...the little group moved up the nave, enveloped in an atmosphere of its own, of which wet overcoats and umbrellas were resolvable constituents.
What a poor phrase!
You were right to look for context clues to try to explain the meaning, but I just went to the dictionary and tried to pick out definitions that would work in that context.
I think he means to say that wet overcoats and umbrellas were (resolvable constituents) expected components of that whole scenario.
Maybe someone else will have another idea. . . .