Cyclones: Types and Mechanism

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Cyclone is a system of low Atmospheric Pressure in which the barometric gradient is steep. Cyclones represent circular fluid motion rotating in the same direction as the Earth. This means that the inward spiralling winds in a cyclone rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale cyclonic circulations are centered on areas of low atmospheric pressure. The cyclones can be tropical cyclones or Temperate Cyclones (extra-tropical cyclones).

Basic difference between Tropical Cyclone and Extra-tropical Cyclone

The term “tropical cyclone” is used to refer to warm-core, low-pressure systems that develop over tropical or subtropical Oceans. This definition differentiates tropical cyclones from extra tropical (midlatitude) cyclones that exhibit a cold-core in the upper Troposphere and often form along fronts in higher latitudes. Subtropical cyclones are hybrid systems that exhibit some characteristics of tropical cyclones and some characteristics of extra-tropical cyclones. Tropical cyclones extract much of their energy from the upper layer of the ocean, while extratropical cyclones derive much of their energy from the baroclinic temperature gradients in which they form.

Tropical Cyclones

The tropical cyclone is a system of low pressure occurring in tropical latitudes characterized by very strong winds. Here are the important notes which you must note about the Tropical Cyclones:

Distribution

The tropical cyclones are found over the North Atlantic Ocean, Southern Atlantic Ocean, the eastern, central and western North Pacific Ocean, the central and western South Pacific Ocean and the northern and southern Indian Ocean.

Formation in Low Pressure areas

All tropical cyclones are formed in areas of low atmospheric pressure in the Earth’s Atmosphere.

Minimum Pressure is at centre

The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth’s surface at sea level.

Driver is the Large Heat of Condensation

Tropical cyclones are driven by the release of large amounts of Latent heat of Condensation, which occurs when moist air is carried upwards and its water vapour condenses. This heat is distributed vertically around the center of the storm. Thus, at any given altitude, Environment inside the cyclone is warmer than its outer surroundings.

Eye is the sinking air

There is an area of sinking air at the center of circulation, which is known as Eye. Weather in the eye is normally calm and free of clouds, although the sea below it may be extremely violent. Eye is normally circular in shape, and is typically 30–65 km in diameter.

Stadium Effect

The mature tropical cyclones sometimes exhibit an outward curving of the eye wall’s top, making it resemble an arena football stadium. It is called Stadium Effect.

Greatest Wind speeds are at eye walls

Greatest wind speeds in a tropical cyclone is found at the eye wall, which is a circle of strong thunderstorms that surrounds the eye. Here, the clouds reach the highest, and Precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclones eye wall passes over land.

Source of the huge Energy

Primary energy source is the release of the heat of condensation from water vapour condensing, with solar heating being the initial source for Evaporation. So a tropical cyclone can be visualized as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation (Coriolis force) and gravity of the Earth. Inflow of warmth and moisture from the underlying ocean surface is critical for tropical cyclone strengthening.

Impact of Earth’s Rotation

The rotation of the Earth causes the system to spin (Coriolis Effect) giving it a cyclonic characteristic and affecting the trajectory of the storm. In Northern Hemisphere, where the cyclone’s wind flow is counterclockwise, the fastest winds relative to the surface of the Earth occur on the eastern side of a northward-moving storm and on the northern side of a westward-moving one; the opposite occurs in the Southern Hemisphere, where the wind flow is clockwise.

Movement of Clouds

In Lower troposphere, motion of clouds is toward the center. At upper-level, there is outward flow of clouds.

Formation in Northern Atlantic Ocean

Northern Atlantic cyclone season occurs from June 1 to November 30, sharply peaking from late August through September. The statistical peak of the Atlantic hurricane season is 10 September.

Formation in North East Pacific

The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic.

Formation in North West Pacific

The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March and a peak in early September.

Formation in North Indian basin

Storms are most common from April to December, with peaks in May and November.

Formation in Southern Hemisphere

Tropical cyclone year begins on July 1 and runs all year-round and encompasses the tropical cyclone seasons, which run from November 1 until the end of April, with peaks in mid-February to early March.

