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Cyclones as natural hazards

Prof.  A.Balasubramanian
Prof.  A.Balasubramanian
Prof. A.Balasubramanian Professor at University of Mysore

In this episode, the following aspects of cyclone are discussed: 1. Origin of Cyclones 2. Types of cyclonic storms and their physical characteristics 3. Distribution of Cyclones 4. Environmental impacts of cyclones 5. Cyclone disaster Management.

Cyclones as natural hazards

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1
CYCLONES AS NATURAL HAZARDS
By
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science,
University of Mysore
Mysore
2
Introduction:
Earth's Natural hazards include
earthquakes, volcanic eruptions, floods,
droughts, desertification, tsunamis and landslides.
In addition to these, Cyclones have also created
severe damage to the life and properties directly
or indirectly.
The occurrence of unpredictable violent storms
brings devastating effects to the coastal regions
and on islands located in their paths.
3
High winds, heavy storms and intensive rainfalls
are all the powerful collective mechanisms of a
cyclone causing the damage.
In this episode, the following aspects of cyclone
are discussed:
1. Origin of Cyclones
2. Types of cyclonic storms and their physical
characteristics
3. Distribution of Cyclones
4. Environmental impacts of cyclones
5. Cyclone disaster Management.
4
1. Origin of Cyclones
A Cyclone is defined as any large system of
winds that rotates about a centre of low
atmospheric pressure with a speed over 100
kmph.
This swirling action happens in a
counterclockwise direction north of the Equator
and in a clockwise direction to the south.
5
As the sun warms the oceans, evaporation and
conduction transfer heat to the atmosphere.
This happens so rapidly that the air and water
temperatures rarely differ by more than a degree
F.
The water vapor generated by such evaporation is
the driving force for a tropical storm. As the vapor
condenses into clouds and precipitation, it releases
enormous amounts of heat into the cyclone.
This has indicated a major point that cyclones can
not develop when the ocean temperature is below
24 deg. C.
6
The diameter of the ring of swirling winds may
range from 500 to 1500 km.
It is a highly dynamic movement and the direction
of its transgression is not precisely predictable.
The barometric pressure will be low at the centre
of this mass and that acts as a chimney through
which air rises, expands, cools dynamically and
produces precipitation after condensation.

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Cyclones as natural hazards

  • 1. 1 CYCLONES AS NATURAL HAZARDS By Prof. A. Balasubramanian Centre for Advanced Studies in Earth Science, University of Mysore Mysore
  • 2. 2 Introduction: Earth's Natural hazards include earthquakes, volcanic eruptions, floods, droughts, desertification, tsunamis and landslides. In addition to these, Cyclones have also created severe damage to the life and properties directly or indirectly. The occurrence of unpredictable violent storms brings devastating effects to the coastal regions and on islands located in their paths.
  • 3. 3 High winds, heavy storms and intensive rainfalls are all the powerful collective mechanisms of a cyclone causing the damage. In this episode, the following aspects of cyclone are discussed: 1. Origin of Cyclones 2. Types of cyclonic storms and their physical characteristics 3. Distribution of Cyclones 4. Environmental impacts of cyclones 5. Cyclone disaster Management.
  • 4. 4 1. Origin of Cyclones A Cyclone is defined as any large system of winds that rotates about a centre of low atmospheric pressure with a speed over 100 kmph. This swirling action happens in a counterclockwise direction north of the Equator and in a clockwise direction to the south.
  • 5. 5 As the sun warms the oceans, evaporation and conduction transfer heat to the atmosphere. This happens so rapidly that the air and water temperatures rarely differ by more than a degree F. The water vapor generated by such evaporation is the driving force for a tropical storm. As the vapor condenses into clouds and precipitation, it releases enormous amounts of heat into the cyclone. This has indicated a major point that cyclones can not develop when the ocean temperature is below 24 deg. C.
  • 6. 6 The diameter of the ring of swirling winds may range from 500 to 1500 km. It is a highly dynamic movement and the direction of its transgression is not precisely predictable. The barometric pressure will be low at the centre of this mass and that acts as a chimney through which air rises, expands, cools dynamically and produces precipitation after condensation.
