Hurricane Michael's Cyclonic Eye, photographed by NASA astronaut Serena Auñón-Chancellor

When in Unknown Territory, Knowledge is Everything.

(By NASA, Cyclone-Image https://www.nasa.gov/image-feature/tropical-cyclone-chapala-over-the-gulf-of-aden)

Knowledge is everything. Travellers are acutely vulnerable during periods of extreme weather as everything about their travel destination is foreign to them, including likely the impact of weather on their specific locality, the robustness of the buildings and infrastructure surrounding them, the readiness of civil defence in the area (if it exists at all), and the capability of national, and local administrations to cope with extraordinary events. Consequently, travellers should make themselves more aware of the occurrence of and context around such circumstances, and to become more informed about requisite conditions for the formation of, timeline, and repercussions of extreme weather.

Understanding natural phenomena, ensuring that pre-trip planning takes into account risk assessment, and implementing adequate measures to minimise foreseeable impacts are critical elements to successful travel. This is particularly true when travelling to developing countries, where authentic travel experiences are increasingly being sought.

This article looks at understanding natural phenomena, in particular, the formation and course of the life of thunderstorms and tropical cyclones.

Article Contents.

Natural Disasters, a Low Concern for Travellers? : Think Again ......

Untitled NASA - Recorded Density of Lightning Stikes for Period 1995-1998
(By NASA, Earth Data Optical Transient Detector (OTD) Data Summaries https://ghrc.nsstc.nasa.gov/lightning/data/data_otd_summaries.html)

While it may seem extraneous for travellers to worry about natural disasters, taking in only 2017 and 2018, the ramifications to travellers of extreme events prove otherwise ; recent fires in California, and Australia; Earthquake and Tsunami in Sulawesi, Indonesia; Volcanic eruptions in Bali and Iceland; Super Typhoons Mangkhut in the Pacific, Philippines and Southern China; Super Typhoon Yutu in Saipan in the Pacific; Typhoons Trami, Jebi, Talim, and Lan in Japan; Typhoon’s Tembin, Nock-ten in the Philippines; Typhoon’s Hato, Haitang, and Khanun in Southern China, Typhoon Doksuri in Vietnam; Flooding and Mudslides in South-Western Japan, Colombia and Sierra Leone; Massive Flooding in Bangladesh; Hurricane Florence along the US East Coast and Harvey in Texas; Hurricane Maria in the Dominican Republic and the massive Category 5 Hurricane Irma which caused massive destruction within the Caribbean and Southern US; Cyclone Josie in Fiji, a Bomb Cyclone in New England, USA.

While there is a diverse catalogue of natural disasters, a significant proportion are related to stormy weather with Tropical Cyclones (Cyclones, Hurricanes, and Typhoons) being dominant. Thunderstorms are ever present within Tropical Cyclones, have a number of other manifestations, and can be associated with massive flooding, mudslides, and lightning strikes. Collectively, stormy weather makes up the overwhelming majority of extreme events that affect travellers, for which some forewarning and planning can take place. Lightning strikes are an excellent indicator of location and density of thunderstorms across the globe. A perusal of the NASA lightning strike data for 1995-98 shows that particular regions sustain high densities of lightning strikes and by inference thunderstorms. Equatorial and Northern Africa, Indochina, Equatorial America, along with various coastal and continental North America are all subjected to a high number of thunderstorms.

UntitledFigure 2, Economic Losses,Poverty & Disasters 1998-2017 prepared Jointly Through CRED + UNISDR (https://www.emdat.be/)

UntitledFigure 3, Economic Losses,Poverty & Disasters 1998-2017 prepared Jointly Through CRED + UNISDR (https://www.emdat.be/)

UntitledFigure 14, Economic Losses,Poverty & Disasters 1998-2017 prepared Jointly Through CRED + UNISDR (https://www.emdat.be/)

More importantly for travellers, data from combine CRED and UNISDR research indicates that for the 20 year period between 1998-2017, the overwhelming majority of disasters were climate related, with roughly 350 disasters happening annually, equating to one disaster every day of the year somewhere in the world. Drilling down into these figures shows that 43% are due to flooding and 28% are due to storms, making up over 70% of all recorded disasters. It is clear that these two occurrences are caused by physical phenomena associated with wet weather, and are distinguished by a gradual and comparatively predictable process that can be recognised by travellers who have prepared sufficiently.

