I'd love to go back there on a cloudy,cool day in January...and I'd love to hit Herculaneum as well.
Visited Herculaneum a couple weeks ago. It was mid-70's very pleasant for a walking tour.
Supposedly, people in Herculaneum saw the eruption, but many thought they were safe, as the cloud went south, and they were to the west. Later, the winds shifted and Herculaneum was also covered by 60 feet of ash.
What I found remarkable is that coast of the Gulf of Naples was reformed about half a mile, and has remained that was for two thousand years. The city of Ercolano was essentially rebuilt on top of the old port city of Herculaneum.
Of course, throughout Rome, and even under the Vatican, vast parts of the city were filled in (by nature or man) and layers and layers of the city were rebuilt on top of the remains.