Requirements for formation

Water temperatures of at least 26.5 °C down to a depth of at least 50 m, so that it may cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.
Rapid cooling with height, so that it may cause release of the heat of condensation that powers a tropical cyclone.
High humidity
Low amounts of wind shear as high shear is disruptive to the storm’s circulation.
A distance from the Equator is necessary, which should be at least 555 km or 5° of latitude, so that it allows the Coriolis Effect to deflect winds blowing towards the low pressure center and creating a circulation. Because the Coriolis effect initiates and maintains tropical cyclone rotation, tropical cyclones rarely form or move within about 5° of the equator, where the Coriolis effect is weakest.
A pre-existing system of disturbed weather.

Movement

Coriolis Effect causes cyclonic systems to turn towards the poles in the absence of strong steering currents. The pole ward portion of a tropical cyclone contains easterly winds, and the Coriolis effect pulls them slightly more pole ward. The westerly winds on the Equatorward portion of the cyclone pull slightly towards the equator, but, because the Coriolis effect weakens toward the equator, the net drag on the cyclone is pole ward. Thus, tropical cyclones in the Northern Hemisphere usually turn north (before being blown east), and tropical cyclones in the Southern Hemisphere usually turn south (before being blown east) when no other effects counteract the Coriolis Effect.

High speed of rotation

It is caused by Coriolis effect as well as energy released by heat of condensation.

Fujiwhara effect

When two cyclones approach one another, their centers will begin orbiting cyclonically about a point between the two systems. The two vortices will be attracted to each other, and eventually spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This phenomenon is called the Fujiwhara effect.

Impact on passing over land

We should note that the deep convection is a driving force for tropical cyclones. The convection is strongest in a tropical Climate; it defines the initial domain of the tropical cyclone. This is a major difference between the Tropical cyclones with other mid-latitude cyclones as the later derive their energy mostly from pre-existing horizontal temperature gradients in the atmosphere. To continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the needed atmospheric moisture to keep the positive feedback loop running. When a tropical cyclone passes over land, it is cut off from its heat source and its strength diminishes rapidly. The moving over land deprives it of the warm water it needs to power itself, quickly losing strength. Thus, most strong storms lose their strength when the pass on to land, but if it manages to move back to ocean, it will regenerate.

Impact of passing over cold water

When a tropical storm moves over waters significantly below 26.5 °C, it will lose its strength. This is because of losing its tropical characteristic of the warm core.

Project Stormfury

The United States Government attempted in 1960s and 1970s to artificially weaken the Cyclones. During this project, Cyclones were seeded with silver iodide. It was thought that the seeding would cause supercooled water in the outer rainbands to freeze, causing the inner eye wall to collapse and thus reducing the winds. The Hurricane Debbie lost as much as 31% of its strength, when seeded with Silver Iodide in this project but Debbie regained its strength after each of two seeding forays. So, it was not a good idea. There were some more ideas applied which were as follows:

Cooling the water under a tropical cyclone by towing icebergs into the tropical oceans and covering the ocean in a substance that inhibits evaporation
Dropping large quantities of ice into the eye at very early stages of development (so that the latent heat is absorbed by the ice, instead of being converted to kinetic energy that would feed the positive feedback loop)
Blasting the cyclone apart with nuclear weapons.
A Project called Project Cirrus involved throwing dry ice on a cyclone.
None of the idea was very much practical because the tropical storms are too large and too momentary.

Naming of Cyclones

Tropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a third group of more intense storms, whose name depends on the region. If a tropical storm in the North-western Pacific reaches hurricane-strength winds on the Beaufort scale, it referred to as a typhoon. If a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in the Atlantic, it is called a hurricane. Neither “hurricane” nor “typhoon” is used in either the Southern Hemisphere or the Indian Ocean. In these basins, storms of tropical nature are referred to simply as “cyclones”.

Types of the Tropical Cyclones

There are three kinds of Tropical cyclones:

Tropical Depression: A tropical depression is a system with low pressure enclosed within few isobars and with the wind speed of 60 kmph. It lacks marked circulation
Tropical Storm: It is a system with several closed isobars and a wind circulation of 115 kmph.
Tropical Cyclone: It is a warm core vortex circulation of tropical origin with small diameter, circular shape and occurs in oceanic areas.