A number of critical aspects for travellers are highlighted within the CRED / UNISDR data; firstly that Asian countries dominate almost all the relevant statistics, and as a consequence travel to Asian countries must be carried out in a circumspect manner; secondly, the fact that 53% of all deaths, and 86% of all people affected due to climate-related events occur in Asia signifies that the region struggles with providing robust infrastructure systems that are resilient to the scale of the occurrences. Inadequacies in building construction, tenacious road and telecommunication networks, along with limited emergency and civil defence facilities point to the likelihood that travellers caught in severe natural events can become marooned, being isolated, unable to communicate sufficiently with both locals and the outside world, and perhaps, more importantly, lacking shelter, food and water.

While the report highlights Asia as a region, it also specifically notes that taking global numbers overshadows important and critical local factors. Perhaps the most notable is that the small gross totals associated with the Pacific disguise the catastrophic damage inflicted by tropical cyclones on small island states in the Pacific. The Pacific simply does not figure simply due to the fact that Pacific populations are not large by global standards, but the effect on their infrastructure and economies is dramatic. A similar effect occurs in the small island states of the Caribbean.

A comparison can be made to more developed nations, with the Americas, and the US in particular being prominant. The US is subjected to some of the worst recorded hurricanes and the America's has recorded over half the global economic losses over the past 20 years. However, despite this only 15% of deaths and 6% of affected populations are recorded, indicating that both infrastructure and civil defence services are more resilient, and able to cope with natural disasters.

What Countries are most Likely to experience Tropical Cyclones.

UntitledAfter NHC-NOAA:Tropical Cyclone Formation regions with Mean Tracks

Countries that have Experienced the Most Tropical Cyclones Since 1970.






5.United States






After HRC-NOAA : FAQ Which Countries Have Had The Most tropical Cyclones Hits?

Generally, it is recognised that the Western Pacific generates the most cyclonic activity, both in terms of frequency and in mean intensity. It is calculated that between 1981 to 2010 the Pacific has has 25.6 named tropical storms per year compared to 12.0 in the Atlantic. Further, between 2000 and 2014 there were 41 super typhoons, the largest magnitude tropical cyclone in the western Pacific, compared to 12 equivalent magnitude Hurricanes in the Atlantic. Numerous factors contribute to this :

(a) Greater Ocean Surface Area.
The Western Pacific possesses over twice the surface area of tropical and sub-tropical ocean as the Atlantic, enabling a much greater opportunity for conditions required for tropical cyclone formation to occur.
(b) Geographical Obstacles.
There are geographical obstacles in the Atlantic, mountainous islands such as Cuba inhibit the free formation of Cyclones, and the Gulf of Mexico can divert cyclonic activity away from land. Conversely, in the Pacific, the ratio of land to ocean surface area is considerably less.
(c) Waters Stay Warmer for Longer.
Waters in the Pacific generally stay warmer for longer than the Atlantic, and while it is a gross generalisation, cyclones can occur over the shoulder seasons in the Pacific, but less likely so in the Atlantic.
(d) Impact of Dry Air over Atlantic.
Cyclones depend for their formation on moist air, which is plentiful and relatively uninhibited in the Pacific, while the geographically significant Sahara Desert on the African Continent impacts climate leading into the North Atlantic Basin.

Rain-Cloud and ThunderStorm 101 - Basics Explained.

UntitledTypical Thunderstorm Formation
taken after Depatment of Earth Sciences, San Francisco State University (Modified from http://tornado.sfsu.edu/)

UntitledCumulus Rain-cloud Formation.

UntitledCumulonimbus Cloud Formation.

