seasonmap field guide

What is the polar vortex?

"Polar vortex" refers to two distinct circulations. This page covers the stratospheric one: what it is, how it breaks down, and why the surface cold that follows arrives on a delay of weeks, not days.

Two vortices, one name

The tropospheric vortex is the ordinary ring of jet-stream westerlies around the pole in the layer where weather lives; its day-to-day wobbles are routine winter variability. The stratospheric polar vortex is a separate structure: a band of westerlies, often exceeding 100 mph, circling the pole from roughly 10 to 30 miles up. It forms each autumn as the pole loses sunlight and dissipates each spring.

A strong, circular stratospheric vortex confines the coldest air to the pole, and the eastern US tends to run mild. The forecasting interest is in what happens when that vortex is disrupted.

A third polar-sky phenomenon gets conflated with both: the aurora. The northern lights are not weather in this sense at all — they are emission at roughly 60–250 miles altitude, far above the stratosphere, driven by solar-wind electron precipitation rather than by anything on this page. Geomagnetic storms and SSWs are unrelated events on unrelated clocks. We track that domain at The Space Weather Observatory, a sister project built from DMSP/SSUSI, POES, and AMPERE measurements, with live and 3-night aurora forecasts. The one operational overlap: seeing an aurora requires clear skies, which is what this site's satellite layers are for.

Sudden stratospheric warming

Tropospheric ridges — often amplified by tropical forcing and East Asian mountain-torque events — launch planetary-scale waves that can propagate upward into the stratosphere. When a large enough wave breaks there, stratospheric temperatures rise by tens of degrees within a few days, and the vortex is either displaced off the pole or split into two lobes. This is a sudden stratospheric warming (SSW); the formal criterion is a reversal of the 10 hPa zonal wind at 60°N from westerly to easterly.

Major SSWs occur in roughly six of ten Northern Hemisphere winters.

The lag to the surface

After a disruption, the easterly wind anomaly descends through the stratosphere over days to weeks. When it reaches jet-stream level, it weakens the westerlies, favors high-latitude blocking (a negative Arctic Oscillation), and allows polar air to spill into the mid-latitudes. The documented window for surface impact is two to six weeks after the SSW.

Cases on record: the January 2014 cold outbreaks, the late-winter 2018 "Beast from the East" in Europe, and February 2021, when the descending response to a January SSW coincided with −2°F in Dallas and the Texas grid failure, roughly a month after the stratospheric reversal.

Caveats

• Coupling is not guaranteed. Roughly a third of SSWs produce little surface response; the disruption has to couple downward, and some stay confined to the stratosphere.
• Location varies. Splits and displacements favor different regions; Europe and Siberia are affected about as often as North America.
• An intact vortex is also informative. A strong, undisturbed vortex shifts the odds toward a milder eastern US.