  Supersonic flows are expected to exist in the atmospheres of irradiated
exoplanets, but the question of whether shocks develop lingers. Specifically,
it reduces to whether continuous flow in a closed loop may become supersonic
and if some portions of the supersonic flow steepen into shocks. We first
demonstrate that continuous, supersonic flow may exist in two flavors:
isentropic and non-isentropic, with shocks being included in the latter class
of solutions. Supersonic flow is a necessary but insufficient condition for
shocks to develop. The development of a shock requires the characteristics of
neighboring points in a flow to intersect. We demonstrate that the intersection
of characteristics may be quantified via knowledge of the Mach number. Finally,
we examine 3D simulations of hot Jovian atmospheres and demonstrate that shock
formation is expected to occur mostly on the dayside hemisphere, upstream of
the substellar point, because the enhanced temperatures near the substellar
point provide a natural pressure barrier for the returning flow. Understanding
the role of shocks in irradiated exoplanetary atmospheres is relevant to
correctly modeling observables such as the peak offsets of infrared phase
curves.
