  The aim of this note is to describe several meteorological properties shown
in a conference talk (Marquet, January 2022) for the moist-air specific
entropy, the associated potential temperature ($\theta_s$) defined in Marquet
(2011) and the associated potential vorticity $PV(\theta_s)$ defined in Marquet
(2014), with in particular regions of negative values of $PV(\theta_s)$ close
to the cold fronts that seems to indicate regions of symmetric instability.
