def nondimensionalSTD(zeta, x=None):
    """
    The nondimensional standard deviation function, defined as:

    phi_c(zeta) = sigma_c / c_star

    From zahn.ea
    """
    if zeta < 0:
        if x == 'u' or x == 'w':
            return 1.25 * (1.0 - 3.0 * zeta) ** (1.0 / 3.0)
        else:
            return 2.0 * (1.0 - 9.5 * zeta) ** (-1.0 / 3.0)
    elif x == 'u' or x == 'w':
        return 1.25
    else:
        return 2.0
