see blocking) for a significant fraction of a day or longer. This often occurs, for example, when a cold front approaches the barrier. The component of the large-scale flow perpendicular to the ridge forces the flow to ascend the barrier. Because the air column is stable, the air layer near the surface is potentially colder (by definition) than the air layer above it, and the stratification opposes and retards the upslope flow. As the colder air ascends, it produces higher pressure along the slope than at the same level over the plain, and consequently also a pressure-gradient force directed away from the mountains. If this pressure configuration lasts for several hours or more, Coriolis deflection accelerates the flow with a component perpendicular to the pressure gradient, that is, in the along-barrier direction. At timescales greater than a pendulum day—that required for geostrophic adjustment—these processes produce a persistent barrier jet at heights below the level of the mountain. The process of geostrophic adjustment also brings the flow in the jet into balance with the thermal wind, so an argument based on thermal wind reasoning also explains the barrier jet. Barrier jets have been documented windward of the Sierra Nevada in California, to the north of the Brooks Range in Alaska, and in Antarctica along the Antarctic Peninsula and the Transantarctic Mountains. Maximum speeds, which generally occur at heights just below the midway level of the mountains, reach 15–30 m s-1, and the jet can extend laterally 100 km or more upstream of the barrier. The strong shear in the jet is capable of producing moderate to severe turbulence for low-flying aircraft.
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