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ATMOSPHERIC CIRCULATION AND WEATHER SYSTEMS 83 which is not present in the tropical cyclones. A schematic representation of the vertical They cover a larger area and can originate over structure of a mature tropical cyclonic storm thelandandsea.Whereasthetropicalcyclones is shown in Figure 9.10. originate only over the seas and on reaching A mature tropical cyclone is characterised by the strong spirally circulating wind around the land they dissipate. The extra tropical cyclone affects a much larger area as the centre, called the eye. The diameter of the compared to the tropical cyclone. The wind circulating system can vary between 150 and 250 km. velocity in a tropical cyclone is much higher and it is more destructive. The extra tropical The eye is a region of calm with subsiding cyclones move from west to east but tropical air. Around the eye is the eye wall, where there cyclones, move from east to west. is a strong spiralling ascent of air to greater height reaching the tropopause. The wind Tropical Cyclones reaches maximum velocity in this region, reaching as high as 250 km per hour. Tropical cyclones are violent storms that Torrential rain occurs here. From the eye wall originate over oceans in tropical areas and rain bands may radiate and trains of cumulus move over to the coastal areas bringing about and cumulonimbus clouds may drift into the large scale destruction caused by violent outer region. The diameter of the storm over winds, very heavy rainfall and storm surges. the Bay of Bengal, Arabian sea and Indian This is one of the most devastating natural ocean is between 600 - 1200 km. The system calamities. They are known as Cyclones in the moves slowly about 300 - 500 km per day. Indian Ocean, Hurricanes in the Atlantic, The cyclone creates storm surges and they Typhoons in the Western Pacific and South inundate the coastal low lands. The storm peters out on the land. China Sea, and Willy-willies in the Western Australia. Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourablefortheformationandintensification of tropical storms are: (i) Large sea surface with temperature higher than 27° C; (ii) Presence of the Coriolis force; (iii) Small variations in the vertical wind speed; (iv) A pre-existing weak- low-pressure area or low-level-cyclonic circulation; (v) Upper divergence above the sea level system. Theenergythatintensifiesthestorm,comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm. With continuous supply of moisture from the sea, the storm is further strengthened. On reaching the land the moisture supply is cut off and the storm dissipates. The place where a tropical cyclone crosses the coast is called the landfall of the o cyclone. The cyclones, which cross 20 N latitude generally, recurve and they are more Figure 9.10 : Vertical section of the tropical cyclone destructive. (after Rama Sastry) Reprint 2026-27