The pressure gradient normal to the flow within a boundary layer is almost zero for low to moderate hypersonic Mach numbers , increasing the boundary layer 's temperature and decreasing the layer 's density and thus so expanding the layer 's bottom that the boundary layer thickens and often merges with the leading edge 's shock wave .
Because the pressure gradient normal to the flow within a boundary layer is zero , the increase of temperature through the boundary layer causes the density to fall .