The incident wave (1) from a burst point above the ground is reflected in a regular reflection (2) until the angle it makes with the ground plane becomes large enough to allow the reflected wave to overtake and coalesce with the incident wave, forming a Mach reflection (3). As the Mach stem grows, the contact surface (4), separating air that has experienced only the Mach shock from air that has been through both the incident and reflected shocks, trails behind and connects the triple point (5) with the surface. However, air flow above the contact surface has not been completely turned parallel to the surface by the reflected shock and retains some downward momentum. This results in an eventual compression at the surface (6) as the downward-moving double-shocked air above the contact surface is stopped. This compression (7) distorts and chews up the tail of the contact surface stream line (6) and raises the pressure. The Mach shock itself repres~nts a lower pressure than that behind the reflected wave in the regular reflection regIon, but the second shock quickly builds to appreciably higher values
at that transition.