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Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth (outside the tropics) having neither tropical nor polar characteristics, and are connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".^[1] Extratropical cyclones are the everyday phenomena which, along with anticyclones, drive the weather over much of the Earth, producing anything from cloudiness and mild showers to heavy gales and thunderstorms.

Extratropical cyclones encompass a class of storms with many names. Although they are sometimes referred to as "cyclones", this is imprecise; cyclone applies to numerous types of low pressure areas. The descriptor extratropical signifies that this type of cyclone generally occurs outside the tropics in the middle latitudes of Earth. The term mid-latitude cyclones may be used because of where they form; "post-tropical cyclones" if extratropical transition has occurred.^[1][2] Weather forecasters and the general public often describe them as "depressions" or "lows". Terms like frontal cyclone, frontal depression, frontal low, extratropical low, non-tropical low are often used as well.

Extratropical cyclones are classified mainly as baroclinic, because they form along zones of temperature and dewpoint gradient known as frontal zones. They can become barotropic late in their life cycle, when the distribution of heat around the cyclone becomes fairly uniform with its radius.^[3]

Extratropical cyclones form anywhere within the extratropical regions of the Earth (usually between 30° and 60° latitude from the equator), either through cyclogenesis or extratropical transition. A study of extratropical cyclones in the Southern Hemisphere shows that between the 30th and 70th parallels, there are an average of 37 cyclones in existence during any 6-hour period.^[4] A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter.^[5]

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