.... the vision or dream of the black skeleton like cloud blowing a white vapor...

But just before that, he sees a black figure forming out of
a cloud, skeleton like, blowing a white vapor, which he interprets
as chemical warfare.
Maybe this black figure is more than symbolic.

... This white vapor could be the winter storm centered in Chicago and goes coast to coast....   It is a deep freeze from coast to coast ... ie  a white vapor..
It is an "Artic Blast" as our local TV stations have been reporting here in the Pacific Northwest.

Whatcha think?, for he interprets it as "chemical warfare" I think it is of a "winter's blast of cold air...

Don Best