area in the atmosphere in which the pressures are lower than those of the surrounding region at the same level; it is represented on a synoptic chart by a system of isobars at a specified altitude level (or a system of contours at a specified pressure level) which enclose relatively low values of pressure (or altitude); a cyclone begins when a wave (young) cyclone forms and moves along a front; a mature cyclone has well-developed warm sectors and both cold and warm fronts; an occluded cyclone is that within which there has developed an occluded front.