What is Cyclone | Types of Cyclone

Cyclones, What is Cyclone, Types of Cyclone, Extra-Tropical Cyclone, Tropical Cyclone, Naming of Tropical Cyclones, Thunderstorms and Tornadoes, What Causes Lightening and Thunder, Why Thunders are Cause of Concerns

What is Cyclones

The atmospheric disturbances which involve a closed circulation about a low pressure centre, anticlockwise in the northern atmosphere and clockwise in the southern hemisphere are call as cyclones. They fall into the following two broad categories: (a) Extra-tropical and (b) tropical cyclones.

Types of Cyclones (What is Cyclone)

1. Extra-Tropical Cyclones

Extra-tropical cyclones are the weather disturbances in the mid and high latitude, beyond the tropics. These latitudes are an area of convergence where contrasting air masses generally meet to form polar fronts.

Initially, the front is stationary.
In the northern hemisphere, warm air blows from the south and cold air from the north of the front. When the pressure drops along the front, the warm air moves northwards and the cold air move towards, south setting in motion an anticlockwise cyclonic circulation.
The cyclonic circulation leads to a well developed extra tropical cyclone, with a warm front and a cold front.

The warm air glides over the cold air and a sequence of clouds appear over the sky ahead of the warm front and cause precipitation.
The cold front approaches the warm air from behind and pushes the warm air up. As a result, cumulus clouds develop along the cold front.
The cold front moves faster than the warm front ultimately overtaking the warm front. The warm air is completely lift up and the front is occlude and the cyclone dissipates.

There is great degree of variation in shape and size of extra-tropical cyclones. Generally, the isobars are almost circular or elliptical. However, in certain depressions, the isobars take the shape of the letter ‘V’. Such storms are called V-shaped depression. At times, the cyclones become so broad and shallow that they are referred to as troughs of low pressure.

Features of Extra-Tropical Cyclone

1. Paths and movement of extra-tropical cyclone

The general direction of movement of temperate cyclones is from west to east with frequent trends towards the southeast to northeast. They are subject to the general westerly flow of atmosphere in temperate zone. The heavy concentration of storms tracks in the vicinity of the Aleutian(Islands west to the Alaska Peninsula) and Icelandic lows is the most important feature of the distribution of extra-tropical cyclones. During winter months, the opposing air masses have greater contrasts in their properties. So the winter cyclones are greater in number and are more intense. On an average cyclone may cover a distance of about 1000 km per day. Cyclones invariably move towards higher latitudes.

2. Secondary cyclones

Under the normal conditions, in the later stages of occlusion the cyclone weakens and ultimately dissipates. But sometimes, during the late maturing stage of a cyclone, a new low develops on the equatorward margin of the original cyclone. Thus, a secondary cyclone is form which passes through different stages of its life cycle and matures very rapidly. It may follow the tract of primary cyclone or may move along new path.

3. Cyclone families

It is found that an extra-tropical cyclone never appears alone. It is usually followed by three or four cyclones forming a series. The primary or the leading cyclone gets occluded, while the new ones originate on the trailing front and are in an incipient stage. In the rear of the last member of the cyclone family there is an outbreak of polar air which builds up an anti-cyclone. Original cyclone would be in high latitudes and each secondary cyclone would follow progressively a more southerly path. Frequent the oceans in a larger number.

4. Extra-tropical cyclone and Jet stream

There is a close relationship between the flow aloft and the cyclonic storm at the surface. Rossby waves produced at the top of troposphere helps in transporting large bodies of polar air to the lower latitudes and tropical air masses are carried to the higher latitudes. This results in the intensity of surface cyclonic activity. There are instances when extra-tropical cyclones form without the prior existence of a polar front. These depressions are actually initiated by a trough in the upper-air westerlies. Once such storms originate in the lower atmosphere they attract different air masses together which leads to the generation of fronts.

2. Tropical Cyclones

The tropical cyclone develops from the ‘warm core’ of extremely low pressure area in the tropical oceanic areas. They are energize from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm. The arrangement of isobars is almost circular. With continuous supply of moisture from the sea, the storm is further  strengthen. 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 cyclone. The conditions favourable for the formation and intensification of tropical storms are:

–  Large sea surface with temperature higher than 27° C
Presence of the Coriolis force
  Small variations in the vertical wind speed
A pre-existing weak low-pressure area or low-level-cyclonic circulation
Upper divergence above the sea level system.

