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    A tropical cyclone is a low-pressure place in the atmosphere located near the equator. It can be recognized by its structure and origin.

    Tropical cyclones have an unquestionable impact on the lives of people living in the tropics. But why do they appear only in certain places and how are the tropical cyclones formed in the first place?

    photo: NASA’s Aqua satellite;desc: Tropical cyclone Winston;link: https://en.wikipedia.org/wiki/List_of_retired_South_Pacific_cyclone_names#/media/File:Winston_2016-02-20_0130Z_(cropped).jpg;licence: cc;

    How tropical cyclones are formed

    Tropical cyclones can be formed only in the tropics because they need a huge amount of energy, which can be present only in the tropics above an ocean when six conditions are satisfied:

    1. Temperature – the temperature of the ocean must be above 26 °C (79 °F). This condition leads to strong evaporation of the oceanic water and the water vapour is the most important source of energy for the hurricanes through its condensation.

    2. Distance from the equator – the hurricane cannot be created right on the equator. Hurricanes need some force to start it's spinning and that force is the Coriolis force. Coriolis force is zero on the equator and rises when moving towards the poles. That is why hurricanes can’t be present within 5 degrees latitude.

    3. Saturated lapse rate gradient near its centre – for the efficiency of using energy. When water vapour is present in the centre of the cyclone, it condensates and releases latent heat at a maximum rate due to a saturated lapse rate.

    4. High relative humidity – for evaporated water and avoiding trade wind inversion. Trade wind inversion means subsidence of air mass which is present mainly in anticyclones. That destroys cyclones. And evaporated water must be present, but with low humidity, the ocean evaporates more and its temperature decreases and can drop below the condition one temperature.

    5. Low vertical wind shear – a low difference of wind speed and direction vertically. If there is a strong wind in the upper atmosphere or in the opposite direction, the tropical cyclones won’t form in the upper levels and will lose energy quickly.

    6. Final condition is the presence of a tropical wave. It can be any disturbance in the atmosphere, for example, a thunderstorm that moves off the coast of Africa to the east.

    When all these conditions are met, a tropical cyclone is created.

    photo: NASA;desc: Hurricane Florence;link: https://www.flickr.com/photos/gsfc/42828604840;licence: cc;

    Moreover, we can distinguish between two types of cyclones - tropical and extratropical. A subtropical cyclone is something in between.

    Tropical cyclones are created a few degrees from the equator and are quite small on the horizontal scale. They are deeper, which means that there is a bigger gradient of pressure. They cannot have any frontal system and they move from east to west because they are pushed by the trade winds flowing close to the equator.

    In their centre, tropical cyclones are warm because they collect energy from condensing water droplets and the process of condensation means warms the surrounding air.

    Tropical cyclones are dangerous because of its huge energy hidden in water vapour that can release its potential by condensation and the heat can be transformed to lower pressure causing really strong winds capable of taking even cars and houses.

    When a tropical cyclone is fully developed, with its eye, eyewall, wind speed above 74 mph (119 km/h), it is classified depending mainly on the wind speed and its location as a hurricane, typhoon or simply cyclone.

    photo: David Hayden;desc: Tropical cyclone;link: http://www.restaurantlaughs.com/wp-content/uploads/2011/02/perfect-storm.jpg;licence: cc;

    Before we get to hurricanes and typhoons, we’ll examine the anatomy of the tropical cyclone. See you in the next chapter soon.

    Next chapters:

    • Four stages of Tropical Cyclone. From Tropical Disturbance to a Hurricane
    • What is the difference between tropical and extratropical cyclone
    • What’s the difference between hurricane, typhoon, cyclone

    Previous chapters:

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