  • 7. 7 Anticyclones have a flow opposite to that of cyclones. -i.e., an outward-spiraling motion, with the winds rotating clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. They are usually not as strong as the cyclonic storms and bring no rainfall. Both are regions of relatively low and high pressure, respectively. They occur over several
  • 8. 8 parts of the globe in a variety of sizes ranging from larger to smaller dimensions. Cyclones of a somewhat different character occur closer to the Equator, forming in latitudes 10 to 15 N and S over the oceans. They are an essential part of the mechanism by which the excess heat received from the Sun in the equatorial belt is conveyed toward higher latitudes. These higher latitudes radiate more heat to space than they receive from the Sun.
  • 9. 9 Strong horizontal temperature gradients are responsible for the formation of cyclones. These are known as the tropical cyclones. Those cyclones that form outside the equatorial belt are known as extra-tropical cyclones. The name of cyclone differs from region to region. The tropical cyclones are called as hurricanes in the Atlantic and Caribbean,
  • 10. 10 as typhoons in the western Pacific , Phillipines and China , and as willy-willies off the coasts of Australia. These storms are of smaller in dimension than the extratropical cyclones, ranging from 100 to 500 kilometres in diameter. Based on the their velocities, wind speeds are classified into various kinds. According to the famous Beaufort wind scale,
  • 11. 11 A Gentle breeze having a wind velocity of 12-19 kmph can move leaves and small twigs A Strong breeze having a wind velocity of 39- 49kmph can make Large branches sway and make it difficult to use umbrellas A Moderate galehaving a wind velocity of 50- 61kmph, will make a person difficult to walk against the wind. A Whole gale having a wind velocity of 89- 102kmph, will uproot the trees A Storm having a wind velocity of 103- 117kmph, will create a widespread damage
  • 12. 12 A Hurricane having a wind velocity of more than 117kmph will lead to a violent destruction. Tropical cyclones are accompanied by winds of extreme violence. Storms usually occur with cyclones. It is defined as a circular strom with rotating wind speeds in excess of 32 metres per second. The life span of a tropical cyclone is on an average, six to nine days until it enters the land. No tropical cyclone follows the same track.
  • 13. 13 Falling atmospheric pressure generally indicates the arrival of a bad weather. But cyclones do not bring any such condition. It is necessary to understand the role of Coriolis force during cyclones. It prevents winds from the North and South poles and the equator from moving directly north or south. Winds that blow toward the equator seem to curve toward the west. Winds that move away from the equator seem to curve to the east. The Coriolis force also influences the direction of
  • 14. 14 ocean currents. This has a profound effect on the movements of cyclonic storms. The Coriolis force is proportional to both the latitude and the angular velocity (rotational speed) of the Earth. A tropical cyclone is likely to occur whenever several of the following prerequisites occur simultaneously: (1) latitude sufficiently high (5-6) for the Coriolis force to be appreciable;
  • 15. 15 (2) a warm-water surface of at least 27 deg. C, of sufficient area to supply the overlying air with large amounts of vapour (3) pronounced instability in the air column or relatively low pressure at the surface and (4) little or no vertical wind shear. Due to strengthening of convection, the centripetal wind flow gains speed. Very soon, the angular velocity component of the Coriolis force becomes sufficient to impart a
  • 16. 16 definite cyclonic curvature to the air flow, resulting in a cyclone The input of warm, very moist air continues. Large-scale condensation of moisture occurs during the ascent, and enormous amounts of previously accumulated latent energy are released. This energy results in stronger winds, which in turn lead to the intake and uplift of larger amounts of humid air, with a further release of energy. The evolution from a tropical depression to a violent tropical cyclone takes four to eight days.
  • 17. 17 Sometimes, strong vertical wind shear (jet streams) would impede convection and prevent the development of the cyclone. 2. Types of cyclonic storms and their physical characteristics Cyclones may be either warm-core or cold-core types. Warm-core cyclones are warmer at the centre than near the edges. They are fairly shallow and become weaker in the upper atmosphere. They often occur over especially warm land areas.
  • 18. 18 Cold-core cyclones are coldest near the centre and warmer near the edges. These cyclones may be very deep, and are more intense several thousand feet in the air than they are at the surface of the earth. All cyclones have two characteristics as one is the atmospheric pressure which is lowest at the centre, and the other one is the inward spiraling of winds. The cyclonic storms may bring winds upto 290 kilometres an hour alongwith terrific rains,
  • 19. 19 violent thunder, and lightning. They measure 320 to 480 kilometres across. Hurricanes: A Hurricane is a powerful, whirling storm that measures 320 to 480 kilometres in diameter. The winds near the centre of a hurricane blow at speeds of 120 kilometres per hour or more. Many hurricanes have caused widespread death and destruction.