With increasing temperature, air will expand and become less dense, and when located over ocean regions the higher temperature air will carry with it an increased amount of water vapour that has evaporated off the sea. ( A basic principle of physics surrounds the Gas laws, in this case Charles Law whereby the volume of a gas is directly proportional to the temperature of the gas).

Being less dense than colder air higher up in the atmosphere, the warm air will rise and the colder air will effectively sink. As the warm air rises it cools and the water vapour contained within it condenses into cloud formations. Flat bottomed cumulus “puffy” clouds that are often seen as a consequence reflect sunlight and effectively shield the earth's surface, thereby producing a cooling effect.

Thunderstorms occur when warm moist air rises quickly upwards, condensing on reaching the cooler air in the higher reaches of the atmosphere, forming dense tower like cloud formations with a large thunderhead at the top of the tower formation. These Cumulonimbus clouds can form alone or alternatively they can be grouped in clusters or along a squall line. Thunder and lightning occur when different cloud regions possess vastly diverse electrical charge characteristics.

Formation of Thunderstorms.

UntitledCumulonimbus Cloud Formations - Australia
taken after Thennicke (Modified Wikimedia CC BY-SA 3.0)
UntitledSingle Cell Thunderstorm
taken after Department of Earth Sciences, San Francisco State University (Modified from http://tornado.sfsu.edu/)
UntitledMature Thunderstorm
taken after National Weather Service (Modified from https://www.weather.gov/jetstream/life)
UntitledSingle Cell Thunderstorm with vertical Shear
taken after Department of Earth Sciences, San Francisco State University (Modified from http://tornado.sfsu.edu/)
UntitledMulticell Thunderstorm
taken after Department of Earth Sciences, San Francisco State University (Modified from http://tornado.sfsu.edu/)

UntitledMature Multicell Thunderstorm
taken after Department of Earth Sciences, San Francisco State University (Modified from http://tornado.sfsu.edu/)
UntitledDissippating Thunderstorm
taken after National Weather Service (Modified from https://www.weather.gov/jetstream/life)

Generally thunderstorms progress through a life cycle involving three stages; The developing Cumulus Stage, the fully formed Mature Stage, and finally the Dissipating Stage where the formation downgrades and disintegrates. The creation of thunderstorm formations requires both atmospheric instability, and lift (which releases the instability). Instability occurs when a warmer body of air rises up through is the surrounding environment of cooler mass.

The Cumulus Stage.

Given favourable conditions, when sufficient warm moist air rises at a certain height, puffy cumulus clouds form due to the condensing water droplets. The droplets are initially minuscule, and are easily suspended within the cloud. The condensation of moisture into droplets releases latent heat which adds to the warming process within the cloud adding to its growth and allows it to be buoyant to great heights within the atmosphere. The cumulus cloud will continue to grow as long as warm air from below continues to rise. Warm air will lift in a variety of ways :

(a) Air near the earth’s surface heats due to the heat from the sun heating the surface, causing a temperature gradient with the air above.
(b) Air is forced up over higher ground such as mountainous regions.
(c) At a weather front, where masses of warm and cold air meet, the lighter warm air is forced up over the cooler air.

The Mature Stage - Establishment of Updraft and Downdraft.

An important characteristic of thunderclouds is the advent movement of the warmer and cooler air within it. A thunderstorm can consist of a collective of cells, all in various stages of development. The movement of the warmer air upwards is termed the “updraft”, while the corresponding movement downwards of the cooler air is the “downdraft”. Within a mature thundercloud, the updraft and downdraft occur in different sections of the cloud, depending on the stage of its development. Within a group of thunderstorm cells, each cell may be in a state of updraft (due to warm air rising), or downdraft (due to cooler air and rain sinking)

Establishment of a Downdraft
As the cumulus cloud grows, the water content in some cells densifies within the cloud mass and the individual size of water droplets increases. As a consequence, the colour of the cloud darkens to a heavy grey and the cloud is no longer able to support the weight of the water. Raindrops fall through the cloud when the rising warm air is no longer able to support them, and impart a frictional drag on the cooler surrounding air. Once a certain threshold is reached, a downdraft becomes established and the downdraft can become firmly entrenched. The frictional drag cause a partial evaporation of some moisture, and the loss of energy in the main rain mass causes it to cool further increasing the downdraft effect.

