The “calm before the storm” typically refers to a period of relative quiet or relaxation prior to a time of intense activity or uproar. This stage of activity may be seen as a positive, such as the time before a rush of business for a company, or as a negative, such as the wait for the influx of patients at a hospital after a disaster. This phrase is typically used as an analogy between the literal calm that can often be felt prior to the arrival of a powerful meteorological event and a period of quiet before a flurry of events or excitement.
As an idiom, the “calm before the storm” is often used interchangeably with similar phrases that replace the word “calm” with “quiet” or “lull.” Regardless of the exact word choice, however, the meaning is the same. This phrase refers to a period of calm and quiet that a person might experience prior to a time of great activity, though it is not necessarily meant to literally indicate that a storm is approaching. Instead, the “calm before the storm” typically refers to a symbolic calm or storm, such as the time when a parent is relaxing prior to his or her children coming home from school.
There is also a literal calm before the storm in some instances, which is the basis for this particular idiom. As storms develop and move, especially thunderclouds and large storm fronts, air is typically pulled up into them from below. This warm, moist air is cooled and moves upward through the clouds, leaving a vacuum in its place, which is then filled by warm air coming from the top of the storm front. When this occurs in the direction a storm is heading, it can produce an environment that is calm and quiet, sometimes eerily calm, which is quickly followed by the onset of the storm itself.
There are other, similar idioms often used in place of this idiom, including “after a storm comes a calm.” Rather than referring to the sense of quiet that can precede a period of great activity, this instead indicates that after this period of activity there may be another time of quiet. This later period, however, may be either a positive or negative concept, since it might indicate a time of peace and relaxation or a time of quiet contemplation of the devastation wrought by a powerful storm. The “calm before the storm” itself, however, is typically considered positive, though it may have ominous connotations for the event to come.