What is the general setting of Act 1 of Macbeth?

The play begins on a "blasted heath" or "deserted place," depending on which version you may have. However it is said, we find ourselves at the beginning of Act One on a battlefield where the witches have appeared. I always imagine dead bodies strewn about and fog and smoke and the smell of death is in the air. And, as whenever the witches appear, there is thunder, lightning and rain. Indeed. I see the whole play like this: gray and nasty, murky and muddy, dark and bloody. As for when all this messy business takes place, historically: there really was a Macbeth, and he killed King Duncan in the year 1040.