Tropical cyclone

Tropical Cyclone – UPSC Indian Geography Notes

In this piece, you will explore the Tropical Cyclone, understanding the favorable conditions leading to its formation, the distinct stages of formation, as well as delving into its structure and characteristics. A Tropical Cyclone is a meteorological occurrence characterized by a swiftly rotating storm system featuring a low-pressure center, potent winds, and intense thunderstorms resulting in substantial rainfall.

Tropical Cyclone

Tropical cyclones, formidable storms born in tropical oceanic regions, traverse towards coastal zones, inflicting widespread devastation through formidable forces like violent winds (squalls), torrential rainfall, and storm surge.

These tempestuous events manifest as irregular wind movements, showcasing a closed air circulation around a central low-pressure center. The genesis of this circulatory motion lies in the rapid ascent of hot air, subject to the influential Coriolis force. The resulting low pressure at the core dictates the intensity of the accompanying wind speeds.

Squall refers to an unexpected and violent gust of wind or a concentrated storm, typically accompanied by precipitation such as rain, snow, or sleet.

A torrent signifies a robust and swiftly flowing stream of water or any other liquid.

  • Cyclonic wind movements exhibit an anti-clockwise direction in the northern hemisphere and a clockwise direction in the southern hemisphere, a phenomenon attributed to the Coriolis force.
  • The presence of an anticyclone is a distinctive feature often observed between two cyclones.
  • Tropical cyclones materialize within the equatorial region, spanning from 5° to 30°, with nomenclature variations based on their global origin.
  • On average, a tropical cyclone can cover approximately 300 to 400 miles per day, reaching a total distance of around 3,000 miles before dissipating.

Conditions Favorable for Tropical Cyclone Formation

  1. Expansive sea surface maintaining a temperature surpassing 27°C.
  2. Sufficient Coriolis force presence, inducing the creation of a cyclonic vortex.
  3. Minimal fluctuations in vertical wind speed.
  4. Existence of a preceding weak low-pressure area or low-level cyclonic circulation.
  5. Upper-level divergence above the sea-level system.

Good Source of Latent Heat

  • Ocean waters with temperatures ≥ 27°C serve as the moisture source for storms.
  • Condensation of moisture releases latent heat, driving the storm’s formation.
  • Warm water depth (26-27°C) should extend 60-70 m from ocean surface to prevent mixing with cooler water below.
  • Western tropical oceans exhibit this condition due to warm ocean currents generated by easterly trade winds, forming a thick layer with temperatures > 27°C.
  • Cold currents in eastern tropical oceans lower surface temperatures, rendering them unsuitable for cyclonic storm breeding.

Coriolis Force (f)

  • The Coriolis force is absent at the equator, leading to no cyclones due to its zero value.
  • The Coriolis force increases with latitude, becoming significant at and capable of generating a storm [cyclonic vortex].
  • Approximately 65 percent of cyclonic activity takes place within the latitude range of 10° to 20°.

Low-level Disturbances

  • The origin of cyclones involves a precursor – low-level disturbance (thunderstorms), acting as the seeds of cyclones, typically in the form of easterly wave disturbances within the Inter-Tropical Convergence Zone (ITCZ).
  • Local variations in water and air temperatures result in small low-pressure centers, where a weak cyclonic circulation emerges.
  • Rapid development of a true cyclonic vortex occurs due to the rising warm, humid air. However, only a select few of these disturbances progress into full-fledged cyclones.
  • The process involves a cycle: rising humid air triggers an adiabatic lapse rate, leading to a temperature drop and moisture condensation. The release of latent heat of condensation further heats and lightens the air, causing uplift. This cycle continues, with more moisture available for condensation.

Temperature contrast between air masses

  • Trade winds from both hemispheres converge along the inter-tropical front.
  • Temperature contrasts between these air masses are crucial, especially when the ITCZ is farthest from the equator.
  • The convergence of air masses with differing temperatures and the ensuing instability are essential conditions for the initiation and intensification of violent tropical storms.

Upper Air Disturbance

  • The remnants of an upper tropospheric cyclone from the Westerlies migrate deeply into tropical latitudes.
  • Divergence dominates on the eastern side of the troughs, fostering a rising motion that sparks the formation of thunderstorms.
  • These discarded troughs, remnants of temperate cyclones, typically possess cold cores, indicating a steeper and unstable environmental lapse rate below. This instability serves as a catalyst for the development of thunderstorms (child cyclones).

