"erratic behavior"
"fickle weather"
"mercurial twists of temperament"
"a quicksilver character, cool and willful at one moment, utterly fragile the next"
relating to or under the (astrological) influence of the planet Mercury;
"the Mercurial canals"
relating to or having characteristics (eloquence, shrewdness, swiftness, thievishness) attributed to the god Mercury;