Large and continuous supply of warm and moist air from the ocean provides necessary energy in the form of latent heat of condensation. Coriolis force causes cyclonic circulation. At the equator, the Coriolis force is zero and the wind blows perpendicular to the isobars. The low pressure gets filled instead of getting intensified. That is the reason why tropical cyclones are not formed near the equator.

Because of small variations in the vertical wind speed or wind shear, cyclone formation processes are limited to latitudes equatorword of the sub-tropical jet stream. It is the pre existing low pressure area which intensifies and develops as cyclone. It must be pointed out that only a few of these disturbances develop into true tropical cyclones. Upper divergence helps in ascending air currents to be pumped out to maintain the low pressure at the centre of the cyclone.

Features of Tropical Cyclones

1. Eye : It is the centre of cyclone around which strong spirally winds circulate in a mature tropical cyclone. It is a region of calm with subsiding air.

2. Eye Wall: There is a strong spiraling ascent of air to greater height reaching the tropopause. The wind reaches maximum velocity in this region, reaching as high as 250 km per hour. Torrential rain occurs here. From the eye wall rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region

3. The diameter of the circulating system can vary between 150 and 250 km.

4. The diameter of the storm over the Bay of Bengal, Arabian Sea and Indian Ocean is between 600 – 1200 km. The system moves slowly about 300 – 500 km per day.

Impact of Tropical Cyclones on humans

This is one of the most devastating natural calamities. They move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges.
The cyclones, which cross 20o N latitude generally, re-curve and they are more destructive.
Trees are uprooted and broken and the loose objects swept away.
A particular location on the land surface encounters opposite winds twice from the circular fashion of the cyclone. These winds create more damage to objects.
Torrential rains that occur in the towering cumulonimbus clouds inundate the low-lying areas, cause floods and landslides resulting in great loss of life and property damage.
Strom waves of great heights are great hazard to shipping. These are called storm surge whose height may go up to 20 meters. If cyclone wave combines with the spring tide, the result is disastrous.

Naming of tropical cyclones

In the beginning, storms(tropical cyclone) were named arbitrarily. Then the mid -1900’s saw the start of the practice of using feminine names for storms. In the pursuit of a more organized and efficient naming system, meteorologists later decided to identify storms using names from a list arranged alphabetically.

There is a strict procedure to determine a list of tropical cyclone names in an ocean basin(s) by the Tropical Cyclone Regional Body responsible for that basin(s) at its annual/biennial meeting. There are five tropical cyclones regional bodies. The Regional Specialized Meteorological Centre (RSMC) – Tropical cyclones is responsible for monitoring and prediction of tropical cyclones over their respective regions. They are also responsible to name the cyclones.

Cyclones

In general, tropical cyclones are named according to the rules at a regional level. The WMO/ESCAP Panel on Tropical Cyclones at its twenty-seventh Session held in 2000 in Muscat, Sultanate of Oman agreed in principal to assign names to the tropical cyclones in the Bay of Bengal and Arabian Sea. After long deliberations among the member countries, the naming of the tropical cyclones over north Indian Ocean commenced from September 2004.

The list of names India has added to the database includes Agni, Akash, Bijli, Jal (cyclones which have all occurred since 2004). The Indian names in the queue are Leher, Megh, Sagar and Vayu, while those suggested by Pakistan include Nilofar, Titli and Bulbul. If public wants to suggest the name of a cyclone to be include in the list , the propose name must meet some fundamental criteria. The name should be short and readily understood when broadcast. Further the names must not be culturally sensitive and not convey some unintend and potentially inflammatory meaning. A storm causes so much death and destruction that its name is consider for retirement and hence is not come in use repeatedly. Names are usually assign to tropical cyclones with one-, three-, or ten-minute sustain wind speeds of more than 65 km/h depending on which area it originates.

Importance for naming tropical cyclones

It would help identify each individual tropical cyclone.
It helps the public to become fully aware of its development.
Local and international media become focused to the tropical cyclone.
It does not confuse the public when there is more than one tropical cyclone in the same area.
The name of the tropical cyclone is well remembered by million of people as it is unforgettable event whose name will long be remembered.
–  Warnings reach a much wider audience very rapidly.
It heightens interest in warnings and increases community preparedness.

3. Thunderstorms and Tornadoes

What is Thunderstorms

Unlike Tropical Cyclones, thunderstorms and tornadoes are highly localize weather phenomenon. They are of short duration, occurring over a small area but are violent. They are so small and short lived as to make their prediction very difficult.

