on Helping Writers become Authors:

Every person or character, at any given time, is in a particular mood. Generally, mood is a person’s state of mind, but it’s more than that. Mood can also describe the disposition of a collective of people, a certain time in history, or the essence of a place.

Regardless what kind of mood we speak of, it’s always subjective. Ten people can be experiencing the same event at the same place and time, yet depending on their perspectives, their individual mood will differ.

It’s easy to think of a setting as a fixed element. Paris is Paris, right? While the location itself may be fixed, the city can look vastly different when certain variables are altered. The time of day, the weather, the season, and what a character is dealing with can make a setting look very different than it did just the day before. And no factor has more influence over the setting than mood.