def refractivity_dry_continuum(ν, θ, pd, e):
    """Complex refractivity due to dry air continuum terms.

    ν       GHz     frequency at which refractivity is evaluated
    θ       -       reciprocal temperature
    pd      hPa     pressure of dry air
    e       hPa     pressure of water vapor

    Liebe et al. (1993).
    """
    S0 = 6.14e-11 * pd * θ*θ
    γ0 = 0.56e-3 * (pd+e) * θ**0.8
    F0 = -ν/(ν + 1j*γ0)
    Sn = 1.40e-18 * pd*pd * θ**3.5
    Fn = ν/(1 + 1.9e-5*ν**1.5)
    return S0*F0 + 1j*Sn*Fn