46 What is Cyclone, Different forms of cyclone, Distribution of cyclone


  • Cyclones are the most terrible storms on Earth. A cyclone is a system of counter-clock-wise circulation of winds around a core of low pressure (eye of a cyclone) in the Northern Hemisphere and opposite conditions in the southern hemisphere winds move clockwise direction around the eye of the cyclone (due to Coriolis effect).
  • Moving winds rise and cool, forming clouds and precipitation (rain). There are two types of cyclones: Mid-latitude cyclones and Tropical cyclones.
  • Mid-latitude cyclones are the main cause of mid-latitude winter storms.
  • Tropical cyclones are also known as hurricanes.
  • An anticyclone is the opposite of a cyclone. In the Northern Hemisphere, its wind moves clockwise around the center of high pressure. Air enters from above and falls on the ground. High-pressure centers usually have good weather.

As per the figure above mentation the situation of wind movement In a cyclone system, the low pressure is at the center which is known as the eye of the cyclone which results the pressure gradient being straight inward but in the case of an anticyclone wind system, the high pressure located is at the center, resulted the gradient is straight outward. Due to the rightward movement caused by Coriolis force and friction with the surface, the movement of the surface air deflects at an angle across the gradient, creating a counter-clockwise inspiraling motion and a clockwise out spiraling motion.


The above describe figure reflects the movement of wind in cyclone and anticyclone systems in the southern hemisphere of the earth. The condition of wind movement in the southern hemisphere for the cyclone is spiral will be clockwise due to the Coriolis effect deflect to the left. For anticyclones, the movement of the wind system is the opposite.

A General overview on Coriolis effect

Earth rotates from west to east, it will create a effect which is well known as the effect or force of Coriolis, cause to deflect winds and ocean currents to the right direction in the northern hemisphere and to left in the southern hemisphere. Tt was first introduced by the French scientist Gaspard-Gustave de Coriolis in 1835.

Coriolis Force

The above figure represents the concept of the Coriolis effect on the natural moveable things on the earth. The Blue arrows reflect the direction of starting motion, and the red arrows show the direction of motion apparent to the Earth observer. The Coriolis effect is strongest near the poles and decreases to zero at the Equator.

Mid-Latitude Cyclones

Mid-latitude cyclones, sometimes called extratropical cyclones, form at the polar front when the temperature difference between the two air masses is very large. It is mainly dominated in the middle and high latitudes. These air masses move in different directions from each other. The Coriolis effect (Northern Hemisphere) causes the circulation of the winds to turn to the right so that the circulation of the winds hits the polar front at an angle. Hot and cold fronts form next to each other.

Most mid-latitude winter storms, including much of the United States and Europe, are caused by mid-latitude cyclones. Warm air at the cold front rises and forms a low-pressure cell. The winds move to lower pressure and create an upward column of air. The wind rotates counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Since the rising air is humid, rain or snow.

Mid-latitude cyclones form in the mid-latitudes in winter and move eastward with westerly winds over Earth. These two- to five-day heavy storms can reach 1,000 to 2,500 km (625 to 1,600 mi), become smaller and larger in diameter, and produce winds of up to 125 km (75 mi) per hour. Like tropical cyclones, they can cause widespread beach erosion and flash floods.

Mid-latitude cyclones are particularly severe in the Mid-Atlantic and New England states where they are called noreasters because they come from the northeast. About 30 Nor’easters attack the region every year.

Tropical Cyclones

Tropical cyclones are one of the most destructive weather events and are also known as typhoons or hurricanes. Let us study in detail the tropical cyclone, how it is formed, and what are its effects through this article. Tropical cyclones are intense circular storms that occur in warm tropical oceans with speeds exceeding 119 kilometers per hour and are accompanied by heavy rainfall. Primarily, the greatest loss of life and property is not caused by wind, but by other secondary events including storm surges, floods, landslides, and tornadoes.

According to the origin, tropical cyclones are known by different names in the world. In the Atlantic Ocean and the eastern North Pacific Ocean, it is known as Hurricane. Hurricanes occur in tropical latitudes (between 10° and 25°N) in summer and autumn when sea surface temperatures are 28 °C (82 °F) or higher. Warm seas form a large humid air mass. Warm air rises and forms a low-pressure cell, known as a tropical depression. Thunderstorms form around a tropical depression. If the temperature reaches or exceeds 28 °C (82 °F), the air begins to revolve around the low pressure (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere).

As the air rises, water vapor condenses, releasing latent heat energy. With mild wind shear, a storm turns into a hurricane within two to three days. Hurricanes are huge with strong winds. The exception is a relatively calm storm center when the wind rises. Precipitation can reach 2.5 cm (1 inch) per hour, resulting in the release of about 20 billion metric tons of water per day during a storm. The release of latent heat generates an enormous amount of energy, roughly equal to the total annual electricity consumption in the United States during a hurricane. Hurricanes can also trigger tornadoes.

