Extratropical cyclones occur outside of the tropical zones in the mid-latitude regions and higher. The storms can occur in both the Northern Hemisphere and the Southern Hemisphere. The term 'extratropical cyclone' is often used to indicate any regional low pressure system. In the mid-latitudes, temperature differentials on either side of differing air masses can cause large low pressure systems to develop. One of the best and most famous examples of an extratropical cyclone was the October Nor'easter
commonly known as The Perfect Storm
The National Weather Service uses the following definition in their advisories and forecasts to indicate any time a storm develops as, or becomes, extratropical: The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes.
Source: NASA Visible Earth: Extratropical Cyclones in the Southern Ocean
Also Known As: Baroclinic storms, Mid-latitude cyclones
Alternate Spellings: Extra-tropical cyclones
Dual systems of low pressure can be seen in this image of extratropical cyclones outside of Iceland