Potential vorticity on two separate vertical sections taken through a
tropopause fold encountered during the SONEX aircraft campaign
(October-November 1997, the dashed white line shows a flight path). High ozone
stratospheric air (bright colours) extends far into the troposphere
(blues/purples) and becomes mixed over the course of a few
days. Kelvin-Helmholtz waves causing mixing were observed by the aircraft at
the location of the black square (see Esler and Polvani, 2004).