When the cooler downdraft hits the surface it spreads out as a density mass from its landing location, which drives surrounding warmer air up into surrounding cells in updraft conditions. This phenomenon of “downbursts’ can produce damaging winds at or near ground level, and the gust front can appear in much the same fashion as desert thunderstorms.

Effect of Wind Shear.
Wind shear, or the change in wind velocity with height has a major influence on the severity of thunderstorms, and can either enhance or diminish the severity of developing storms. Three effects are noted :

Extremely High Wind Shear :
In this case extreme wind at the head of the thunderstorm blows the storm off its base, effectively destroying it. The thunderstorm takes on an extremely sloped formation.

Optimum Wind Shear :
There is an optimum level of Wind Shear that removes the precipitation away from the head of the store facilitating the processes involved in transitioning warm moisture into future rain. The thunderstorm takes on a tilted formation, that facilitates multi-cell creation.

Weak Wind Shear :
Where visually no tilting of thunderstorms occur, virtually no wind shear occurs. Such storms have short lifespans since the precipitation in the downdraft quickly suffocates the updraft, terminating the thunderstorm mechanism. Any severity occurs just prior to dissipation.

Single-Cell and Multi-Cell Thunderstorms
While the life of a Single Cell thunderstorm can last an hour or more, Multi-Cell storms can last much longer, over several hours and consists of the effect of an aggregate of cells. To an observer, the cloud mass appears to be a single storm, where the makeup of cells within the storm is changed constantly by the cloud spreading out into the surrounding boundary, creating new cells. A developing cell will be completely experiencing updraft, a mature cell having distinct regions of updraft and downdraft within it, and Dissipating cells completely within downdraft.

Dissipation Stage.

Thunderstorms experience constantly changing states of updraft and downdraft conditions, and when the later become dominant the storm begins to weaken. As the updraft regions die away, warm moist air cannot rise to the same extent and the driving engine behind the storm diminishes. This is ultimately reflected in the downdraft and precipitation eases to light rain, the cloud disappears at the base of the cloud formation, with its dissipation gradually making its way to the top of the cloud. For typical thunderstorms, the process from creation through its final acts takes about an hour.

Precipitation can take the form of either rain or hail, depending on the latitude of the storm. In East Asia, severe hail has been noted from Thailand through to Beijing, although rare lower than Hanoi, Vietnam.

Observations of Thunderstorms.

UntitledThunderstorm Updraft / Downdraft Formations 1
taken after Department of Atmospheric Sciences,
University of Illinois at Urbana Champaign (from http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/comp/up/home.rxml)

UntitledThunderstorm Updraft / Downdraft Formations 2
taken after Department of Atmospheric Sciences,
University of Illinois at Urbana Champaign (from http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/comp/up/home.rxml)

Theorising the nature of any particular storm, consideration of the relative strength of updrafts and downdrafts provides some elucidation of storm manifestation :

Weak Updraft - Weak Downdraft.
Typically, a weak updraft will lead to low level cloud formation due to the lack of any high level glaciation and wind shear effects, storm manifestation will occur through non-severe actions and ordinary rain occurrences.

Strong Updraft - Weak Downdraft.
Usually occurring when drier conditions are prevalent, such formations result from unstable air that is able to provide a limited volume of moisture, but as a consequence has strong downdraft due to high evaporation of precipitation into the dry air. They are particular week, providing limited microbursts, generally occurring over land.

Strong Updraft - Strong Downdraft.
Formations possessing both strong updraft and downdraft provide the ultimate mechanisms of destructive storms. These can produce destructive downburst, hail, heavy rain, and tornadoes and are considered the severest of storms.