A storm accompanied by thunder and lightning is call as thunderstorm. It is associates with the cumulonimbus clouds.. Thunderstorms are cause by intense convection on moist hot days. When the clouds extend to heights where sub-zero temperature prevails, hails are form and they come down as hailstorm. If there is insufficient moisture, a thunderstorm can generate dust storms.

Different Stages of Thunderstorms

1. Cumulus stage

Warm, moist air rises in a buoyant plume or in a series of convective updrafts. As this occurs the air begins to condense into a cumulus cloud. As the warm air within the cloud continues to rise, it eventually cools and condenses. The condensation releases heat into the cloud, warming the air. This, in turn, causes it to rise adiabatically. The convective cloud continues to grow upward, eventually growing above the freezing level where super-cooled water droplets and ice crystals coexist Precipitation begins once the air rises above the freezing level.

2. Mature stage

It is characterize by the presence of both updrafts and downdrafts within the cloud. The downdrafts are initiate by the downward drag of falling precipitation. Cold descending air in the downdraft will often reach the ground before the precipitation. As the mature-stage thunderstorm develops, the cumulus cloud continues to increase in size, height and width. Cloud to ground lightning usually begins when the precipitation first falls from the cloud base. During this phase of the life cycle, the top of the resulting cumulonimbus cloud will start to flatten out, forming an anvil shape often at the top of the troposphere

3. Dissipating stage

It is characterize by downdrafts throughout the entire cloud. Decay often begins when the super-cooled cloud droplets freeze and the cloud becomes glaciated, which means that it contains ice crystals. The cloud begins to collapse because no additional latent heat is release after the cloud droplets freeze, and because the shadow of the cloud and rain cool downdrafts reduce the temperature below the cloud.

What Causes Lightning and Thunder

The rising air in a thunderstorm cloud causes various types of frozen precipitation to form within the cloud. Included in these precipitation types are very small ice crystals and much larger pellets of snow and ice. The smaller ice crystals carry upward toward the top of the clouds by the rising air while the heavier and denser pellets are either suspend by the rising air or start falling toward the ground. Collisions occur between the ice crystals and the pellets, and these collisions serve as the charging mechanism of the thunderstorm. The small ice crystals become positively charged while the pellets become negatively charged. As a result, the top of the cloud becomes positively charged and the middle to lower part of the storm becomes negatively charged. When the strength of the charge overpowers the insulating properties of the atmosphere, lightning happens.

At the same time, the ground underneath the cloud becomes charged oppositely of the charges directly overhead. When the charge difference between the ground and the cloud becomes too large, a conductive channel of air develops between the cloud and the ground, and a small amount of charge (step leader) starts moving toward the ground. When it nears the ground, an upward leader of opposite charge connects with the step leader. At the instant this connection is made, a powerful discharge occurs between the cloud.

The channel of air through which lightning passes can be heat to 50,000°F—hotter than the surface of the sun! The rapid heating and cooling of the air near the lightning channel causes a shock wave that results in the sound we know as “thunder.

Why Thunders are Cause of Concern

Each year, many people are kill or seriously injured by severe thunderstorms despite the advance warning. While severe thunderstorms are most common in the spring and summer, they can occur at just about any time of the year. Cloud-to-ground lightning, Hail, Tornadoes and waterspouts, Flash flood and Downburst are some of the hazards associate with thunderstorm. There is no safe place outside during a thunderstorm but building constructed according to current guidance could provide safe seltor and avoid injury or death.

What is Tornado

From severe thunderstorms sometimes spiraling wind descends like a trunk of an elephant with great force, with very low pressure at the centre causing massive destruction on its way. Such a phenomenon is call a tornado. Excessive instability and steep lapse rate in the atmosphere are necessary pre-requisite for the development of a tornado. Tornadoes generally occur in middle latitudes. The tornado over the sea is call as water sprouts.

Features of Tornadoes

Tornado’s funnel can have size of 90-460m in diameter.
Tornadoes generally occur in middle latitudes.
Tornadoes are the most violent of all the storms.
They are very small in size and of short duration which makes weather prediction difficult.
The velocity of winds revolving tightly around the core reaches more than 300 km per hour.
It causes massive destruction on its way.
When looked at from the ground, the funnel appears dark because of the presence of condensed moisture and the dust and debris picked up from the ground by the whirling tornado.
Tornadoes may be found to be moving singly or in families of several individual tornadoes.
These generally move in straight paths.

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