subject to change; "a changeable climate"; "the weather is uncertain"; "unsettled weather with rain and hail and sunshine coming one right after the other"
With regard to geomagnetic activity, a descriptive word defining conditions between quiet and active, specifically meaning that the A index is between 8 and 15.
To present participles which come from intransitive verbs, or are themselves employed as adjectives, to mark the absence of the activity, disposition, or condition implied by the participle;