 Extrotropical cyclones, sometimes called mid-latitude cyclones or weight cyclones, are low-pressure aureus which, along with the anticyclones of high-pressure aureus, drive the weather over much of the earth. Extrotropical 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 weathered systems that occur in the middle latitudes of the earth. In contrast with tropical cyclones, extra-tropical cyclones produce rapid changes in temperature and do point along broad lines, called weather fronts, about the center of the cyclone. The term cyclone applies to numerous types of low-pressure aureus, one of which is the extra-tropical cyclone. The descriptor extra-tropical signifies that this type of cyclone generally occurs outside the tropics and in the middle latitudes of earth between 30 degrees and 60 degrees latitude. They are termed mid-latitude cyclones if they form within those latitudes or post-tropical cyclones if a tropical cyclone has intruded into the mid-latitudes. Weather forecasters and the general public often describe them simply as depressions or lobes. Terms like frontal cyclone, frontal depression, frontal lobe, extra-tropical lobe, non-tropical lobe and hybrid lobe are often used as well. Extra-tropical cyclones are classified mainly as baroclinic because they form along zones of temperature and viewpoint radiant known as frontal zones. They can become barotropic late in their life cycle when the distribution of heat around the cyclone becomes fairly uniformed with its radius. Extra-tropical cyclones form anywhere within the extra-tropical regions of the earth, usually between 30 degrees and 60 degrees latitude from the equator either through cyclone genocide or extra-tropical transition. A study of extra-tropical cyclones in the southern hemisphere shows that between the 30th and 78th parallels, there are an average of 37 cyclones in existence during any six-hour period. A separate study in the northern hemisphere suggests that approximately 234 significant extra-tropical cyclones form each winter.