indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm
frontal boundaries. The following types of cyclones are identifiable in synoptic charts.
Surface-based types There are three main types of surface-based cyclones:
extratropical cyclones,
subtropical cyclones and
tropical cyclones.
Extratropical cyclone An extratropical cyclone is a
synoptic scale low-pressure weather system that does not have
tropical characteristics, as it is connected with
fronts and horizontal
gradients (rather than vertical) in
temperature and
dew point otherwise known as "baroclinic zones". "
Extratropical" is applied to cyclones outside the tropics, in the middle latitudes. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" when a tropical cyclone has moved (
extratropical transition) beyond the tropics. They are often described as "depressions" or "lows" by weather forecasters and the general public. These are the everyday phenomena that, along with
anticyclones, drive weather over much of the Earth. Although extratropical cyclones are almost always classified as
baroclinic since they form along zones of temperature and dewpoint gradient within the
westerlies, they can sometimes become
barotropic late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform with radius. An extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone, if it dwells over warm waters sufficient to warm its core, and as a result develops central convection. A particularly intense type of extratropical cyclone that strikes during winter is known colloquially as a
nor'easter.
Polar low in December 2009 A polar low is a small-scale, short-lived atmospheric
low-pressure system (depression) that is found over the ocean areas poleward of the main
polar front in both the Northern and Southern Hemispheres. Polar lows were first identified on the meteorological satellite imagery that became available in the 1960s, which revealed many small-scale cloud vortices at high latitudes. The most active polar lows are found over certain ice-free maritime areas in or near the Arctic during the winter, such as the
Norwegian Sea,
Barents Sea,
Labrador Sea and
Gulf of Alaska. Polar lows dissipate rapidly when they make landfall. Antarctic systems tend to be weaker than their northern counterparts since the air-sea temperature differences around the continent are generally smaller. However, vigorous polar lows can be found over the Southern Ocean. During winter, when cold-core lows with temperatures in the mid-levels of the troposphere reach move over open waters, deep convection forms, which allows polar low development to become possible. The systems usually have a horizontal length scale of less than and exist for no more than a couple of days. They are part of the larger class of
mesoscale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. Polar lows have been referred to by many other terms, such as polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least .
Subtropical in the north Atlantic Ocean in January
2016 A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form between the equator and the 50th parallel. As early as the 1950s, meteorologists were unclear whether they should be characterized as tropical cyclones or extratropical cyclones, and used terms such as quasi-tropical and semi-tropical to describe the cyclone hybrids. By 1972, the
National Hurricane Center in the United States officially recognized this cyclone category. Subtropical cyclones began to receive names off the
official tropical cyclone list in the Atlantic Basin in 2002. This means that subtropical cyclones are more likely to form outside the traditional bounds of the hurricane season. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.
Tropical viewed from the
International Space Station A tropical cyclone is a
storm system characterized by a
low-pressure center and numerous
thunderstorms that produce strong winds and flooding rain. A tropical cyclone feeds on heat released when moist air rises, resulting in
condensation of
water vapour contained in the moist air. and their dependence on
Maritime Tropical air masses for their formation. The term "cyclone" refers to the storms' cyclonic nature, with
counterclockwise rotation in the
Northern Hemisphere and clockwise rotation in the
Southern Hemisphere. Their winds increase the wave size, and in so doing they draw more heat and moisture into their system, thereby increasing their strength. They develop over large bodies of warm water, and hence lose their strength if they move over land. This is the reason coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from strong winds. They also carry heat and energy away from the tropics and transport it toward
temperate latitudes,
Upper level types Polar cyclone A polar, sub-polar, or Arctic cyclone (also known as a
polar vortex) A polar cyclone is a low-pressure
weather system, usually spanning to , in which the air circulates in a counterclockwise direction in the northern hemisphere, and a clockwise direction in the southern hemisphere. The Coriolis acceleration acting on the air masses moving poleward at high altitude, causes a counterclockwise circulation at high altitude. The poleward movement of air originates from the air circulation of the
Polar cell. The polar low is not driven by convection as are tropical cyclones, nor the cold and warm air mass interactions as are extratropical cyclones, but is an artifact of the global air movement of the Polar cell. The base of the polar low is in the mid to upper troposphere. In the Northern Hemisphere, the polar cyclone has two centers on average. One center lies near Baffin Island and the other over northeast Siberia. In the southern hemisphere, it tends to be located near the edge of the
Ross ice shelf near 160 west longitude. When the polar vortex is strong, its effect can be felt at the surface as a westerly wind (toward the east). When the polar cyclone is weak, significant cold outbreaks occur.
TUTT cell Under specific circumstances, upper level cold lows can break off from the base of the tropical upper tropospheric trough (TUTT), which is located mid-ocean in the Northern Hemisphere during the summer months. These upper tropospheric cyclonic vortices, also known as TUTT cells or TUTT lows, usually move slowly from east-northeast to west-southwest, and their bases generally do not extend below in altitude. A weak inverted surface trough within the
trade wind is generally found underneath them, and they may also be associated with broad areas of high-level clouds. Downward development results in an increase of
cumulus clouds and the appearance of a surface vortex. In rare cases, they become warm-core
tropical cyclones. Upper cyclones and the upper troughs that trail tropical cyclones can cause additional outflow channels and aid in their intensification. Developing tropical disturbances can help create or deepen upper troughs or upper lows in their wake due to the outflow jet emanating from the developing tropical disturbance/cyclone. ==Non-synoptic types==