What is cyclone?

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    What is a Cyclone?



    According to GA (2008) and BOM (1994), Tropical Cyclones (TC) are low pressure systems which develop in the tropics, in the southern hemisphere.  They are sufficiently intense to produce sustained gale force winds of at least 63 km/h on average that rotates in clockwise circulations.  The severe tropical cyclone is also known as a Hurricane or Typhoon (GA, 2008; BOM, 1994).  


    Australia experiences TC’s regularly around the Northern coastline region due to its proximity to the tropics than the remainder of the country.  As a result, many people are exposed to cyclones during the cyclone season between November and April, and a major precautionary system has to be working and reliable to ensure their safety.  For six months of the year, approximately 20,000 km of Australia’s coastline and 20 per cent of the population are vulnerable to the devastating winds, high seas and flood rains of a tropical cyclone (BOM, 1994; 2008).


    Furthermore, the events of global warming and climate change have increased the possibility of cyclones to occur more often at a larger scale (Zakey et al, 2008).  In addition to this, El Nino and La Nina events that changed the currents and sea temperature also increase the numbers of cyclones occurring along the Australian Coast (Ibid, 2008).


    This means that current technologies needs to be manages, maintained and utilised in order to protect the exposed population.  In order to achieve such an outcome, characteristics of a TC have to be understood (Ibid, 1994), including:

    1. Sustained wind speeds of more than 120 km/h surrounding the centre

    2. Very low atmospheric pressure system

    3. Increase sea level, storm surge and wave heights

    4. Eye diameters are 40 km on average

    5. Eye wall marks the strongest wind and heaviest rainfall

    6. Spiral rain band clouds that extend over 1000 km from the eye


    A cyclone is formed when a warm temperature of the sea reaches a threshold level and the wind structure is rising.  In other words, TC’s derive their energy from the warm tropical oceans and do not form unless the sea-surface temperature is above 26.5°C.  However, once formed they can persist at lower temperatures and dissipate over land or colder oceans (BOM, 1994).  GA (2008) describes it this way,

    1. “The development of a tropical cyclone also relies on favourable broad-scale wind regimes and can persist for several days with many following quite erratic paths. They lose their source of energy when they move over land or colder oceans causing them to dissipate. Weakening may occur also if the cyclone moves into an unfavourable wind regime which disrupts the structure of the system. Sometimes a decaying tropical cyclone may interact with a weather system in higher latitudes to cause impacts far from the tropics” (Ibid, 2008).


    The eye of the cyclone is the centre of the cyclone where the focus lies.  The areas surrounding the eye will be most affected because of the strong wind.  This is illustrated in Figure 3.1, where the cyclone is rising with a clockwise direction at the bottom and an anti-clockwise direction as it rises to the sky, forming a Tropical Cyclone.




    Figure 3.1 – Eye of a Cyclone

    (Geoscience Australia, 2008)



    In addition, GA (2008) described how the cyclone could affect the environment and the community through strong winds, heavy rainfall and storm surge.   Possibilities of other factors may also come into effect such as the heavy rain that will lead to flooding, rise of water tables and sea tides that also lead to flooding, damages to agriculture, farms and crops over a large area, fatalities and injuries to the surrounding population and economic impact on businesses, infrastructure and the surrounding properties.  In extreme cases, gusts in excessive circumstances can reach up to 360 km/h (Ibid, 2008).  For this reason, the BOM communicate weather warning systems through wind speed factors due to the potentially devastating impact.


    The cyclone aftermath includes heavy rainfall which causes direct damage to the community.  GA (2008) describes the heavy rainfall as “severe impact” that can cause flood and landslides,

    1. “Heavy rain from tropical cyclones or tropical depressions often can reach Australia’s more southerly latitudes where the rainfall is a major source of water for the country’s inland river systems. Flooding can wreak havoc over vast areas, inundating land, isolating communities and destroying infrastructure” (Ibid, 2008).


    There is also a possibility of a storm surge occurring during cyclone events. This effect is high regarding the damage it can cause to the community,

    1. “Potentially, the most dangerous hazard associated with tropical cyclones which make landfall in storm surge. The phenomenon has been responsible for more deaths than any other feature of cyclones. Storm surge is a raised dome of water about 60 to 80 kilometres across and typically about two to five metres higher than the normal tide level. It is caused by a combination of strong winds driving water onshore and the lower atmospheric pressure in a tropical cyclone. In the southern hemisphere the onshore winds occur to the left of the tropical cyclone’s path. In Australia, this is the east side on the north west and north coasts and the south side on the east coast” (GA, 2008).


    To understand further, there are four stages that form a cyclone which include:

    1. Formative Stage

    2. Immature Cyclone

    3. Mature Cyclone

    4. Decay stage


    The precautionary warning of cyclones is usually made during the formative stages.  Then, if necessary, an evacuation will take place during the immature stages.  The most dangerous stage is the mature progress, where the cyclone reaches the peak limit of its strength cause the most damage.  Finally, the cyclone will ease into the decay stage and dissipate.


    Cyclones are measured by a category classification, as illustrated in Table 2.3 (BOM, 2008).  Additionally, the cyclone category can vary from different cyclone stages.  Category 1 would be of minimal damage with wind gusts up to 125 km/h, and Category 5 would be of destructive and maximal damage with wind gusts exceeding 280km/h.  Table 2.3 also describes the characteristics and effects of each category with two estimations of the damages it may cause.  This estimation is especially important for an accurate prediction and appropriate precautions to be taken.  Naturally, as cyclone intensity increase between category 1 to 5, the risk of damage to shorelines, crops, property and life increase significantly.  For example, Cyclone Winifred is classified as Category 3, while Cyclone Larry and Cyclone Tracy are Category 4 cyclones.



    The Tropical Cyclone Severity is measured differently in the Southern and Northern hemisphere, as shown in Figure 3.2.  The Southern hemisphere usually follows the Australian Categories, while the Northern hemisphere follows the US Saffir Simpson Categories.  The wind gusts are derived from measuring the 10 minute mean wind and the 1 minute mean wind.  While the maximum gusts is recorded at the peak of the cyclones.  Cyclones at different seasons and time occur at various categories where the track of the cyclone resulted in a combination of categories as it changes between the stages. 




    Figure 3.2 – Tropical Cyclone Severity Categories

    (NOAA, 2008)


    As a result, the damage analysis from each of the different cyclones will correlate to each of the categories.  The prediction model will revolve around these categories with different extrapolation of wind gusts for each different category.

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