Manifestation of Lightning

UntitledCloud to Ground Lightning
(from https://www.nasa.gov/centers/marshall/news/news/releases/2016/earths-new-lightning-capital-revealed.html)

UntitledThunderstorm Updraft / Downdraft Formations 1
Cloud to Cloud (from https://www.nssl.noaa.gov/education/svrwx101/lightning/types/)

As the air masses move past each other, differentiated areas of charge build up and some regions become negatively charged while others become positively charged. Initially, air acts as an insulator between differently charged sections of air masses and charge potential continuously build up, eventually breaking the insulating action of the air. Lightning discharge can happen between different sections of the cloud mass, or between the cloud and the ground effectively neutralising regional electrical charge in the thunderstorm.

A number of theories exist in regard to the differentiated charges. Precipitation theory advances charge transfer as occurring through collision and interaction of the variously sized raindrops, with the larger heavier particles carrying negative charge lower. Conversely, convection theory explains the effect of updrafts carrying more positively charged air upwards, and downdrafts drag negatively charged air downwards.

It is of intense interest to understand circumstances where lightning strikes occur more often, and three factors are believed to increase the presence of lightning :

High Instability.
Instability occurs where the ambient atmospheric temperature decreases rapidly with height. In such cases the heat release in updrafts will enable high intense thunderstorm columns to develop, with the top sections becoming supercooled or “glaciated” forming the anvil-like structure at the storm’s top. The glaciation process enhances charge differential, and the magnitude of the charge difference is dependent on the speed at which glaciation develops, the depth of the ice layer amongst other factors.

High Moisture Content.
High levels of moisture content in the lower atmosphere will fuel updrafts, and providing greater volumes of water for the glaciation process.

Wind Shear.
Wind shear is manifested when horizontal wind speed changes significantly with height, which separates and displaces updraft and downdraft sections more widely apart. This enables thunderstorms to last longer, often in the form of multi-celled storms, with the greater lifespan enabling greater charge differentials to develop.

Cyclones, Hurricanes and Typhoons.

UntitledCyclone Particia, the Second Largest Tropical Cyclone on Record.

Physically cyclones, hurricanes and typhoons are the same thing, a rotating mass of atmospheric air, clouds that possess tremendous amounts of energy manifested by extraordinary winds that rotated around a central eye. In this article they are collectively all called Tropical Cyclones, with their names being associated with the region in which they occur rather than any displayed physical differences. They occur in tropical and sub-tropical regions either side of the equator, generally between 5˚ and 30˚ associated with the south-easterly trade winds and are created from the interaction of warm sea temperatures with atmospheric winds.

Hurricanes prevail either side of the North American continent, Typhoons in western Asia in the Northern Hemisphere, while all occurrences in the Southern Hemisphere are Cyclones. After forming, the movement of tropical cyclones can be extremely erratic, exhibiting unstable wobbly tracks and have a have been known to stop completely and reverse their direction of of travel. Typically however, they proceed in a westerly direction and ultimately move towards the polar regions dissipating along the way.

Formation of Tropical Cyclones.

UntitledCross Section of a Tropical Cyclone.

It is difficult to separate the formation of tropical cyclones from a description of their structure as they are intrinsically connected, and a discussion of one invariably incorporates major aspects of the later. As a result, any discourse on the subject is faced with a chicken and egg situation … which to begin with.

Most literature on the topic of tropical cyclones will invariably attempt to simplify what is a complex mechanism dependent on a number of atmospheric characteristics including, at an embryonic stage, the organisation of many nearby thunderstorms into an organised formation. As with thunderstorms, tropical cyclones have a requisite set of criteria before they are able to form, and similarly, they have a lifecycle which is much extended to that of any thunderstorms. Provided no destructive wind shear occurs, cyclones typically last between three and ten days. Dominant elements include the relatively calm central Eye, the destructive Eyewall region that is located adjacent, and the much larger Rainband region that is often dismissed in cyclonic discourse.

