WHAT IS A CYCLONE?
When the wind is spirally moving in towards a low-pressure center, it is known
as a “Cyclone”. A cyclone has a low pressure at the center and high pressure on the surrounding sides. The wind movement in a cyclone is anti-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
SHAPING PROCESS OF CYCLONES
The atmosphere above the vast oceanic surface is the breeding ground for cyclones. If a new pressure pocket exists in the upper layers, it has a suction effect on the naturally rising air from the warm tropical sea surface. The spinning motion of the earth imparts a rotational motion to this rising column of air and it then ascends in a spiral. It cools as it rises and this causes increased suction from blow. The ocean surface provides an inexhaustible supply of moisture. Gradually the rotational motion increases in intensity and the cyclone develops a curved funnel shape.
INTENSITY OF CYCLONES
The initial rotating wind system is known as “Deep Depression” and its rotation velocities are less than 33 knotts or 60 km/hour. Above 33 knotts, it acquires cyclonic intensity. Further intensification makes it a severe cyclonic storm with spinning wind velocity above 100 km/hour. This already menacing (threatening) formation can become even more terrible with the onset of “hurricane winds”, the rotational velocity rises above 120 km/hour.
MOVEMENT OF CYCLONES
The fierce spinning momentum of cyclones makes the whole system capable of linear action as well. It slowly moves (speed 10-20 km/hr) towards west or north-west in the Northern Hemisphere, but the cyclones formed over the Southern Hemisphere move towards west or south-west.
HEIGHT OF CYCLONES
Vertically, the cyclonic circulation reaches phenomenal heights of 6 km. Above this level, the inner circulation around the eye persists upto 17 km (the Tropopause level) but the outer circulation peters out (gradually comes to an end) and is replaced by anti-cyclonic circulation above 9 km.
FEATURES OF A FULLY DEVELOPED CYCLONES
A severe cyclone, when fully developed, is a vast violent whirl of air which may extend from 150 to 1,000 km across, 10-17 km high with fierce winds spiraling around a central low pressure area. The whirl moves slowly over the surface of the ocean at a speed of 300 to 500 km/day. The speed of wind of a mature storm can be 150 to 200 km/hour and accounts for most of its destructive potential.
A mature cyclone consists of a calm central area of 15-20 km in diameter where very light winds with clear or partly cloudy skies prevail. This is also the area of lowest pressure and is called Eye of the storm.
2. Eye Wall
The calm eye is surrounded by a ring of hurricane winds, exceeding 90 km/hour and extending from 30-50 km beyond the eye. This is an area where torrential (stormy) rains occur under circular wall of clouds surrounding the eye. This is called the “Eye Wall” and is the most dangerous part of the cyclones.
3. Rain Bands
Beyond the eye wall, spiraling rain bands occur upto 400 km or so and winds decrease outwards. Stronger winds occur to the right of the track of the cyclone than to the left.
PRESSURE SYSTEM OF THE CYCLONE
As a cyclone approaches a station, the surface pressure diminishes gradually till the eye wall passes over it and steeply from the eye wall to the eye. The pressure in the eye in mature cyclones may be about 70 mb (5.2 cm) less than at the periphery.
The thermal structure of cyclone is also typical. The core, including the eye and the eye wall, is warm compared to its surroundings. The temperature in the core of a mature cyclone may be 8o to 10oC warmer than its surroundings. The warmer core extends upto about 15 km above which the core is colder than its surroundings. At the Tropopause level of 17 km, the core of the cyclone can be 8o -10oC colder.
ENERGY SOURCE OF CYCLONES
Since the moisture is picked up from the underlying sea surface, the warm tropical ocean is the ultimate energy source of the cyclones. The heat energy is converted into kinetic energy which, in a moderate cyclone, is equivalent to the energy released by about 400 hydrogen bombs.
EXTRA-TROPICAL, TEMPERATE, WAVE CYCLONE
The dominant type of weather disturbance of middle and high latitudes is the Wave Cyclone, a vortex (whirlpool) that repeatedly forms, intensifies and dissolves along the frontal zone between cold and warm air masses. Temperate or extra-tropical cyclones are most dominant over the North Atlantic Ocean and specially during the winter season. They are much larger in diameter but posses weaker pressure gradients. The shape and arrangement of isobars is v-shaped and they are accompanied with anti-cyclones. The speed is also different and there is a marked temperature and rainfall difference between their front and back. They are, however, not born of convection and earth’s rotation.
