def pressure_ratio_across_normal_shock(
        mach_upstream,
        gamma=1.4
):
    """
    Computes the ratio of fluid static pressure across a normal shock.

    Specifically, returns: P_after_shock / P_before_shock

    Args:
        mach_upstream: The mach number immediately before the normal shock wave.
        gamma: The ratio of specific heats of the fluid. 1.4 for air.

    Returns: P_after_shock / P_before_shock

    """
    m2 = mach_upstream ** 2
    return (
                   2 * gamma * m2 - (gamma - 1)
           ) / (
               (gamma + 1)
           )