high-pressure system sits over Quebec/New England, it drives cold air southward along the eastern side of the Appalachians. This dense, cold air cannot rise over the mountains, causing it to bank against the windward side and accelerate as a northeasterly barrier jet.
• Impact: These jets are associated with severe winter weather, creating significant wind shear and heavy precipitation along the east coast affecting the airports associated with major US cities.

Gulf of Alaska / Southeast Alaska Coast 

• Context: Frequently observed along the steep coastal terrain of Southeast Alaska.
• Mechanism: Strong onshore flow, driven by landfalling storms (cyclones) in the Gulf of Alaska, hits the coastal mountains (e.g., Mount Fairweather, Valdez–Cordova mountains).
• Impact: These are among the most persistent barrier jets, with winds often exceeding 45 kts. They are most common during the winter. 

Taiwan Central Mountain Range

• Context: Identified during the Taiwan Area Mesoscale Experiment (TAMEX) in 1986 which studied heavy rain-producing mesoscale weather systems.
• Mechanism: During the early summer rainy season (Mei-Yu season), southwesterly monsoon flow hits the island. The central mountain range blocks this flow, forcing it to turn and accelerate northward along the northwestern coast of Taiwan.
• Impact: The barrier jet creates strong convergence and is a major factor in heavy rainfall events over northwestern Taiwan. 

Sierra Nevada Mountains (California) 

• Context: Studied extensively in the Western United States.
• Mechanism: When air masses with a westerly component (e.g. behind a cold front) approach the Sierra Nevada, the low-level flow is blocked, resulting in a northerly, coast-parallel barrier jet.
• Impact: These jets are known for causing high wind events, such as the northerly barrier jet that develops along the western portion of the Sacramento Valley. 

Antarctic Peninsula (Larsen C Ice Shelf) 

• Context: Observed along the eastern coast of the Antarctic Peninsula.
• Mechanism: Airflow approaching the steep mountains of the peninsula is blocked, creating a strong, cold southerly or southwesterly flow along the eastern, windward side.
• Impact: These jets are critical drivers of local climate and are associated with foehn winds, which contribute to the melting of the Larsen C Ice Shelf. 

Colorado Rockies Front Range 

• Context: Occurs on the eastern slope of the Front Range.
• Mechanism: Similar to the Appalachian case, a cold-frontal passage brings easterly, cold, and stable air that is trapped against the mountains.
• Impact: This specific jet is associated with narrow, intense bands of snowfall.