They are all tropical cyclones. Tropical cyclones are great masses of warm, humid, rotating air. They occur in all tropical oceans except the equatorial South Atlantic. Large tropical cyclones are called hurricanes (Hurricanes = the god of the wind of the Caribbean Taino people) in the North Atlantic and the eastern Pacific, Typhoons (Tai- Fung = Chinese god for great wind) in the western Pacific, tropical cyclones in the Indian Ocean and the Willi- Willis in the waters near Australia.

Hurricanes Cyclone

Hurricanes are huge with strong winds. The exception is a relatively calm storm center when the wind rises. Precipitation can reach 2.5 cm (1 inch) per hour, resulting in about 20 billion metric tons of water per day in the storm. The release of latent heat generates an enormous amount of energy, roughly equal to the total annual electricity consumption in the United States during a hurricane.

Hurricanes can also trigger tornadoes. Storms are strange creatures because they are deadly monsters, but they have a soft but cold heart. The anatomy of a hurricane is quite simple, although the processes involved are quite complex. When a low-pressure disturbance forms, warm humid air moves to low pressure, rising upward and forming a violent thunderstorm. The low-pressure disturbance is surrounded by a wall of clouds called the wall of eyes. The inside of the eyeball has the highest wind speed, the highest cloudiness, the lowest atmospheric pressure, and the most intense precipitation.

The center or heart of the storm is called the eye. In the center of the storm, there is a light breeze, minimal rainfall, and sometimes clear skies over the sky. It is a calm tropical storm area, but that is why it is so dangerous. Many people go outside when their eyes look up because they believe the storm is over. But some do not understand that the “second round” is coming from behind. Away from the eyewall, strong thunderstorms form a storm, called a spiral rain band, which wraps around the eyewall of the storm. First, these lines of rain Hurricanes are classified according to wind speed. The categories are listed on the Saffir–Simpson scale.

CategoryMPHEstimated Damage
1 (Weak)74–95Above normal; no read damage to structures
2 (Moderate)96–110Some property damage; considerable damage to vegetation
3 (Strong)111–130Some buildings were damaged; mobile homes destroyed
4 (Very strong)131–156Complete roof failure on small residences; major beach erosion of beach areas
5(Devastating)Over 156Complete roof and some building failure on most residential and industrial buildings

Typhoons Cyclone

  • A cyclone is a mature tropical cyclone that develops between 180° and 100° east longitude in the Northern Hemisphere.
  • This region is known as the Pacific Northwest Basin, and is the most active tropical cyclone basin on Earth, accounting for about a third of the world’s annual tropical cyclones. For organizational purposes, the North Pacific Ocean is divided into three regions: east (140° W to North America), central (140° W to 180 E), and west (180° to 100 E).
  • The Regional Specialized Meteorological Center (RSMC) for Tropical Cyclone Forecast is in Japan, and the Joint Typhoon Warning Center for the Pacific Northwest in Hawaii, the Philippines and Hong Kong.
  • While the RSMC names each system, the main rosters themselves are coordinated among the 18 countries whose territories are at risk of typhoons each year.

Willy-Willy Cyclones

Willy-Willy is a tropical cyclone in northwestern Australia. This is a storm that begins to spin to form a tornado. Willies are formed by local winds. There may be a storm or gust of wind that helps them. The rising air column is caused by local hotspots, which may be like roads.

Indian Cyclones

Cyclones are caused by atmospheric disturbances around low-pressure areas characterized by rapid and often destructive air circulation. Cyclones are usually accompanied by severe storms and bad weather. Air circulates in a counterclockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere. Cyclones are classified as

(i) extratropical cyclones (also called moderate cyclones).

(ii) tropical cyclones. The word Cyclone comes from the Greek word Cyclos which means snake scroll. It was created by Henry Paddington because tropical storms in the Bay of Bengal and the Arabian Sea look like coiled sea snakes.


Cyclones are classified as extratropical cyclones (also called moderate cyclones); and tropical cyclones. The World Meteorological Organization (WMO, 1976) uses the term ‘Tropical Cyclone’ to cover weather systems where winds exceed the ‘Gale Force’ (minimum 34 knots or 63 kph). Tropical cyclones are derivatives of the oceans and atmosphere, which are powered by heat from the ocean; and driven by temperate east and west trade, high planetary winds, and their ferocious energy. In India, cyclones are classified by:

  • Related wind power
  • Storm waves
  • Incredible rainfall events.

Indian Meteorological Department

The following criteria have been prepared by the Indian Meteorological Department (IMD), which classifies low-pressure systems in the Bay of Bengal and Arabian Sea based on potential damage adopted by WMO.

