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The polar front and its associated jet stream have a major influence on the weather conditions surrounding it. Many storms form along the polar front in the vicinity of the jet stream's maximum winds. (For more on this see my previous Suite 101 article The Highs and Lows of Weather: Part 2 -- The Low) Because of the association of the jet stream with the polar front, media weather maps often include the jet stream's position as a rough indicator of continental divisions between warm and cold air masses. Here is why.
From a climatological viewpoint, the position of the polar front forms a more-or-less even band around the globe at mid-latitudes in both hemispheres. The average polar-front position slips north and south with the seasons in response to the annual hemispheric heating cycle. Like my waistline's annual cycle, the polar frontal belt shrinks poleward in the summer months and then expands toward the subtropics during the winter. Global Pattern of Long (Rossby) Waves Wave troughs are indicated by red lines, polar front by grey line. From a meteorological viewpoint, however, the polar front is not an evenly encircling latitude belt but an undulating zone around the globe that is ever-changing as masses of warm and cold air push away from their regions of origin. Of particular interest to meteorologists and weather forecasters is the ever-changing pattern of long-waves that form around the polar front. These long-waves, very visible on polar projection maps, undulate around the hemisphere with three to six wave cycles. (Long waves are also known as Rossby waves in honour of Swedish meteorologist C.G. Rossby who gave us many early insights into the impacts of upper atmosphere features on weather.) At times, that global belt fits tight, having three or four, small-amplitude undulations (little north-south latitude variation) around the hemisphere. At other times, it has as many as six large-amplitude loops (great north-south variation). How those wave loops sit over the hemisphere, or portion thereof, determines what temperature regimes are experienced on the surface below. When a long section of the polar front, as seen on continental weather maps, is smooth like the surface of a calm sea, the upper-level winds, including associated jet streams, run generally parallel to the latitude lines, a condition called zonal flow. Under zonal-flow, north-south undulations of the frontal boundary are small, and the surface temperatures across the continent, as seen in the isotherm (lines of equal temperature) pattern on the weather map, layer in zonal (east-west) bands with warm air to the south and cold air to the north.
The copyright of the article Jet Streaming Along the Polar Front in Meteorology is owned by . Permission to republish Jet Streaming Along the Polar Front in print or online must be granted by the author in writing.
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