  • 20. 20 The strongest winds and heaviest rain of a hurricane occur within its wall clouds.
  • 21. 21 Hurricanes occur in the North Atlantic and North Pacific oceans from June to November--most of them in September. On an average, about six to eight hurricanes form in these regions. Tornadoes: A Tornado is a powerful, twisting windstorm. These are the most violent winds that occur on the earth. They whirl around the centre at more than 320 kilometres per hour speed.
  • 22. 22 Most of them measure several hundred metres in diameter. It becomes a rotating funnel cloud that extends downward from a mass of dark clouds. Some of them do not reach the earth. Others may strike the surface of the earth, withdraw into the dark clouds above, and then dip down and strike the earth again. It happens in the United States, and in parts of Australia. Most tornadoes last less than an hour.
  • 23. 23 Some of them last several hours and measure up to about 2.5 kilometres in diameter. Statistics show that about 700 tornadoes have been reported annually in the United States since the mid-1950's. Tornadoes can uproot large trees, overturn railway carriages, and carry cars hundreds of metres away.
  • 24. 24 Typhoons: A Typhoon is a violent, low-pressure tropical storm that occurs in the western Pacific Ocean. They are similar to hurricanes, beginning near the equator and move westward. They advance slowly, usually at about 15 to 25 kilometres per hour speed. The circular winds around the centre are very strong, often reaching speeds of 240 kilometres per hour. The diameter of a typhoon can be as large as 480 kilometres.
  • 25. 25 Typhoons are associated with heavy rains and powerful winds causing severe land and property damage and loss of life. The destructive rush of seawater, called a storm surge, often accompanies a typhoon as it moves onto coastal lands. One of the Cyclonic storm surges which occurred in the east coast of India during 1977, was reported to have been 5.7 m high, 80 km long, 16 km wide with a speed of 190kmph.
  • 26. 26 3. Distribution of Cyclones Cyclonic winds move across nearly all regions of the Earth except the equatorial belt. Most of them are generally associated with heavy rain or snowfall. Cyclones occur chiefly in the midlatitude zones of both the hemispheres.
  • 27. 27 In the Southern Hemisphere, where most of the terrestrial surface is covered by the oceans, they are distributed in a relatively uniform manner through various longitudes. As trade winds, certain tracks are favoured by these wind systems also. The principal tracks lie over the oceans. The ocean surface being smoother than that of the land, offers less resistance to the strong winds to move around.
  • 28. 28 Extratropical cyclones The extratropical cyclones range from nearly 1,000 to 4,000 kilometres. Whereas, the tropical cyclones measure only about 100 to 500 kilometres in diameter. They are more violent than those occurring in the midlatitudes and can cause considerable damage. These are transient cyclones. They are the most abundant phenomena and exert influence on the broadest scale, affecting the largest percentage of the Earth's surface.
  • 29. 29 It has an impact on the day-to-day weather changes. The life cycle of these events is typically several days, during which the cyclone may travel from several hundred to a few thousand kilometres. In its path and wake occur dramatic weather changes.
  • 30. 30 4. Environmental impacts of cyclones Cyclones have a destructive power. They attack coastal regions with sever storms and rainfall. Activated by the rainstorms, the can easily remove huts, small shelters, trees and plants, boats and other vehicles. Every storm changes the distribution pattern of surface water bodies, groundwater systems, affect the growth of freshwater crops, disrupt the normal cycle of life and occupation.
  • 31. 31 It has been noticed that due to these, several villages have been washed away, and sealed off by sandcasting. Flooding, storm surges, sandcasting, erosion and deposition of silt are the impacts of these processes. A storm surge can rise several feet above the normal ground and cause floods. It is very destructive if it occurs at high tide level.
  • 32. 32 When a hurricane moves over land, strong winds and heavy rain hit the area for several hours. It weakens as it moves over the land, but the heavy rain continues even after the winds decrease. The winds may exert a pressure of more than 400 kilograms per square metre on tall structures and can flatten weak buildings at first impact. Tropical cyclones can cause immense damage, both directly (by wind, pressure, and rain) and indirectly (mainly through storm surges and floods).
  • 33. 33 Roofs and windows are damaged by the suction produced by strong winds on the downwind side. All loose objects are lifted by the wind. It also causes injuries and deaths by toppling structures and hurling loose and torn off objects with enormous force. Torrential rain may erode the soil, causing landslides in mountainous regions and making the streams and reservoirs overflow.
