What is the meaning of it looked threatening as it hung in the clear blue sky?

It means that something in the blue sky above appears threatening. Whatever the thing in the sky may be is causing you fear! It suggests a horrible danger to come. You better run before it's too late.

Have a good day!

one of the central themes of edward abbey's work has been his staunch opposition to the glen canyon dam on the colorado river that created lake powell, destroying stunning scenery and wildlife habitat, endangering many rare species of fish and animals, and also irreparably destroying many important archaeological sites which provided information about the early inhabitants of the united states.