Type of DisturbancesWind Speed in Km/hK
Low PressureLess than 31Less than 17
Deep Depression49-6127-33
Cyclonic Storm61-8833-47
Severe Cyclonic Storm88-11747-63
Super CycloneMore than 221More than 120

Effects of Tropical Cyclone

As discussed above the effects of a tropical cyclone and the expected damage depend not only on wind speed but also on other factors including speed, duration of strong wind and precipitation during and after rainfall, direction, and intensity of movement. Sudden changes in structure, like the size and intensity of tropical cyclones, and also affect human response to tropical cyclone disasters. Tropical cyclones directly or indirectly affect health in several ways:

– It aggravates cases of drowning and other physical trauma.

– It also increases the risk of water and vector-borne infectious diseases.

– Enhances the psychological effects which are also related to emergencies.

– Disrupts health systems, facilities, and services. This would make it difficult for communities to access health care when they need it most.

– Damage to basic facilities including food and water supplies and safe shelter.

It is also said that the impact of a tropical cyclone also depends on the number of people living in the low-lying areas within the direct path of the storm, the built environment like the design of the building, and the adequate time for warning and evacuation. Flooding and seawater flow due to tropical cyclones increases the risk of drowning and water or vector-borne diseases.

Floodwaters can contain sewage and chemicals, hide sharp objects made of metals or glass, and power lines or dangerous snakes or reptiles can also fall into the stream, resulting in injury, electrocution, bites, and even diseases. can. According to the WHO, about 726 million people were affected by hurricanes worldwide between 1998-2017.

How do Tropical Cyclones work?

Tropical cyclone winds operate in the central region of atmospheric pressure. The wind is caused by a low-pressure core and Earth’s rotation, which additionally deflects the air path due to a phenomenon known as the Coriolis force. In the Northern Hemisphere, a tropical cyclone rotates counterclockwise or in a cyclonic direction, and the Southern Hemisphere it rotates in a clockwise or anticyclonic direction.

How do Tropical Cyclones form?

Tropical cyclones form when water vapor and heat are transferred from a warm ocean to evaporating air, primarily through evaporation from the ocean surface. As we know, hot humid air rises, expands, and cools, saturates quickly, and releases latent heat due to the condensation of water vapor. In this process, the air beneath the developing condyle becomes hot and humid. This creates a difference in temperature between the warm air and the cold atmosphere, which enriches the air and pushes it upwards.

If the ocean surface is too cold, the heat will not be enough and the rate of evaporation will be low to promote a tropical cyclone. Even the insufficient depth of the hot water layer will reduce the energy supply. This is because the tropical system is changing the underlying ocean. If the storm generates more disturbance, the ocean surface becomes cooler with rain from deep concentrated clouds and strong winds in the center. If this resulting mixture carried cold water from below the surface to the surface, the fuel supply for the tropical system would be reduced or stopped.

Rising air warms the core, removes latent heat and direct heat transfer from the ocean surface, and decreases atmospheric pressure at the center of the turbulence. As a result, the pressure decreases and causes an increase in surface air, which, in turn, increases steam and heat transfer, and also promotes the rise of air. The core heats up and the amplification of surface air reinforces or reinforces each other in a positive feedback mechanism.

Blizzards and Lake Effects

blizzard is distinguished by certain conditions:

  • Temperature is below -7 ° C (20 ° F); -12oC (10 ° F) for severe blizzard.
  • Wind speed exceeds 56 km / h (35 mph); 72 km / h (45 mph) in severe blizzard.
  • Snow is so heavy that visibility is impaired by 2/5 km (1/4 mile) or at least three hours; Almost zero visibility due to a strong blizzard.
  • Blizzards occur in mid-latitudes and closer to the poles, usually as part of a mid-latitude cyclone. Snowstorms most often occur in winter, when the jet stream has moved south and cold northern air mass comes into contact with warm semi-tropical air mass. Very strong winds occur because of the pressure gradient between the low pressure storm and the high pressure west of the storm. The snow generated by the storm is caught by the winds and moves almost horizontally. Snowstorms can also cause sleet or freezing rain. In winter, continental polar air masses descend from Canada. When cold air crosses one of the Great Lakes, it heats up and absorbs moisture. When the air mass reached the shore of the lake, it was very unstable and a large amount of snow fell. This lake-like snow falls in the most snow-covered metropolitan areas in the United States: Buffalo and Rochester, New York.

Heat Wave

Even more deadly is a seasonal phenomenon – heat waves. Heatwaves are different for different places; It is a period of hot weather that extends to at least 86 °F (30 °C) for at least three days in colder places, but longer in warmer places. In recent years, there has been an increase in the frequency and duration of heatwaves.

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