Cyclone

    A cyclone is a general term for a weather system in which winds rotate inwardly to an area of low atmospheric pressure. The rotation is usually counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth.

Size

    The largest low-pressure systems are cold core polar cyclones and extra tropical cyclones which lie on the synoptic scale (up to 5000km in diameter). 

    Warm-core cyclones such as tropical cyclones, mesocyclones, and polar lows lie within the smaller mesoscale (up to 500km in diameter). Subtropical cyclones are of intermediate size (upto 2000km in diameter).

Terminologies

    Hurricanes and typhoons are the same weather phenomenon: tropical cyclones.  A tropical cyclone is a generic term used by meteorologists to describe a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has closed, low-level circulation. 

    The weakest tropical cyclones are called tropical depressions. If a depression intensifies such that its maximum sustained winds reach 39 miles per hour, the tropical cyclone becomes a tropical storm.  Once a tropical cyclone reaches maximum sustained winds of 74 miles per hour or higher, it is then classified as a hurricane, typhoon, or tropical cyclone, depending upon where the storm originates in the world.  In the North Atlantic, central North Pacific, and eastern North Pacific, the term hurricane is used. The same type of disturbance in the Northwest Pacific is called a typhoon. Meanwhile, in the South Pacific and Indian Ocean, the generic term tropical cyclone is used, regardless of the strength of the wind associated with the weather system.

STRUCTURE OF CYCLONE

    

The main parts of a tropical cyclone are the rainbands, the eye, and the eyewall. The hurricane's center is a relatively calm, generally clear area of sinking air and less wind region is known as the eye. The strong wind gets as close as it can is the eyewall. Curved bands of clouds and thunderstorms that trail away from the eye wall in a spiral fashion. These bands are known as rainbands.

    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 Coriolis force must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure. The wind flow around a large cyclone is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere as a result of the Coriolis effect.(An anticyclone, on the other hand, rotates clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere.)

There are six main types of cyclones:

1. Polar cyclones, 
2. Polar lows, 
3. Extratropical cyclones
4. Subtropical cyclones, 
5. Tropical cyclones, and 
6. Mesocyclones

There are six main requirements for tropical cyclogenesis: 

• sufficiently warm sea surface temperatures,
• 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, and 
• low vertical wind shear. 

    An average of 86 tropical cyclones of tropical storm intensity form annually worldwide, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones (at least Category 3 intensity on the Saffir-Simpson Hurricane Scale).

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Reference:

1. Atmosphere, Ocean, and Climate Dynamics - An Introductory Text (John Marshall & R. Alan Plumb).

2. Climatology – second edition – (John E Oliver, John J Hidore)

3. Atmospheric Thermodynamics - John M. Wallace, Peter V. Hobbs.