[HD1200]

“because all action must, to a certain extent, be planned in a mere twilight, which in addition not infrequently — like the effect of a fog or moonshine — gives to things exaggerated dimensions and unnatural appearance.””

He writes of planning not the physical activity of a war.

Unexpected and unknown physical attributes of a war are termed FRICTION.

Don’t argue with me; argue with my one son’s USAF & my other son’s Marine books.

[dr_lew]

Well, a perusal of an online pdf version of Vom Kriege, ( as opposed to the photocopy version I dug up previously,) shows that Friktion is a basic concept in the work, whereas Nebel (fog) is used metaphorically to express the contribution of uncertainty to the general operation of the former principle.

I actually see two places where Nebel is used figuratively in this way. In addition to the Wikipedia citation I mentioned, there is this passage:

Der Krieg ist das Gebiet der Ungewißheit; drei Vierteile derjenigen Dinge, worauf das Handeln im Kriege gebaut wird, liegen im Nebel einer mehr oder weniger großen Ungewißheit.

"War is the realm of uncertainty, three quarters of the things upon which the practice of war depends, lie in a fog of more or less total uncertainty."

This is probably a better source for the popular idea of "the fog of war" than the other quote.

BTW, Clausewitz does cite weather as a contribution to friction, and gives the example of an artillery signal not being heard due to fog, but fog is used metaphorically as well, and in a way consistent with the popular conception of "the fog of war".