Morning fog shrouds a runner on a cold November day in Madison, Wis.

Q: Can weather in the stratosphere affect us near the ground?

A: One potential phenomenon that can encourage a winter-like cold air outbreak is a sudden warming of the lower stratosphere, the layer of the atmosphere between 6 and 20 miles above the surface.

Every year, as the sun heads toward the horizon in polar regions at the autumnal equinox, a strong wind maximum develops on the edge of the cold, stratospheric air that is left in the expanding polar night. This jet is known as the polar night jet (PNJ). It is usually fairly east-west oriented with few undulations in it, and it provides dynamical support for containing cold air in the lower troposphere at high latitudes.

The PNJ can weaken rapidly when it is showered with wave energy that emanates from a parade of storms occurring in the underlying troposphere. Rapid weakening of the PNJ is often associated with lower stratospheric temperature increases of as much as 50 degrees in one day — sudden stratospheric warmings (SSWs).

With a weaker PNJ, the containment of cold polar air at lower altitudes is also weakened, and so locations in the middle latitudes are suddenly more susceptible to dramatic cold air outbreaks, often delayed by a few weeks from when the SSW occurs.

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"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences.


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