With regard to the origin of the temperate cyclones, two theories are advanced. One is the Dynamic Theory of Lampert and Shaw; the other is the Polar Front Theory of J. Bjerkenes.
1. Dynamic Theory
It was formulated on the basis of a study of North Atlantic disturbances. It holds that the cyclones are developed due to the crowding of different currents of air against the high pressure belt. The difference in temperatures of different air masses provides the energy for cyclonic movement. According to this theory, the cyclone originates at a height of 7,500 ft., just blow the region of Cirrus clouds. When the current of cold air meets the warm rising air, an eddy is formed which begins to descend. Hence the cyclone is cone shaped with the narrow edge upwards.
2. Polar Front Theory
The Norwegian meteorologist, J. Bjerkenes, at the time of World War One, recognized the existence of atmospheric fronts and developed his “Wave Theory of Cyclones”.
This theory states that depressions are formed due to the meeting of a warm sector, consisting of light, warm, equatorial air, with the heavy and cold polar air. The cold heavy air will try to force its way under the warm air and will raise it. This theory holds that there are two currents of air having different temperatures and velocities. They are flowing side by side and the surface of discontinuity separating the two is known as the Polar Front.
Due to the distribution of land and sea and their unequal heating, this surface of discontinuity is always wavy or irregular. Over this irregular and highly changeable polar front, exist two sectors – warm and cold. The warm air pushes the cold air in front of it and, being light, rises above it. Simultaneously, the cold air behind the warm sector pushes underneath. The result is that the warm air mass is not only reduced in extent but is also lifted upwards. The pressure will be reduced and gradually the area of low pressure would increase. Consequently on this, the winds would begin to move forcefully.
1. Early Stage
At the start of the cycle, the polar front is a smooth boundary along which air is moving in opposite directions. In the polar front a bulge or wave begins to form. Cold air is turned in a southerly direction, warm air in a northerly direction, as if each would invade the domain of the other.
2. Open Stage
In this stage, the wave like disturbance along the polar front has deepened and intensified. Cold air is now actively pushing south-westward along the cold front; warm air is actively moving north-eastward along the warm front. The zone of precipitation is now considerable, but wider along the warm front than along the cold front. Each front is convex along the path of motion.
3. Occluded Stage
In this stage, the more rapidly moving cold front has reduced the zone of warm air to a narrow sector. In this stage, the cold front has overtaken the warm front, producing an Occluded (plugged, choked) Front and forcing the warm air mass off the ground, isolating it from parent region of warm air to the south.
4. Dissolving Stage
In this stage, the source of moisture and energy cuts off because of the movements of cold and warm fronts in the previous stages. The cyclonic storm gradually dies out and the polar front is re-established as originally.
Normally, these cyclones travel with the prevailing westerlies and their usual direction is from west to east. But in the northern hemisphere, they dip to the south over the continents while they are deflected towards north over the oceans. These depressions may move in any direction but they always tend a little towards east. The following three facts are worth considering:
1. If the direction of the polar front is west-east, the warm air from the south is continually lowering the pressure towards the east and the center shifts to the east. They can enter the continent to the farthest extent.
2. If the direction of the polar front is towards north, the center of the cyclone is not able to move to a great distance. Within 2 or 3 days, the difference in pressure vanishes and the cyclone also disappears.
3. Polar fronts having a southward dip are also quite permanent and they are able to move forward over quite an extensive surface. The Mediterranean cyclones are an example of this.
The temperate cyclone is quite unsymmetrical in outline. It is formed like a spearhead and has the shape of an upturned “V”. At the center of this depression lies the low pressure area, while to the north-west lies the cold sector and to the north-east is the warm sector. In the cold sector, cold winds prevail and the line towards which cold air seems to be moving is known as the Cold Front. This is ultimate boundary or limit of the cold air. In the north-east sector, prevail the warm winds and the line towards which they blow is known as the Warm Front.
1. Warm Sector
In the north-eastern or warm sector, the warm air which is full of water vapors continually meets the cold air and over runs the latter. As it rises high, dense Nimbus clouds are formed and are followed by heavy downpour of rain or snowfall. As we proceed from the warm front in the direction of north-east, there are rain giving Nimbus clouds or Nimbostratus clouds on the warm front. Next to these at higher levels are the dark gray Altostratus clouds, then are Cirrostratus clouds and finally at the front of the storm are the Cirrus clouds spreading out as wisps (fluffy) and plumes (feather).
