Craig Berry wrote:
 > Actually, to get full dimming you need a much bigger cloud...
 > You need the cloud shadow radius to approximate earth's radius
 > to get the full effect of the cloud's opacity.

Yes, I was wrong. You are right for the full dimming case.

(Using the earth as an example:
radius ~ 150 million km
arc length ~ 12800 km

angle subtended is ~8.5 x 10^(-5) radians.

If radius is 149 million km, arc length is ~12715 km.)

My incorrectly sized cloud (diameter 9300 km) casts a slightly larger
shadow (~9360 km) over the earth. Darkness over half the earth and
dimming over the rest will cause serious cooling if you can maintain the
effect for long enough.

Needing a bigger cloud makes the logistics worse for someone who wants
to completely darken a planet by this method.

Which gets us to Tim's:
 > My translation of that is: unlikely in real life but just
 > about within the margins of fictional drama.

Rob O'Connor