Which word effectively describes someone whose temperament is subject to rapid and unpredictable changes?

The word that effectively describes someone whose temperament is subject to rapid and unpredictable changes is "mercurial." This term originates from the qualities associated with the Roman god Mercury, who was known for his speed and fluidity. In the context of personality, a mercurial individual is one whose moods and behaviors can shift swiftly and capriciously, making them difficult to predict. This characteristic captures the essence of someone whose emotional state is unstable or changeable.

In contrast, "morose" refers to a sullen or gloomy disposition, which does not imply variability but rather a consistent sadness. "Mediocre" describes something of only average quality, without any implication of emotional changes. Lastly, "perfunctory" pertains to an action performed with minimal effort or reflection, indicating a lack of engagement rather than an unstable mood. Thus, "mercurial" is the most accurate term for describing unpredictable temperament.