Both thunderstorms and tropical cyclones require moisture, energy and certain wind conditions to develop. They are related insomuch as tropical cyclones occur when many thunderstorms organise into a larger system and begin flowing in a circular pattern around a low-pressure centre (Nasa-earth observatory). The life-cycle of severe cycles ( category 3 and higher cyclones) generally proceed through four distinct stages; Formative, Immature, Mature, and Decay Stages (BOM).

The fact that tropical cyclones require fairly specific conditions to develop, leads to the general category of “convective disturbances” possessing a wide range of attributes regarding physical size, wind strength, movement and lifecycle manifestation. Classifications in order of increasing severity include; Tropical Disturbances; Tropical Storms; Non Severe Tropical Cyclones (Cat 1 + cat 2); and Severe Tropical Cyclones (Cat 3 - Cat 5). The progression of a convective disturbance into a tropical cyclone is not spontaneous, and not assured by any means. As a generalisation, only 10% of Tropical Disturbances develop into Tropical Storms, but once attaining this strength, 70% of Tropical Storms develop further into Tropical Cyclones (Severe and Non-Severe)(Hawaii). Non severe cyclones fail to develop fully due to unfavourable conditions such as; moving over cooler water that decreases the amount of contributing energy; moving over land that removes moisture from the system; or being subjected to differential wind shear that erodes the cyclones formation structure.

Prerequisite Conditions for a Tropical Cyclone to Occur.

UntitledNASA Photograph of Tropical Cyclone Funso
(from https://earthobservatory.nasa.gov/images/77029/tropical-cyclone-funso)

UntitledLife-Cycle of Cyclone Kona Low, which shows Incipent, Intensifying, Mature, Weaking and Decaying Stages - not the formation of the low level vorticity from the shape of isobar formation. The source of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2017 University Corporation for Atmospheric Research. All Rights Reserved.

A number of climatic conditions are required to be in existence for the development of a Tropical Cyclone to be possible :

Loose Set of Existing Thunderstorms.
The existence of a loosely organised set of atmospheric disturbances (convection disturbances such as thunderstorms), that collectively have a certain degree of spin (vorticity) around an approximate centre or vortex. The nature of the loose arrangement is such that there is a convergence of airflow, around the centre. The aggregate of disturbances acts to ‘seed’ the process of cyclonic formation, and without it no formation is possible. It is imperative that the system has been obtained momentum from a low-level spin phenomenon which can be further progressed. For most cyclonic basins the primary source of vorticity is through the distortion of inflow as a consequence of monsoon troughs.

A Energy Source in the Form of Warm Ocean Mass.
Cyclones require energy to maintain a stable condition which is supplied by warm ocean waters. The threshold ocean temperature is 26.5 degrees Celsius which must remain constant through a sufficient depth of water, generally in the order of around 50 metres. Clearly, higher sea temperatures encourage evaporation and increase moist air levels at surface level. However, critically at temperatures lower than 26.5 degrees Celsius, the atmosphere is able to maintain relative stability, and inhibit warm moist air from attaining positive buoyancy.

Atmospheric Conditions with a Steep Thermal Gradient.
Atmospheric conditions must be prevalent that encourage thunderstorm development. A steep thermal gradient allows warm moist air to rise through its cooler ambient surroundings to the towering heights within the full lower atmosphere (troposphere) required for cyclonic development.

A Moist Mid-Atmospheric Region.
The mid regions of the lower atmosphere (troposphere) must possess sufficient humidity to encourage the further use of warm air upwards (positive buoyancy). The infiltration of dry air into cloud formations (dry entrainment) causes evaporation within the suspended water vapour mass, leading to cooling and the commencement of downdraft within cells. Such action stymies thunderstorm cell updrafts thereby inhibiting ongoing cyclonic development. While there is some resistance within established cyclones, ultimately dry air will wrap into the mechanism weakening the formation as evinced by Hurricane Isabel in 2003 which downgraded from Cat 5 to Cat 2 over a period of two days.

