Interstellar Extinction (Galactic Smog)
=======================================

... is a fractal (cf. the IRAS cirrus maps).  
It is very patchy and nonlinear.

Approximate dependence on gal. latitude |b| :

	A   (b) = A   (90 ) cosec b

where the polar extinction is A   (90 ) ~ 0.15 mag

The local "average" is about A(V) ~ 1 mag/kpc,
and it varies with the galactocentric radius
(a function of the metallicity, etc.?)

Usually expressed as a ratio of total to selective
extinction, in magnitudes:

A(  )/E(B-V) = f (  ; R)

Where the parameter R = A(V)/E(B-V) varies from
here to yonder, depending on the size, shape,
composition, and temperature of the grains, but 
a good (local?) average value may be about 3.2.

The absorbed energy is reradiated in the far-IR
(IRAS!).