Wind Shear

  • Wind Shear refers to variations in wind speeds at different heights.
  • Tropical cyclones emerge under conditions of uniform wind.
  • Cyclone formation is constrained to latitudes equatorward of the subtropical jet stream due to weak vertical wind shear.
  • In temperate regions, the presence of high wind shear caused by westerlies acts as an inhibiting factor for the formation of convective cyclones.

Upper Tropospheric Divergence

Well-developed divergence in the upper layers of the atmosphere is crucial. This ensures that rising air currents within the cyclone are continuously expelled, maintaining a low pressure at the center.

Humidity Factor

  • High humidity (approximately 50 to 60 percent) is essential in the mid-troposphere, facilitating the formation of cumulonimbus clouds in the presence of moist air.
  • These conditions prevail over the equatorial doldrums, particularly on the western margins of oceans. This is attributed to the east to west movement of ocean currents, which continuously replenish the air with substantial moisture, creating favorable conditions for cyclone development.

Origin and Development of Tropical Cyclones

  • Tropical cyclones: Thermal origin, form over tropical seas in late summers (August to mid-November).
  • Strong local convectional currents: Acquire a whirling motion due to the Coriolis force.
  • Cyclones development: Advance until they locate a weak spot in the trade wind belt.

Origin

  • Favorable conditions: Multiple thunderstorms form over the oceans.
  • Thunderstorms merge: Create an intense low pressure system (wind is warm and lighter).

Early Stage

  • In the thunderstorm, air is uplifted as it is warm and light.
  • At a certain height, due to lapse rate and adiabatic lapse rate, air temperature falls, and moisture undergoes condensation.
  • Condensation releases latent heat of condensation: Air becomes warmer and lighter, further uplifted.
  • Space filled with fresh moisture-laden air. Condensation occurs, cycle repeats with ongoing moisture supply.
  • Due to excess moisture over oceans, thunderstorm intensifies, sucking in air at a much faster rate.
  • Surrounding air rushes in, undergoes deflection due to Coriolis force, creating a cyclonic vortex (spiraling air column, similar to a tornado).
  • Due to centripetal acceleration (force pulling towards the center countered by centrifugal force), air in the vortex forms a region of calmness – an eye at the center of the cyclone.
  • Inner surface of the vortex forms the eyewall, the most violent region of the cyclone.
  • Wind carried upwards loses moisture, becomes cold and dense. Descends through the cylindrical eye region and at the edges of the cyclone.
  • Continuous supply of moisture from the sea is the major driving force behind every cyclone.
  • On reaching land, moisture supply is cut off, and the storm dissipates.
  • If the ocean can supply more moisture, the storm will reach a mature stage.

Mature stage

  • At this stage, spiraling winds create multiple convective cells with successive calm and violent regions.
  • Regions with cumulonimbus cloud formation (rising limbs of convective cell) are called rain bands, where intense rainfall occurs.
  • Ascending air loses moisture, descends through calm regions (descending limbs of convection cell – subsiding air) between two rain bands.
  • Cloud formation is dense at the center, decreasing from center to periphery.
  • Rain bands mostly consist of cumulonimbus clouds; those at the periphery have nimbostratus and cumulus clouds.
  • Dense overcast at upper troposphere due to cirrus clouds, mostly composed of hexagonal ice crystals.
  • Dry air flowing along central dense overcast descends at the periphery and the eye region.

Structure of a Tropical Cyclone

Eye

  • A mature tropical cyclone is characterized by strong spirally circulating wind around the center, known as the eye.
  • The “eye” is a roughly circular area with comparatively light winds, clear skies, and fair weather at the center of a severe tropical cyclone.
  • Little or no precipitation; sometimes blue sky or stars are visible.
  • The eye has the lowest surface pressure and warmest temperatures aloft (in the upper levels).
  • Eye temperature may be 10°C warmer or more at an altitude of 12 km than the surrounding environment, but only 0-2°C warmer at the surface in the tropical cyclone.
  • Eyes range in size from 8 km to over 200 km across, but most are approximately 30-60 km in diameter.

