def vapor_flux(air_density, k, q_dif, z_dif):
    """water vapor flux (kg/(m^2 sec)) between two layers

    Args:
        air_density (float or array): air density (kg/m^3)
        k (float or array): diffusion coef. (m^2/sec)
        q_dif (float or array): specific hum. diff between layers (kg/kg)
        z_dif (float or array): absolute distance between layers (m)

    Returns:
        float or array: water vapor flux
    """
    return air_density * k * (q_dif/z_dif)