Meteorologists typically assign large low pressure systems to one of three types.
1. Tropical Cyclones
2. Extratropical Cyclones
3. Subtropical Cyclones
A tropical cyclone is a warm-core, non-frontal low pressure system of synoptic scale that develops over tropical or subtropical oceans. Tropical cyclones tend to have a more circular shape with spiral bands of showers and thunderstorms rotating around the center of circulation. More intense tropical cyclones may develop a clear area at the center known as an eye. Rising motion in the thunderstorms near the center may create a cirrus canopy of diverging air at the top of the circulation. Tropical cyclones may be called tropical depressions, tropical storms, hurricanes or typhoons depending on their intensity and location.
An extratropical cyclone is a cold-core synoptic scale low pressure system whose primary source of energy is baroclinic (the difference between warm and cold air). It will often be associated with fronts. The development of an extratropical cyclone is often linked to a trough in the middle and upper troposphere. Extratropical cyclones are the primary large scale storm systems in the middle and higher latitudes. They may also be called mid-latitude cyclones.
A subtropical cyclone is a low pressure system that develops over subtropical oceans and initially has a non-tropical circulation structure, but it exhibits some elements of the cloud patterns associated with tropical cyclones. It is often considered a hybrid system with some characteristics of tropical cyclones and some characteristics of extratropical cyclones.
It is important to remember that these types are just names meteorologists use to characterize low pressure systems. A low pressure system may change structure as it moves from one environment to another and meteorologists may change its type. For example, often when a tropical cyclone moves to a higher latitude and a colder environment it makes a transition to an extratropical cyclone.