An average model for the galactic absorption

It is found from a comparison of optical and radio emission that planetary nebulae south of the galactic plane exhibit lower average extinction at H-beta than those to the north. Suggestions are that either (1) the absorbing material is distributed with a scale height higher in the north than in the south, or (2) a large absorbing cloud immediately above the sun has increased the average extinction in the north. In either case the z-distribution of absorbing material is close to an exponential model, with extinction of 0.7 bel/kpc along the galactic plane and scale height of about 100 pc.