Cyclones

In meteorology, cyclones are  large scale air masses that rotate around a strong center of low atmospheric pressure

Low_pressure_system_over_Iceland

An extratropical cyclone near Iceland on September 4, 2003. image: wikipedia/NASA

Cyclones are characterized by inward spiraling winds that rotate about a zone of low pressure. 

Direction of rotation of cyclones

Because of the Coriolis effect, the wind flow around large cyclones are counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

The largest cyclones are generally away from the tropics

 The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale (the synoptic scale meaning over 1000km). 

comparison of extra tropical cyclone and tropical cyclone. image: Japan Meteorological Agency/wikipedia

The more intense cyclones though are tropical due to heat

Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. 

Mesocyclones, tornadoes and dust devils lie within the smaller mesoscale. 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. 

Origin of the word cyclone

It was Henry Piddington who published 40 papers dealing with tropical storms from Calcutta between 1836 and 1855 in The Journal of the Asiatic Society.coined the term cyclone, meaning the coil of a snake. In 1842, he published his landmark thesis, Laws of the Storms. 


 Cyclogenesis

This describes the process of cyclone formation and intensification. 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 (overtaken by another front) 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.


Tropical cyclogenesis 

This describes the process of development of tropical cyclones. Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core. 

Cyclones can transition between extratropical, subtropical, and tropical phases. Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. 

Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.

Diagram of the structure of a tropical cyclone. image: wikipedia/Kelvinsong 

 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. 

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


Cyclones have also been seen on extraterrestrial planets, such as Mars and Neptune. 

Source adapted from: Wikipedia contributors. (2018, September 14). Cyclone. In Wikipedia, The Free Encyclopedia. Retrieved 05:09, September 15, 2018, from https://en.wikipedia.org/w/index.php?title=Cyclone&oldid=859459684


Commentaries

Cyclones are an important topic in natural disasters or described as a natural hazards.