2. Cold Sector
In the north-western or cold sector, the cold air pushes the warm air front beneath and although clouds are formed, the extent of clouds and rainfall is very limited. Cumulonimbus clouds dominate here and bring heavy downpour. Accompanied with thunder and lightening, there may be hail storm as well. Due to the cold winds the temperature is continually declining.
The approach of a temperate cyclone is heralded by:
1. The wisps and plumes of the cirrus clouds spread over the sky like a thin veil (covering) which, spreading upwards from the western horizon, give a milky appearance to the blue sky.
2. The long streaks of thin white clouds run parallel to one another and spread like radii from some point in the horizon.
3. Fall of mercury in the barometer.
4. Shift in the direction of wind.
5. Formation of a halo (ring of light) round the sun or the moon.
As soon as the cyclone approaches, there is a drizzle, which turns into a heavy downpour. The force and the velocity of the wind also increases. On the approach of the warm front, the fall of mercury in the barometer stops and rainfall ceases. The sky becomes clear and fine weather is ushered in. This shows that the center of the cyclone is reached and the weather will remain clear till the warm sector has passed. Immediately after this, the temperature begins to fall, the sky becomes cloudy and it begins to rain. The rain brings hailstorms and is accompanied by lightening and thunder but soon the rain stops and the sky begins to clear.
One of the most powerful and destructive types of cyclonic storms is the tropical cyclone otherwise known as the Hurricane or Typhoon.
Tropical cyclones are typically broad, shallow, low pressure centers that are found mainly within the Inter-Tropical Convergence Zone (ITCZ) between trade winds. The absence of strong fronts between air masses in this zone, due to small differences in temperature, prevents the development of cyclones like those of higher latitudes. The tropical cyclone has rarely a pressure range of more than about 3-5 mb and is much larger in size than the extra-tropical type, often reaching 1500-2500 miles in diameter. They rarely develop occlusions (trouble, impact) and, generally, slowly weaken and disappear. Their direction is from east to west within the general easterly flow of low latitudes. Movement is slow and they frequently remain motionless for several days. Weather changes accompanying them are little more than “spells of weather” with periods of overcast skies and fairly steady rains that may last for several days. They appear to be more equinoctial periods but may occur in any season.
The cyclones of the tropics are thermal in the origin and develop due local convection currents, acquiring a whirling motion on account of rotation of the earth. These develop at special seasons over the sea within the tropics. The conditions most suitable to their development and origin are: quiet air; highly saturated atmosphere; and great heat. These conditions are found in the doldrums (parts of the ocean near the equator where there is little or no wind and ships with sails cannot move) or the equatorial calms, specially over the sea. Due to the presence of trade winds, the atmosphere of the western margins of such ocean surfaces possess the greatest capability for absorbing water vapor.
The cyclones, therefore, originate on the western margins of the oceanic area within the doldrums but generally along the polar margins of the belt. The time they originate most is when the doldrums are the farthest from the equator, specially during the autumnal equinox. At this position, there are two advantages:
1. The air over the sea is over heated; and
2. The sun is farthest from the equator.
After originating and developing, they advance till they get a weak spot in the trade wind belt.
The tropical cyclones are most dominant in the northern hemisphere and their two breeding grounds are the south-eastern Caribbean Sea and near the Caroline Islands in the Western Pacific. They occur during the months of August and September. These are, however, not experienced in the Southern Pacific and Southern Atlantic. During the months of March and April, similar cyclones visit Mauritius and Mozambique.
SHAPE AND SIZE
The tropical cyclone is almost a circular storm center of extremely low pressure. The tropical cyclone is usually short in size and extent but its diameter varies in length. Storm diameter, near the places of their origin, is 80 km only. But well developed cyclones have their diameter ranging from 300-1500 km. The clouded area is still more extensive – 300 km, if not more.
They are generally circular in form but resemble more like an ellipse than a circle. The two lengths of ellipse have a ratio of 2:3.
VELOCITY AND SPEED
The barometric pressure in the center commonly falls to 965 mb or lower; and the velocity with which they travel varies from storm to storm, and from day to day and sometimes even between different parts of the same storm. It may be zero in some cases, while in others it may be as much as 800 miles/hour. Generally their speed is to be 15 miles/hour. They move at a faster rate over the oceans than over the land because the irregularities of the land surfaces retard their speed.