Anticyclones

An ‘anticyclone’ is opposite to a cyclone, in which winds move into a low-pressure area. In an anticyclone, winds move out from a high-pressure area with wind direction clockwise in the northern hemisphere, anti-clockwise in the southern hemisphere. Such a high pressure area is usually spread over a large area, created by descending warm air devoid of moisture. The absence of moisture makes the dry air denser than an equal quantity of air with moisture. When it displaces the heavier nitrogen and Oxygen, it causes an anti-cyclone.

Temperate Cyclones

Temperate cyclones are generally called depressions. They have low pressure at the centre and increasing pressure outwardly. They are of varying shapes such as circular, elliptical. The formation of tropical storms as we read above are confined to oceans, the temperate cyclones are formed over land and sea alike. Temperate Cyclones are formed in 35-65° North as well as South Latitudes. While the tropical cyclones are largely formed in summer and autumn, the temperate cyclones are formed in generally winter. Rainfall in these cyclones is low and continuous not as furious as in case of tropical cyclones.

Weather

Weather is the state of the atmosphere at a particular place and time, with respect to variables such as temperature, moisture, wind speed and direction, and barometric pressure.

Types of weather

There are many different types of weather, but some of the most common include:

Tropical cyclones are large, rotating storms that form over warm waters in the tropics. They are also known as hurricanes or typhoons. Tropical cyclones can cause extensive damage and loss of life.
Extratropical cyclones are large, rotating storms that form over temperate or polar regions. They are also known as midlatitude cyclones or wave cyclones. Extratropical cyclones can bring heavy rain, snow, and strong winds.
Polar cyclones are small, intense storms that form over polar regions. They are also known as polar lows. Polar cyclones can bring heavy snow and strong winds.
Mesocyclones are small, rotating storms that form within thunderstorms. They are often associated with tornadoes.
Tornadoes are violently rotating columns of air that form from thunderstorms. They can cause extensive damage and loss of life.
Dust devils are small, rotating columns of dust that form on hot, dry days. They are usually harmless, but can be dangerous if they pick up debris.
Waterspouts are small, rotating columns of water that form over water. They are similar to tornadoes, but are usually weaker.
Squall lines are bands of thunderstorms that can form along cold fronts. They can bring heavy rain, hail, and strong winds.
Cold fronts are boundaries between cold and warm air masses. They can bring showers, thunderstorms, and strong winds.
Warm fronts are boundaries between warm and cold air masses. They can bring clouds, rain, and drizzle.
Stationary fronts are boundaries between two air masses that are not moving relative to each other. They can bring clouds and Light rain.
Occluded fronts are formed when a cold front overtakes a warm front. They can bring clouds, rain, and strong winds.
Anafronts are warm fronts that are moving upslope. They can bring clouds, rain, and strong winds.
Katafronts are cold fronts that are moving downslope. They can bring clouds, rain, and strong winds.
Mesoscale convective complexes are large, rotating storms that form over land. They can bring heavy rain, hail, and strong winds.
Mesoscale convective vortices are small, rotating storms that form within thunderstorms. They can cause tornadoes.
Monsoon troughs are areas of low pressure that form near the equator. They can bring heavy rain and flooding.
Intertropical convergence zones are areas of low pressure that form near the equator. They are where the trade winds from the Northern and Southern Hemispheres meet.
Polar fronts are boundaries between cold polar air masses and warm tropical air masses. They form in the mid-latitudes.
Arctic fronts are boundaries between cold Arctic air masses and warm polar air masses. They form in the high latitudes.
Alpine fronts are boundaries between cold air masses that have descended from Mountains and warm air masses that have risen from the valleys. They form in mountainous regions.
Lee waves are waves that form on the leeward side of mountains. They can cause turbulence and downdrafts.
Orographic clouds are clouds that form when air is forced to rise over mountains. They can be cumulus, stratus, or lenticular clouds.
Lenticular clouds are lens-shaped clouds that form when air is forced to rise over mountains. They are often seen in the Alps.
Mammatus clouds are pouch-like clouds that hang from the underside of cumulonimbus clouds. They are often associated with thunderstorms.
Pileus clouds are small, cap-like clouds that form above cumulus clouds. They are often associated with thunderstorms.
Velum clouds are thin, veil-like clouds that form above cumulus clouds. They are often associated with thunderstorms.
Incus clouds are anvil-shaped clouds that form at the top of cumulonimbus clouds. They are often associated with thunderstorms.
Castellanus clouds are turret-shaped clouds that form at the top of cumulonimbus clouds. They are often associated with thunderstorms.
Floccus clouds are small, fluffy clouds that form at low altitudes. They are often seen on sunny days.
Stratus clouds are low, gray clouds that form when air is cooled and moistened. They can bring drizzle or light rain.

