def mean_vector(lons, lats):
    """
    Returns the resultant vector from a series of longitudes and latitudes

    Parameters
    ----------
    lons : array-like
        A sequence of longitudes (in radians)
    lats : array-like
        A sequence of latitudes (in radians)

    Returns
    -------
    mean_vec : tuple
        (lon, lat) in radians
    r_value : number
        The magnitude of the resultant vector (between 0 and 1) This represents
        the degree of clustering in the data.
    """
    xyz = sph2cart(lons, lats)
    xyz = np.vstack(xyz).T
    mean_vec = xyz.mean(axis=0)
    r_value = np.linalg.norm(mean_vec)
    mean_vec = cart2sph(*mean_vec)
    return mean_vec, r_value