The wind velocities range from 75-125 miles/hour, sometimes much more. At the center, the wind force is very light, but as we proceed from the center to the polewards side, wind force goes on increasing till the velocity becomes even 50-60 m/s. This change in velocity is seldom permanent and soon the moderate velocity of 50-60 km/hour is reached.
The storm develops over oceans in latitudes 8o to 15o north and south, but not close to the equator, where the coriolis force is extremely weak. In many cases, an easterly wave simply deepens and intensifies, growing into a deep, circular low. High sea surface temperatures which are over 80oF in these latitudes are of basic importance in the environment of storm origin. Warming of air at low level creates instability and predisposes toward storm formation. Once formed, the storm moves westward through the trade wind belt. It may then curve north-west and north, finally penetrating well into the belt of westerly winds.
A brief description of the life cycle of the Tropical Cyclone at sea might be as follows:
During the day preceding the storm, the air is generally calm, the pressure somewhat above normal, and the sky shows cirrus clouds in long streamers (banner type), seeming to originate from a distant point on the horizon. The cirrus may be veil like, giving a halo to the sun or moon and producing a red sunset. A long swell is felt on the sea, this being the train of dying storm waves that have outrun the slowly moving storm center.
As the storm approaches, barometer begins to fall. Wind springs up. A great dark wall of clouds approaches. Torrential (stormy) rainfall begins. The wind rises quickly to terrifying intensity.
The terrible storm continues for several hours and is abruptly followed by total calm and clearing skies, and sometimes by a sharp rise in temperature. The barometer has now reached its lowest point and it is the calm central eye of the storm. This is merely a hollow vortex (whirlpool) produced by the rapid spiraling of the air in the storm. Although the air is clear and calm, seas are mountainous, and rise in great peak like masses which are of gravest peril (danger).
The period of calm may last for half an hour. Then a great dark wall of clouds strikes and winds of high velocity again set in, but this time in reverse direction to those of the first half of the storm. For several hours more, the full fury of storm rages, then gradually the winds abate, the clouds break and fair weather returns.
World distribution of tropical cyclones is limited to six regions, all of them over tropical and sub-tropical oceans. These are:
1. West Indies, Gulf of Mexico, Caribbean Sea and the Coast of Florida. In these regions they are known as Hurricanes.
2. Western North Pacific including the Philippine Islands, China Sea and Japanese Islands. Here these are known as Typhoons.
3. Arabian Sea and Bay of Bengal. Here these are known as Cyclones.
4. Eastern Pacific / Coastal Region off Mexico and Central America.
5. South India Ocean, off Madagascar.
6. Western South Pacific, in the region of Samoa and Fiji Islands and the East coast of Australia.
Curiously enough, these storms are unknown in the South Atlantic. Tropical cyclones never originate over land, although they often penetrate far into the margins of continents.
The tropical cyclones have their isobars almost circular in pattern and the lowest isobars are always in the middle. The center is known as the “Eye” of the cyclone. Round this eye is the barometric slope, and the pressure gradients are quite steep.
A small patch of blue sky characterizes the eye of the cyclone. Here the air is descending and due to compression it gets warmed up. Hence the sky is clear and there is an extent of calm and dry weather. Round the eye, there is a strong upward movement of air, and this causes the cirrus clouds to spread over the sky like sheep. Heavy clouds and torrential rainfall, accompanied by violent thunder storms surround this central area.
These cyclones bring very destructive weather full of lightening and violent gusts of wind. The weather is generally dull and muggy (humid). The rainfall is more in front than in the rear.
Any place situated in the right hand back corner of the cyclone experiences violent squalls of wind and heavy showers of rain. In between these, there are short periods of lull when there is swift change in wind direction. On the other, hand in the left hand back corner, the rain stops as the trough passes and gradually the sky also clears away.
The following are the chief characteristics of the weather associated with the passage of a tropical cyclone:
1. The approach of a cyclone is heralded by cirrus clouds at the eye of the cyclone.
2. This is followed by gradually darkening clouds till dark nimbus clouds come and blot out the sun.
3. Then there is a heavy downpour of rain, which passes away as the tail is reached.
4. The tail of the cyclone brings sleet and the weather once again is clear.
5. But before the tail of the cyclone actually approaches, there is thunder and lightening combined with an oppressive calm weather.