What is a cyclone?

A cyclone is a large, rotating storm with low pressure at its center. The winds around the center of a cyclone blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

What are the different types of cyclones?

There are three main types of cyclones: tropical cyclones, extratropical cyclones, and mesoscale cyclones.

Tropical cyclones are the most powerful type of cyclone. They form over warm ocean waters and have winds of at least 74 miles per hour. Tropical cyclones are also known as hurricanes, typhoons, and tropical storms.
Extratropical cyclones form over cool or cold ocean waters and have winds of at least 39 miles per hour. Extratropical cyclones are also known as midlatitude storms or low-pressure systems.
Mesoscale cyclones are the smallest type of cyclone. They form over land or water and have winds of at least 20 miles per hour. Mesoscale cyclones are also known as thunderstorms, squall lines, and tornadoes.

How do cyclones form?

Cyclones form when warm, moist air rises from the surface of the Earth. As the air rises, it cools and condenses, forming clouds and rain. The rising air creates an area of low pressure at the surface. This low pressure draws in more warm, moist air, which continues to rise and create clouds and rain. The cyclone continues to grow and intensify as long as there is a source of warm, moist air.

What are the effects of cyclones?

Cyclones can cause a variety of effects, including:

High winds: The strong winds of a cyclone can cause damage to buildings, trees, and other structures.
Heavy rain: The heavy rain of a cyclone can cause flooding, landslides, and mudslides.
Storm surge: The storm surge of a cyclone is a rise in sea level that can cause flooding along the coast.
Tornadoes: Some cyclones can produce tornadoes, which are violently rotating columns of air that can cause extensive damage.

How can people protect themselves from cyclones?

There are a number of things people can do to protect themselves from cyclones, including:

Have a plan: People should have a plan in place in case a cyclone strikes. This plan should include where they will go, how they will communicate with family and friends, and what supplies they will need.
Stay informed: People should stay informed about the latest weather forecasts and warnings.
Prepare their home: People should prepare their home for a cyclone by securing loose objects, clearing gutters, and making sure their home is in good repair.
Evacuate if necessary: If ordered to evacuate, people should do so immediately.

What is the future of cyclones?

The future of cyclones is uncertain. However, Climate Change is expected to make cyclones more frequent and more intense. This means that people living in areas that are prone to cyclones need to be prepared for the possibility of a major cyclone.

Which of the following is not a type of cyclone?
(A) Tropical cyclone
(B) Extratropical cyclone
(C) Polar cyclone
(D) Mesocyclone
Tropical cyclones form over warm ocean waters where the sea surface temperature is at least 26.5 degrees Celsius.
(A) True
(B) False
The strongest tropical cyclones are called hurricanes in the Atlantic Ocean, typhoons in the Pacific Ocean, and cyclones in the Indian Ocean.
(A) True
(B) False
The eye of a tropical cyclone is a region of calm weather at the center of the storm.
(A) True
(B) False
The strongest winds in a tropical cyclone occur near the eyewall, which is a ring of thunderstorms that surrounds the eye.
(A) True
(B) False
Extratropical cyclones form when cold air meets warm air.
(A) True
(B) False
Extratropical cyclones are typically much larger than tropical cyclones.
(A) True
(B) False
Extratropical cyclones can cause heavy rain, strong winds, and flooding.
(A) True
(B) False
Polar cyclones are a type of extratropical cyclone that form over polar regions.
(A) True
(B) False
Mesocyclones are small, rotating thunderstorms that can form tornadoes.
(A) True
(B) False