Eye Wall

  • The eye is surrounded by the “eyewall”, a roughly circular ring of deep convection, the area with the highest surface winds in the tropical cyclone.
  • The Eyewall region experiences maximum sustained winds, the fastest winds in a cyclone occur along this region.
  • The eye consists of sinking air, while the eyewall has a net upward flow due to moderate to occasionally strong convection.
  • Eye’s warm temperatures result from compressional warming (adiabatic) of the subsiding air.
  • Soundings within the eye reveal a low-level moist layer with an inversion above, suggesting sinking typically does not reach the ocean surface but only gets to around 1-3 km of the surface.
  • Maximum wind velocity and torrential rain occur in this region.
  • From the eyewall, rain bands may radiate, and trains of cumulus and cumulonimbus clouds may drift into the outer region.

Spiral bands

  • Another feature of tropical cyclones likely influencing the formation and maintenance of the eye is the eyewall convection.
  • In tropical cyclones, convection is organized into long, narrow rain bands aligned with the horizontal wind.
  • These bands, spiraling into the cyclone center, are termed “spiral bands.”
  • Alongside these bands, low-level convergence is maximum, leading to pronounced upper-level divergence above.
  • A direct circulation forms: warm, moist air converges at the surface, ascends through bands, diverges aloft, and descends on both sides.
  • Subsidence occurs widely outside the rain band but is concentrated in a small inside area.
  • As air subsides, adiabatic warming and drying take place.
  • Concentrated subsidence on the inside causes stronger adiabatic warming inward, creating a sharp pressure contrast across the band.
  • Pressure falls on the inside, causing an increase in tangential winds around the tropical cyclone due to the heightened pressure gradient.
  • Eventually, the band moves toward the center, encircling it, and the eye and eyewall form.
  • The cloud-free eye may result from a combination of dynamically forced centrifuging of mass out of the eye into the eyewall and forced descent due to moist convection in the eyewall.

Vertical Structure of a Tropical Cyclone

  • The vertical structure of tropical cyclones comprises three divisions.
  • The lowest layer, up to 3 km, known as the inflow layer, drives the storm.
  • The middle layer, spanning 3 km to 7 km, is where the primary cyclonic storm occurs.
  • The outflow layer, above 7 km, with maximum outflow at 12 km and above, features an anticyclonic movement of air.

Categories of Tropical Cyclones

  • The Bureau of Meteorology utilizes the following tropical cyclone category system:
  • Category one (tropical cyclone): Strongest winds are GALES with typical gusts over open flat land of 90-125kph.
  • Category two (tropical cyclone): Strongest winds are DESTRUCTIVE with typical gusts over open flat land of 125-164kph.
  • Category three (severe tropical cyclone): Strongest winds are VERY DESTRUCTIVE with typical gusts over open flat land of 165-224kph.
  • Category four (severe tropical cyclone): Strongest winds are VERY DESTRUCTIVE with typical gusts over open flat land of 225-279kph.
  • Category five (severe tropical cyclone): Strongest winds are VERY DESTRUCTIVE with typical gusts over open flat land of more than 280kph.

Favorite Breeding Grounds for Tropical Cyclones

  • In the south-east Caribbean region, they are referred to as hurricanes.
  • In the Philippines islands, eastern China, and Japan, they are known as typhoons.
  • In the Bay of Bengal and the Arabian Sea, they are termed cyclones.
  • Around the south-east African coast and Madagascar-Mauritius islands.
  • In north-west Australia.

Regional names for Tropical Cyclones

RegionsWhat they are called
Indian OceanCyclones
AtlanticHurricanes
Western Pacific and South China SeaTyphoons
Western AustraliaWilly-willies

Characteristics of Tropical Cyclones

The primary features of tropical cyclones include the following.

  • Size and Shape Tropical cyclones exhibit symmetrical elliptical shapes with a 2:3 ratio of length to breadth and steep pressure gradients. They maintain a compact size, ranging from 80 km near the center to potentially expanding between 300 km and 1500 km.
  • Wind Velocity and Strength In tropical cyclones, wind velocity is higher in poleward margins than at the center and is more pronounced over oceans than landmasses, which are scattered with physical barriers. Wind speeds can range from nil to 1200 km per hour.
  • Path of Tropical Cyclones These cyclones initiate with a westward movement but gradually shift northwards around 20° latitude. Further, they turn northeastwards at approximately 25° latitude and then eastwards around 30° latitude. Subsequently, they lose energy and subside. Tropical cyclones follow a parabolic path with the axis parallel to the isobars. The path is influenced by the Coriolis force, Earth’s rotation, and the interaction of easterly and westerly winds.
  • End of Tropical Cyclones Tropical cyclones reach their end at 30° latitude due to cool ocean waters and increasing wind shear caused by westerlies.

