In general, any agitation or disruption of a steady state. In meteorology, this has several rather loose applications: 1) used for any low or cyclone, but usually one that is relatively small in size and effect; 2) applied to an area where weather, wind, pressure, etc. , show signs of the development of cyclonic circulation (see tropical cyclone); 3) used for any deviation in flow or pressure that is associated with a disturbed state of the weather, that is, cloudiness and precipitation; and 4) applied to any individual circulatory system within the primary circulation of the atmosphere. See wave disturbance, instability; compare perturbation.