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In meteorology, a cyclone (/ˈsaɪ.kloʊn/) is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anticyclone).[1][2] Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure.[3][4] The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale (the synoptic scale). Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale.[5] Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale.[6]
Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars, Jupiter, and Neptune.[7][8] Cyclogenesis is the process of cyclone formation and intensification.[9] Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.
Weather fronts mark the boundary between two masses of air of different temperature, humidity, and densities, and are associated with the most prominent meteorological phenomena. Strong cold fronts typically feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Such fronts form west of the circulation center and generally move from west to east; warm fronts form east of the cyclone center and are usually preceded by stratiform precipitation and fog. Warm fronts move poleward ahead of the cyclone path. Occluded fronts form late in the cyclone life cycle near the center of the cyclone and often wrap around the storm center.
Tropical cyclogenesis describes the process of development of tropical cyclones. Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core.[10][11] Cyclones can transition between extratropical, subtropical, and tropical phases.[12] Mesocyclones form as warm core cyclones over land, and can lead to tornado formation.[13] Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.[14] In the Atlantic and the northeastern Pacific oceans, a tropical cyclone is generally referred to as a hurricane (from the name of the ancient Central American deity of wind, Huracan), in the Indian and south Pacific oceans it is called a cyclone, and in the northwestern Pacific it is called a typhoon.[15] The growth of instability in the vortices is not universal. For example, the size, intensity, moist-convection, surface evaporation, the value of potential temperature at each potential height can affect the nonlinear evolution of a vortex.[16][17]
Nomenclature[edit]
Henry Piddington published 40 papers dealing with tropical storms from Calcutta between 1836 and 1855 in The Journal of the Asiatic Society. He also coined the term cyclone, meaning the coil of a snake. In 1842, he published his landmark thesis, Laws of the Storms.[18]
Structure[edit]
There are a number of structural characteristics common to all cyclones. A cyclone is a low-pressure area.[19] A cyclone's center (often known in a mature tropical cyclone as the eye), is the area of lowest atmospheric pressure in the region.[19] Near the center, the pressure gradient force (from the pressure in the center of the cyclone compared to the pressure outside the cyclone) and the force from the Coriolis effect must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure.[20]
Because of the Coriolis effect, the wind flow around a large cyclone is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.[21] In the Northern Hemisphere, the fastest winds relative to the surface of the Earth therefore occur on the eastern side of a northward-moving cyclone and on the northern side of a westward-moving one; the opposite occurs in the Southern Hemisphere.[22] In contrast to low-pressure systems, the wind flow around high-pressure systems are clockwise (anticyclonic) in the northern hemisphere, and counterclockwise in the southern hemisphere.
Formation[edit]
Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere.[9] Cyclogenesis is an umbrella term for several different processes that all result in the development of some sort of cyclone.[24] It can occur at various scales, from the microscale to the synoptic scale.
Extratropical cyclones begin as waves along weather fronts before occluding later in their life cycle as cold-core systems. However, some intense extratropical cyclones can become warm-core systems when a warm seclusion occurs.
Tropical cyclones form as a result of significant convective activity, and are warm core.[11] Mesocyclones form as warm core cyclones over land, and can lead to tornado formation.[13] Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.[14] Cyclolysis is the opposite of cyclogenesis, and is the high-pressure system equivalent, which deals with the formation of high-pressure areas—Anticyclogenesis.[25]
A surface low can form in a variety of ways. Topography can create a surface low. Mesoscale convective systems can spawn surface lows that are initially warm-core.[26] The disturbance can grow into a wave-like formation along the front and the low is positioned at the crest. Around the low, the flow becomes cyclonic. This rotational flow moves polar air towards the equator on the west side of the low, while warm air move towards the pole on the east side. A cold front appears on the west side, while a warm front forms on the east side. Usually, the cold front moves at a quicker pace than the warm front and "catches up" with it due to the slow erosion of higher density air mass out ahead of the cyclone. In addition, the higher density air mass sweeping in behind the cyclone strengthens the higher pressure, denser cold air mass. The cold front over takes the warm front, and reduces the length of the warm front.[27] At this point an occluded front forms where the warm air mass is pushed upwards into a trough of warm air aloft, which is also known as a trowal.[28]
Tropical cyclogenesis is the development and strengthening of a tropical cyclone.[29] The mechanisms by which tropical cyclogenesis occurs are distinctly different from those that produce mid-latitude cyclones. Tropical cyclogenesis, the development of a warm-core cyclone, begins with significant convection in a favorable atmospheric environment. There are six main requirements for tropical cyclogenesis:
- sufficiently warm sea surface temperatures,[30]
- atmospheric instability,
- high humidity in the lower to middle levels of the troposphere
- enough Coriolis force to develop a low-pressure center
- a preexisting low-level focus or disturbance
- low vertical wind shear.[31]
An average of 86 tropical cyclones of tropical storm intensity form annually worldwide,[32] with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones (at least Category 3 intensity on the Saffir–Simpson hurricane scale).[33]
Synoptic scale[edit]
The following types of cyclones are identifiable in synoptic charts.
Surface-based types[edit]
There are three main types of surface-based cyclones: Extratropical cyclones, Subtropical cyclones and Tropical cyclones
Extratropical cyclone[edit]
An extratropical cyclone is a synoptic scale low-pressure weather system that does not have tropical characteristics,[34] as it is connected with fronts and horizontal gradients (rather than vertical) in temperature and dew point otherwise known as "baroclinic zones".[35]
"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.[35][36] 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.[37] 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.[38] A particularly intense type of extratropical cyclone that strikes during winter is known colloquially as a nor'easter.
Polar low[edit]
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 [citation needed]. 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 −45 °C (−49 °F) move over open waters, deep convection forms, which allows polar low development to become possible.[39] The systems usually have a horizontal length scale of less than 1,000 kilometres (620 mi) 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 17 m/s.[40]
Subtropical[edit]
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.[41] 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.[42] By 1972, the National Hurricane Center officially recognized this cyclone category.[43] Subtropical cyclones began to receive names off the official tropical cyclone list in the Atlantic Basin in 2002.[41] They have broad wind patterns with maximum sustained winds located farther from the center than typical tropical cyclones, and exist in areas of weak to moderate temperature gradient.[41]
Since they form from extratropical cyclones, which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required is around 23 degrees Celsius (73 °F) for their formation, which is three degrees Celsius (5 °F) lower than for tropical cyclones.[44] 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.[45]
Tropical[edit]
A tropical cyclone is a storm system characterized by a low-pressure center and numerous thunderstorms that produce strong winds and flooding rain.