Weather, moods, prices and people can all change without warning, and English has a word for each kind of change. Some describe things that swing unpredictably (erratic, volatile, fluctuate); some describe people whose moods or loyalties shift (capricious, fickle, mercurial); and some describe acting without stopping to think (impulsive, impetuous, spontaneous).
Key idea
Changing moods and loyalties: capricious (on a whim), fickle (loyalty that shifts), mercurial (quick mood swings). Unpredictable things: erratic (irregular), volatile (ready to blow up), fluctuate (to rise and fall), vacillate (to waver between choices). Acting without planning: impulsive and impetuous (rash), spontaneous (natural, unplanned, often a good thing).
The ten words
Common mistake
spontaneous is often good (a spontaneous smile); impetuous and impulsive usually warn. vacillate is about indecision; fluctuate is about changing amounts.
| word | what changes | a typical subject |
|---|---|---|
| capricious | choices, on a whim | a ruler, the weather |
| fickle | loyalty and affection | fans, friends, a cat |
| mercurial | mood, quickly | a temperamental boss |
| erratic | behavior or motion, irregularly | an engine, sleep |
| volatile | anything, suddenly and maybe explosively | markets, tempers |
| vacillate | a decision, back and forth | someone who can’t choose |
| fluctuate | amounts, up and down | prices, temperatures |
spontaneous is usually praise (a spontaneous laugh, a spontaneous party), while impulsive and impetuous usually warn: they mean acting before thinking. And vacillate is about indecision in a mind; fluctuate is about numbers and amounts going up and down.