Warning of Tropical Cyclones

Detection of Unusual Weather Phenomena Leading to Cyclones – The detection of unusual phenomena in the weather that may result in cyclones relies on three crucial parameters: a fall in pressure, an increase in wind velocity, and the direction and movement (track) of the storm.

A global network of weather stations systematically monitors the fall in pressure and wind velocities across all countries, encompassing even the Arctic and Antarctic regions. Notably, islands play a vital role in facilitating the monitoring of these weather developments.

In India, the use of detection radars along both coasts enhances the country’s ability to monitor cyclones. Additionally, aircraft equipped with various instruments, including weather radar, contribute to monitoring efforts.

Satellites, equipped with high-resolution radiometers, play a pivotal role in cyclone monitoring. These satellites operate in both the visual and infra-red regions (for night view) of the spectrum, providing detailed images of cloud cover and its structure.

The integration of remote sensing technologies, including radars, aircraft, and satellites, aids in predicting the precise trajectory of the cyclone. This information is crucial for taking proactive measures in various areas, such as:

  • Closing of ports and harbors
  • Suspension of fishing activities
  • Evacuation of the population
  • Stocking of food and drinking water
  • Provision of shelter with sanitation facilities (safety homes)

Remarkably, advancements in technology enable the detection of a cyclone right from its genesis in the high seas, allowing for continuous monitoring and providing a warning at least 48 hours prior to a potential cyclone strike.

However, it’s noteworthy that predictions made only 12 hours in advance of a storm’s course lack a very high rate of precision.

Major Differences between Temperate Cyclone and Tropical Cyclone

Tropical CycloneTemperate Cyclone
Movement: Move from east to westMovement: Move from west to east
Affected Area: SmallerAffected Area: Larger
Wind Velocity: Higher and damagingAir Velocity: Comparatively lower
Formation: Requires seas with temperature > 26-27°C and dissipates on reaching landFormation: Can form on both land and sea
Duration: Doesn’t last more than 7 daysDuration: Can last for 15 to 20 days

In summary, tropical cyclones exhibit an east-to-west movement, affect smaller areas with higher wind velocity, form in seas with temperatures above 26-27°C, and last no more than 7 days. On the other hand, temperate cyclones move west-to-east, affect larger areas with lower air velocity, can form on both land and sea, and have a longer duration of 15 to 20 days.

Tornado

A tornado is an intensely rotating column of air extending from a thunderstorm to the ground, characterized by a vortex of swiftly moving air. The genesis of a tornado is rooted in alterations in wind speed and direction that generate a horizontal spinning effect within a storm cell. This horizontal spin is subsequently transformed into a vertical orientation as rising air ascends through the thunderclouds.

  • Winds within the tornado funnel can exceed 500 kmph.
  • The primary cause of damage in these weather events is the presence of high-velocity winds.
  • Tornadoes also inflict damage through reductions in air pressure.
  • The air pressure at the tornado center is around 800 millibars (average sea-level pressure is 1013 millibars), leading to the collapse of many human-made structures due to significant pressure drops.

Origin of Tornado

Tornado formation typically hinges on four key ingredients: shear, lift, instability, and moisture.

  • Wind shear stands out as the crucial factor in tornado creation, often manifesting as winds rolling into a horizontal column of air.
  • A robust updraft of air, transporting from the ground to the atmosphere, transforms this column into a vertical orientation, marking the initiation of storm development.
  • The evolving storm frequently matures into a supercell thunderstorm, characterized by its rotation. Supercells are distinct, isolated cells not forming part of a storm line.
  • The conjunction of the vertical, rotating column of air and the supercell thunderstorm provides conditions conducive to the emergence of a tornado from the storm cloud.

Tornadoes predominantly occur in spring, with a lower incidence in winter. The seasons of spring and fall witness heightened activity due to the presence of stronger winds, wind shear, and atmospheric instability. Additionally, the occurrence of tornadoes is significantly influenced by the time of day, particularly in response to solar heating.