Presence in a Latitude where a Coriolis Force Exists.
Generally, cyclone formation cannot be maintained without the application of external momentum to maintain the low-level vorticity of the cyclone formation. Because the earth is a rating frame of reference, moving objects are subjected to a Coriolis Force and take on a curved trajectory despite having apparent movement in a straight line. The Coriolis force is zero at the equator and only takes on sufficient magnitude to sustain cyclone vorticity at a latitude of 5 degrees (roughly 500km), increasing in magnitude with increasing distance from the equator. Provided the embryonic cyclone is higher than 5 degrees latitude, and the initial spin is in the same orientation as the Coriolis Force, the vorticity can be maintained.

Weak Vertical Wind Shear.
As with thunderstorm development, strong wind differential with height will impact cyclone structure. Cyclone mechanisms are dependent on strong thunderstorm cells surrounding its eye to enhance its convective action and strong vertical wind shear will blow the high level sections of thunderstorm cells away from its lower level circulation. Effectively, the component parts are separated and will cease to operate as a connected assemblage. Even with moderate wind shear, the reduced effectiveness of thunderstorms will allow an increase e in pressure near the eye, decreasing pressure gradients across the cyclone itself, and the cyclonic winds as a direct consequence.

Tropical Cyclone Lifecycle.

UntitledCharacteristics of Formative Stage of Cyclone Life-Cycle.
(from "The source of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2017 University Corporation for Atmospheric Research. All Rights Reserved."
UntitledBureau of Meteorology : Formative, Immature, Mature, and Decay Stages of Tropical Cyclone Paul Apr 2000 .
UntitledEstablishment of Steady State Tropical Cyclone
"The source of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2017 University Corporation for Atmospheric Research. All Rights Reserved."

[1] Formative Stage (Incipient).
The embryonic stage of cyclone formation occurs when a relatively localised area is subjected to an unusual amount of atmospheric convection, manifested through many fairly closely positioned thunderstorms. The thunderstorms are not generally not organised in any fashion, appear in a haphazard configuration and may not have a discernible focus.

Maximum winds are commonly away from the systems centre, generally less than gale force, and may only be of notable in a single quadrant. Where such winds make landfall they are rarely devastating, but may include short intense squalls and can be associated with heavy rain and flooding.


[2] The Immature Stage (Intensifying).
The arrangement of thunderstorms and atmospheric disturbances becomes more organised around the system's core. As a result more efficient system-wide convection mechanisms leading to an intensification of physical actions. The minimum surface pressure drops markedly, creating a greater pressure gradient from the core outwards, and culminating in the development of gale force winds circling around the core in ever-tightening spirals. The Cyclonic Eye remains covered in cloud (usually high-level cirrus cloud) which may show distinct linear features associated with the outward flowing of air out of the top of the core.

Local damage can be extreme due to a very rapid escalation of the physical manifestation of the formation, although such damage is commonly limited to localised areas.


[3] The Mature Stage .
During the mature stage, the cyclonic system acquires a relatively steady state that allows it to intensify, with only minor swings in its configuration and physical characteristics. The steady-state provides the formation with a more symmetrical configuration, allows the development of maximum winds around the core which in turn develops into a distinct circular eye. The system generally remains at its maximum intensity around a day before weakening unless the elements feeding are sufficiently resourced not to be consumed by the cyclonic system.


[4] The Decay Stage ( Weakening + Dissipating).
The process of weakening begins with pressure gradually rising within the central core, decreasing the pressure gradient across the system. As a result, the tight bands of wind located at the Eye-Wall expands and moves away from the Eye. The organised convection at the centre dissipates and the upper-level cloud formations recede and wither away. Conversely, the lower level clouds associated with downdrafts may remain for some time in their spiral configuration producing heavy rain.

When a decaying formation passes over land rapid disintegration may occur as the elements feeding it are removed. However, the heavy rain associated with its last actions may lead to flooding.

Tropical Cyclone Structure.

