Answer: cold and warm air masses interact in an unstable environment.
Explanation: Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone .According to the polar-front theory, extratropical cyclones develop when a wave forms on a frontal surface separating a warm air mass from a cold air mass. As the amplitude of the wave increases, the pressure at the centre of disturbance falls, eventually intensifying to the point at which a cyclonic circulation begins. The decay of such a system results when the cold air from the north in the Northern Hemisphere, or from the south in the Southern Hemisphere, on the western side of such a cyclone sweeps under all of the warm tropical air of the system so that the entire cyclone is composed of the cold air mass. This action is known as occlusion.Extratropical cyclones arise through a process called cyclogenesis, in which cold and warm air masses interact in an unstable environment.