Distribution of tornadoes

  • Thunderstorms are rare in polar regions and infrequent at latitudes higher than 50° N and 50° S.
  • The most prone regions to thunderstorms are the temperate and tropical regions.
  • Tornadoes have been reported on all continents except Antarctica.
  • The United States experiences the most violent tornadoes globally.
  • Canada follows as the country with the second-largest number of tornadoes.
  • In the Indian sub-continent, Bangladesh stands out as the most prone country to tornadoes.
  • Globally, at any given moment, there are approximately 1,800 thunderstorms in progress.

Differences between Tornado and Cyclone

HeadsTornadoCyclone
DefinitionA rotating column of air, ranging in width from a few yards to more than a mile, whirling at destructively high speeds. Usually accompanied by a funnel-shaped downward extension of a cumulonimbus cloud. Winds range from 40-300+ mph.An atmospheric system of rapidly circulating air massed about a low-pressure center. Usually accompanied by stormy, often destructive weather. Storms in the Southern Pacific are called cyclones.
RotationClockwise in the southern hemisphere and counterclockwise in the northern hemisphere.Clockwise in the southern hemisphere and counterclockwise in the northern hemisphere.
Forms of PrecipitationRainRain, sleet, and hail
FrequencyThe United States records about 1200 tornadoes per year. The Netherlands has the highest number of tornadoes per area compared to other countries. Tornadoes are common in spring and fall but less common in winters.10-14 per year
LocationTornadoes have been spotted on all continents except Antarctica.Southern Pacific Ocean, Indian Ocean. Cyclones in the northwest Pacific that reach (exceed) 74 mph are called “typhoons.”
OccurrencePlaces where cold and warm fronts converge. Can occur almost anywhere.Warm areas

In India, both tornadoes and cyclones make appearances, but with a stark difference in their frequency. While cyclones find their origin in the Bay of Bengal and the Arabian Sea, the occurrence of tornado outbreaks is notably infrequent.

Tornadoes, albeit of weak strength, manifest in the north-western and north-eastern regions of the country, leading to significant damage to both man and material.

Conclusion

In the atmospheric theater of India, the presence of both tornadoes and cyclones adds a layer of complexity to the country’s weather dynamics. While cyclones emerge from the maritime realms of the Bay of Bengal and the Arabian Sea, the infrequent yet impactful tornado outbreaks unfold in the north-western and north-eastern regions. The discrepancy in their frequencies underscores the distinct challenges posed by these atmospheric phenomena. As India continues to grapple with the nuances of its weather patterns, understanding the unique characteristics of tornadoes and cyclones becomes paramount for effective preparedness and mitigation strategies in the face of nature’s dynamic forces.

FAQs on Tropical Cyclone

Q1: What initiates the formation of a tropical cyclone?

Tropical cyclones, hurricanes, or typhoons come into existence when convection prompts the ascent of warm, moist air above the ocean. Originating as a cluster of storms, these phenomena gain momentum when the ocean’s temperature reaches 80 °F (27 °C) or higher. The Earth’s rotation induces the Coriolis effect, setting the winds into rotation. Fueled by warm, humid ocean air, tropical cyclones typically move westward in the tropics, occasionally shifting north or south into the temperate zone. Once making landfall, they gradually lose strength, ultimately dissipating over extended periods on land or cooler ocean waters.

Q2: Where do tropical cyclones predominantly occur?

Tropical cyclones are prevalent around the equator, spanning the range of 5 ° to 30 ° latitude. Their trajectory initially trends westward due to easterly winds and may deviate slightly towards the poles.

Q3: What are the distinct types of cyclones?

There are two primary types of cyclones: middle latitude (mid-latitude) cyclones, responsible for winter storms in mid-latitudes, and tropical cyclones, commonly referred to as hurricanes. Conversely, an anticyclone represents the opposing counterpart of a cyclone.

Q4: What are the five categories defining tropical cyclones?

  • Category One (Tropical Cyclone): Characterized by gales, with typical gusts over open flat land ranging from 90-125kph.
  • Category Two (Tropical Cyclone): Features destructive winds, with typical gusts over open flat land measuring 125-164kph.
  • Category Three (Severe Tropical Cyclone): Showcases very destructive winds, with typical gusts over open flat land ranging from 165-224kph.
  • Category Four (Severe Tropical Cyclone): Exhibits very destructive winds, with typical gusts over open flat land measuring 225-279kph.
  • Category Five (Severe Tropical Cyclone): Boasts very destructive winds, with typical gusts over open flat land exceeding 280kph.

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