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Cyclones and hurricanes


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Cyclones and hurricanes

  1. 1. Cyclones and Hurricanes Cyclones refer to a system of winds that rotate inwards towards an area of low pressure. In temperate latitude, such a weather pattern is called a depression and in tropical area, a hurricane. Cyclones are known by different names in different parts of the world but they are all essentially the same. They are called:  Hurricanes in the Caribbean  Cyclones off Africa  Typhoons in the Far East  Willy-willies in Australia Hurricanes are violent tropical storms with wind speed of at least 120 km/hour. Wind speed may rise as high as 250 km/hour and hurricanes are usually according to their speed from force 1-5. Certain conditions must be met for hurricanes to be formed. They normally occur July and October since they form over tropical oceans where temperature must meet at least 27˚c. Only in the warmer summer months in the northern hemisphere is this possible. Northern Australia is the exception where they are called willy- willies. Characteristics of Hurricanes In the centre of the hurricane, air pressure is very low. It is usually below 985mb (millibars), and sometimes as low as 860-890mb. There are strong updraughts over most of the weather system. Around the centre of the low pressure, there is a strong revolving wind system. In the northern hemisphere, they rotate in an anti- clockwise direction. Wind speeds are very high. Near the centre of the storm, they may reach 360km/hr (100m/sec).
  2. 2. There is a spiral pattern of enormous cumulonimbus clouds. Some of the clouds tower up to a height of 16km. Where the clouds are thickest, there is very little light even at midday. At a high level, there is a canopy of cirrus clouds. Above 9km, the winds spiral outwards from the centre of the hurricane. In some hurricanes, there is very heavy rain. During one hurricane in 1909, more than 2,400 mm of rain was recorded over a four day period at Silver Hill in Jamaica. Right at the centre of the storm there is an eye, 20-50 km across. In the eye, conditions are very different. There are light winds, blowing at perhaps 10-20 km/hr and there may be no rain falling. Around the eye there is a wall of dark clouds. Within the eye, there is a strong downwards current of air. Because of this, the air there is very warm, and there are no low- level clouds. Immediately around the eye is the vortex. In this area moist air is rapidly rising.
  3. 3. Underwater landslides and Tsunami Underwater/ undersea landslide is a form tsunami. Tsunami (also known as tidal waves), is a destructive wave caused by the disturbance on the ocean bed. These can be:  Under-sea earthquakes  Under-sea landslides  Under-sea volcanic eruptions  Under-sea hurricanes and  Other large storms at sea Whatever the cause, large amounts of rock or sediment are moved around the seabed. This under-sea activity displaces a huge amount of water which forms the tsunami. One disturbance usually generates several tidal waves. These can cross ocean very quickly. Tsunami can reach speeds of 750 km per hour. In the open ocean, tsunamis are typically 800 metres long and only a few metres high. When they run onto a shelving shore, they reach heights of 12 metres or more and cause massive destruction. A tsunami may form a wall of water more than 100 feet high when it approaches shallow water near shore. Under-sea earthquakes Tsunamis caused by undersea earthquakes are called seismic sea waves. Scientists using seismographs can accurately predict when a tsunami will arrive at a given seacoast. When a seaquake occurs, seismograph picks up vibrations, called seismic waves that travel outward in all directions from the quake’s point of origin.
  4. 4. Under-sea landslides The technical term for a tsunami caused by landslides, is megatsunami. Undersea landslides have considerably higher amplitude than typical tsunamis because the amount of water displacement increases the wave size more than a submarine earthquake.
  5. 5. Kick-Em-Jenny Kick-'em-Jenny is a submarine volcano located 6 miles (10 km) north of the island of Grenada. It is the southern-most active volcano in the Lesser Antilles and the only active submarine volcano in the arc. Kick-'em-Jenny has erupted 10 times since 1939 with the most recent eruption in 1990. The 1939 eruption sent a black cloud up to 885 feet (270 m) above sea level. During the 1965 eruption, earthquakes of intensity V were felt on Isla de Ronde. During the 1974 eruption, the sea above the volcano was boiling turbulently and spouting steam. Most of the eruptions were detected by submarine hydrophones. The hydrophones detect shock waves from explosive eruptions as they travel through the water. Using data from several hydrophones, volcanologists can determine where an active submarine volcano is located and when it started erupting. There is no indication that eruptions at Kick-'em-Jenny are hazardous. Kick- 'em-Jenny has a basal diameter of about 3 miles (5 km) and rises about 4,300 feet (1,300 m) above the sea floor.