Sets in refers to the start or onset of something, usually a feeling, behavior, or weather condition that gradually becomes more intense or pervasive. There are several synonymous expressions that can be used to describe this concept, such as takes hold, begins, looms, approaches, emerges, dawns, seeps in, infiltrates, creeps up, sneaks up on, settles, establishes itself, ingrains, and becomes entrenched. These phrases convey the gradual or sudden onset of an emotion or sensation, or the slow emergence of a weather condition, making them useful in many contexts, from describing the arrival of a storm to the onset of a pervasive feeling like anxiety or depression.