  • 34. 34 These may, inturn, cause floods. The indirect damages of these are mostly due to storm surges. During these periods, the sea level is raised by up to three to four metres for a period that may last several hours. Notable and extreme tides have been recorded on the Gulf Coast due to hurricanes. Pounding waves are an additional cause of damage to coastal installations and structures.
  • 35. 35 One of the impacts of this is the coastal erosion. Marshes, estuaries and lakes are affected in the same way. Severe flooding is a combined process of the cyclones in coastal zones. 5. Cyclone disaster Management. The Cyclone Intensity to damage potential has been proposed by the National Hurricane Centre of the United States. This is based on three storm intensity classifications as, minimal, major and extreme cyclonic events. :
  • 36. 36 a) Minimal cyclone – 120 to 160 kmph with a maximum central pressure of 983-996 millibars b) Major cyclone – 161 to 220 kmph with a maximum central pressure of 949-982 millibars c) Extreme cyclone – 221 and more kmph with a maximum central pressure of 948 or less millibars Most of the tropical cyclones are a seasonal phenomenon with their frequencies varying from an average of one per year in some places to as many as 20 per years.
  • 37. 37 It occurs in the north Pacific any time of the year. Warning and tracking of tropical cyclones are essential tasks of the people. Cyclone warning can help people to vacate the zones of danger and prepare for advance relief measures. Attempts to forecast tropical cyclones date back to the nineteenth century. The basic criteria to indicate a new cyclone is a) Occurrence of a sub-normal pressure in the low latitudes and b) above normal pressure in high latitudes
  • 38. 38 c) disturbance of atmospheric winds, d) erratic tides, and e) swelling of seas. Because of the small diurnal pressure changes common in the low latitudes, it is difficult to single out changes due to approaching cyclones. A local drop in pressure of more than three to four millibars within a day (or over a distance of 500 kilometres or less) may be considered as a danger sign.
  • 39. 39 The growth of distinct curved isobars moving toward the land is a much more definite indication of an approaching cyclone. The roughly circular shape of the isobars can reveal the position of the cyclone's centre. Humidity is another indicator of these incidents. It is normally high and is a notable and useful indicator. Cyclone and pressure Monitoring ships in the oceans and radar units provide warning of any storm within the range. Earth observation satellites transmit live images of any part of the globe and its cloud systems.
  • 40. 40 Methods to protect the environment against tornadoes are a permanent requirement in some places. Weather monitoring stations obtain information from local observers, radar stations, and the pilots of aircraft when weather conditions appear to be dangerous. The receiving stations issue warning messages to the people in the area of the tornado. People have built mobile homes as a precaution.
  • 41. 41 Global telecommunication systems and global data communication systems will certainly help people to understand the occurrences of these events. Every country having a long coastal belt has to establish a cyclone warning network and research stations. Wind tunnel research is a major activity to propose cyclone resistant houses and structures. It is very difficult to protect the vegetation without constructing sea walls and shelter belts.
  • 42. 42 Ports and harbours need frequent maintenance operations and hence they need to evolve certain precautionary measures. The origin of a cyclone may be spotted out using a satellite image atleast two days in advance. This is one of the major advantages of organising precautionary measures and relief related activities. A cyclone will have immediate and long-term impacts. Continuous and frequent incidences may cause a severe decline in the economy of a region and its country.
  • 43. 43 Whenever a natural disaster occurs, it will certainly cause an economic crisis, starvation, epidemic, migration, landlessness, homelessness, orphanage to some children and massive deaths. Disruption of road networks, railway lines and power supply are notable impacts. Homeless people need to be provided with appropriate shelters, food, clothing and financial support to sustain and recoup their life.
  • 44. 44 Shortage of fuel and food need immediate attention. Control of disease vectors and medical treatments are to be planned in advance. Cyclones disrupt agriculture mainly and destroy the crops. Coastal zone classification, building norms, indoor and out door safety measures and disaster preparedness are the major factors to be considered. Awareness programmes should be carried out to enlighten the issues in all offices, villages forums and in all schools and colleges.
  • 45. 45 Education is a primary mechanism of disaster management. Disasters are recurring features over the globe. There is no place on earth which is free from these types of natural calamities. It is very essential to protect the life mainly from these dangers. Strong will and sound scientific predictions can help us to save the souls.