The following figure is an isentrope altitude cross-section derived from MTP/ER2 data taken during a POLARIS Mission flight that penetrated the vortex.
 

Figure 1.  Isentrope altitude cross-section for flight 1997.04.26, from Fairbanks, Alaska to the North Pole and back. Colored traces are isentrope surfaces, 10 K apart.  The black trace is ER-2 altitude.

The "wind maximum" is at a latitude of 84.0 degrees, which also coincides with the latitude where ClONO2 rises, indicating that the vortex edge was at 84.0 degrees latitude.  The 500 K isentrope is fitted to straight lines for latitude regions either side of the "slope break latitude" of 82.5 degrees, which is 1.5 degrees equatorward of the vortex edge.  The isentrope slope outside the vortex is 25 [meters/degree], whereas inside the vortex the slope is 125 [meters/degree].

Another vortex penetrating flight, ER920213, shows that inside the vortex the isentropes slope 145 [meters/degree].

Figure 2.  One isentrope during a flight north from Bangor, Maine to Canada and back.  ClO changed in a way to indicate that the vortex edge was at 54.9 ks, or 0.8 ks earlier than the isentrope slopes made an abrupt change.

For this flight the polar vortex edge, as revelaed by chlorine monoxide, was encountered before the isentrope slopes changed.  The difference in flight time of 800 seconds between the two events corresponds to 1.5 degrees of latitude.  The return portion of the flight could not be used to make the same comparison because ClO exhibited a broad and confusing transistion between 64.0 and 67.6 ks (at 20 km flight altitude, instead of 19.0 km for the northbound portion).

More coparisons like this need to be made.

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This site opened:  January 14, 2000.  Last Update: March 1, 2000