UntitledCross-section of a tropical Cyclone .
(ABC, & Tim Madden. (2017)- Tropical Cyclones Explained from https://www.abc.net.au/news/2011-02-01/tropical-cyclones-explained/1926870)

(from "The source of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2017 University Corporation for Atmospheric Research. All Rights Reserved."

"The source of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2017 University Corporation for Atmospheric Research. All Rights Reserved."

The Eye and Eyewall of the Tropical Cyclone.
At the core of the tropical Cyclone is its eye which is typically 30-65 km in diameter, but can range to over 350km or more. The core generally possesses dry air, little to no wind, and is associated with brilliant sunshine and settled weather. Over land the eye presents a picture of ultimate calm, while over water chatotic wave motion can make this zone one of the most dangerous. Waves from all directions are created which slam into each other and can manufacture monster wave structures up to 40 metres in height.

The eye is created by convection effects created by the organised formations around it, creating low pressure at the bottom of the eye and the consequent inflow of air from the systems outer region inwards. At the eyewall the converging air spirals outwards further adding to the convection effects. The eyewall contains the tropical cyclones strongest winds and is associated with dense clouds, intense thunderstorms. In the Northern Hemisphere the winds rotate counter-clockwise around the central eye, while in the southern hemisphere they revolve in a clockwise direction.

At the top of the Eyewall most wind moves outward, and away from the eye. However, a small amount of air falls over the eyewall edge and down the core of a forming eye, but only enough to overcome the convection effects. This downward process clears out any cloud within the central area and creates the rain-free eye region.

Cyclonic Rainbands
Away from the Eye of the tropical cyclone, a series of rainbands occur that circulate around it bringing with them heavy rain, severe squalls, and tornados. The experience of travelling away from the eye produces a series of severe cyclone weather at the rainbands, with somewhat calmer and drier intervening periods between rainbands, with the severity of weather gradually easing with distance away from the eye. Rainbands located approximately 80 km or more away from the eye move away from the eye. Rainbands that are closest to the eyewall move closer becoming more intense effectively forming secondary or outer eyewalls. As the inner eyewall erodes it is replaced by the closest outer eyewall during which time the ferocity of the winds are lessened until the new eyewall is firmly established.

Categorisation of Tropical Cyclones -
Hurricane, Typhoon, and Cyclone Categories -
A Confusing Mishmash

Development of the science around Tropical Cyclones has been made difficult by a less than unified approach within the scientific community, that are essentially separated by the different Cyclonic basins concerned. Differences in Tropical Cyclone terminology ( Hurricanes, Typhoons, and Cyclones) is also supported by the magnitude categorisation associated with each. While the categorisation in each cyclonic basis differs, a descriptive commonality in each cyclonic region that a tropical cyclone forms when sustained winds exceed around 65km/hr or 35 knots. Subsequent to that, any co-relation is difficult as each region takes its own defining parameters to determine magnitude categories, which are related to the time period over which the wind measurements are taken. Given that wind speeds fluctuate widely over relatively short periods of time, measurement scales that appraise sustained winds over shorter time periods will generally result in higher wind speeds being reported. As a consequence measurement scales that use shorter measurement periods will incorporate higher wind velocity thresholds to define magnitude categories, and this can be misleading in regard to the destructiveness associated with each.

Briefly, Tropical Cyclones magnitudes are measured within the various cyclonic basins in the following ways :

Hurricanes are categorised by the Saffir-Simpson hurricane wind scale (SSHWS), which assess the estimated maximum sustained winds over a one minute period. A total of five categories are used to define the intensity.

In the western Pacific the ESCAP/WMO Typhoon committee uses the maximum sustained winds over a ten minute period, and utilise a total of four categories.

In the Southern Hemisphere from Bangladesh eastwards, the Australian Tropical intensity Scale is used which incorporates an assessment of maximum sustained winds over a ten minute period, and utilises five magnitude categories.

The India Meteorological department’s scale uses the estimated maximum sustained wind taken over a three minute period.


Comparision of Tropical Cyclone Categories across Cyclonic Basins.

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