• A tropical cyclone is a weather phenomenon that is essentially a rapidly rotating storm system with characteristics such as a low-pressure center, strong winds and thunderstorms that produce heavy rain, among others.

Tropical Cyclones

• Tropical cyclones are violent storms that originate over oceans in tropical areas.
• They move over to the coastal areas and cause large-scale destruction due to violent winds (squalls), very heavy rainfall (torrential rainfall), and storm surge.
• They involve irregular wind movements with closed air circulation around a low-pressure center.
• The closed air circulation (whirling motion) is a result of rapid upward movement of hot air subjected to Coriolis force.
• The low pressure at the center is responsible for the wind speeds.

• Squall is a sudden violent gust of wind or localized storm, especially one bringing rain, snow, or sleet.
• Torrent is a strong and fast-moving stream of water or other liquid.
• Cyclonic wind movements are anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere due to Coriolis force.
• Cyclones are often characterized by the existence of an anticyclone between two cyclones.
• Tropical cyclones occur around the equator at 5°-30° and have varying names depending on where they form.
• An average tropical cyclone can travel about 300 to 400 miles a day or about 3,000 miles before it dies out.

Conditions Favourable for Tropical Cyclone Formation

• Good Source of Latent Heat:
  • Ocean waters with temperatures of 27°C or more provide moisture for storms.
  • The condensation of moisture releases enough latent heat of condensation to drive the storm.
  • Warm water depth should extend for 60-70 m from the surface of the ocean/sea to avoid churning and mixing of cooler water.
  • Western tropical oceans have warm ocean currents, which form a thick layer of water with temperatures greater than 27°C, providing enough moisture for storms.
  • Cold currents lower surface temperatures of eastern parts of tropical oceans, making them unsuitable for cyclonic storms.
• Coriolis Force:
  • The Coriolis force is zero at the equator, but it increases with latitude, creating a cyclonic vortex.
  • About 65% of cyclonic activity occurs between 10° and 20° latitude.
• Low-level Disturbances:
  • Low-level disturbances in the form of easterly wave disturbances in the Inter-Tropical Convergence Zone (ITCZ) should pre-exist for cyclone formation.
  • Small local differences in temperature of water and air produce low-pressure centers of small size.
  • A weak cyclonic circulation develops around these areas due to rising warm humid air.
  • A true cyclonic vortex may develop very rapidly due to rising warm humid air.
  • Few of these disturbances develop into cyclones.
  • Rising of humid air leads to adiabatic lapse rate, fall in temperature of air, and condensation of moisture in air.
  • Latent heat of condensation released, air gets more hot and lighter, air is further uplifted, more air comes in to fill the gap, new moisture available for condensation, latent heat of condensation, and the cycle repeats.
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• Temperature contrast between air masses:
  • Trade winds from both hemispheres meet along the inter-tropical front
  • Temperature contrasts between these air masses must exist when the ITCZ is farthest from the equator
  • Convergence of air masses of different temperatures and resulting instability are prerequisites for violent tropical storms
• Upper Air Disturbance:
  • Remains of an upper tropospheric cyclone from the Westerlies move deep into tropical latitude regions
  • Divergence prevails on the eastern side of the troughs, leading to the development of thunderstorms
  • Old abandoned troughs (remnants of temperate cyclones) usually have cold cores
  • Environmental lapse rate is steeper and unstable below these troughs, encouraging thunderstorms (child cyclones)
• Wind Shear:
  • Wind shear is the differences between wind speeds at different heights.
  • Tropical cyclones develop when the wind is uniform.
  • Weak vertical wind shear is necessary for cyclone formation, limited to latitude equatorward of the subtropical jet stream.
  • Wind shear is high in the temperate regions due to westerlies and this inhibits convective cyclone formation.
• Upper Tropospheric Divergence:
  • A well-developed upper tropospheric divergence is necessary so that the rising air currents within the cyclone continue to be pumped out and a low pressure maintained at the center.
  • 
• Humidity Factor:
  • High humidity (around 50 to 60 percent) is required in the mid-troposphere since the presence of moist air leads to the formation of cumulonimbus clouds.
  • Such conditions exist over the equatorial doldrums, especially in western margins of oceans which have great moisture-carrying capacity because the trade winds continuously replace the saturated air.

Origin and Development of Tropical Cyclones

• The tropical cyclones have a thermal origin, and they develop over tropical seas during late summers (August to mid-November).
• At these locations, the strong local convectional currents acquire a whirling motion because of the Coriolis force.
• After developing, these cyclones advance till they find a weak spot in the trade wind belt.

Origin

• Under favorable conditions, multiple thunderstorms originate over the oceans. These thunderstorms merge and create an intense low pressure system (wind is warm and lighter).
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Early stage

• Warm and light air is uplifted in a thunderstorm and condensation occurs as the temperature falls at a certain height, releasing latent heat of condensation and making the air even lighter and more uplifted.
• Fresh moisture-laden air fills the space and the cycle continues as long as there is moisture supplied.
• Excess moisture over oceans intensifies the thunderstorm and creates a spiraling air column or cyclonic vortex, similar to a tornado, due to the Coriolis force.
• The air in the vortex forms a calm region called the eye at the center of the cyclone due to centripetal acceleration and the inner surface of the vortex forms the eyewall, the most violent region of the cyclone.
• 
• The tangential force acting on wind following a curvy path creates the eye.
• All the wind that is carried upwards loses its moisture and becomes cold and dense. It descends to the surface through the eye region and at the edges of the cyclone.
• A continuous supply of moisture from the sea is the major driving force behind every cyclone, and the storm dissipates when the moisture supply is cut off on reaching the land.
• 
• If the ocean can supply more moisture, the storm can reach a mature stage.

Mature stage

• Spiraling winds create multiple convective cells with calm and violent regions.
• Regions with cumulonimbus cloud formation are called rain bands and have intense rainfall.
• Ascending air loses moisture and descends back to the surface through calm regions between two rain bands.
• Cloud formation is dense at the center and decreases in size towards the periphery.
• Rain bands are mostly made up of cumulonimbus clouds, with nimbostratus and cumulus clouds at the periphery.
• The dense overcast at the upper troposphere is due to cirrus clouds made up of hexagonal ice crystals.
• Dry air flowing along the central dense overcast descends at the periphery and the eye region.

Structure of a tropical cyclone

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Eye

• A mature tropical cyclone has a strong spirally circulating wind around the center called the eye.
• The eye is a circular area of light winds, clear skies, and fair weather found at the center of a severe tropical cyclone.
• There is little or no precipitation, and sometimes blue sky or stars can be seen.
• The eye is the region of lowest surface pressure and warmest temperatures aloft.
• The 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.
• The size of the eye can range 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, which is the area of highest surface winds in a tropical cyclone.
• The eyewall region also sees the maximum sustained winds and fastest winds in a cyclone.
• The eyewall has a net upward flow due to many moderate to occasionally strong updrafts.
• The eye’s warm temperatures are due to compressional warming of the subsiding air.
• Most soundings taken within the eye show a relatively moist low-level layer with an inversion above.
• The sinking in the eye typically does not reach the ocean surface but only gets to around 1-3 km of the surface.
• The wind reaches maximum velocity in the eyewall region, and torrential rain occurs here.
• From the eyewall, rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region.

Spiral bands

• Tropical cyclones have organized convection in long, narrow rain bands that spiral into the center of the cyclone.
• The bands are oriented in the same direction as the horizontal wind and are called “spiral bands.”
• Warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands.
• Subsidence is distributed over a wide area on the outside of the rain band but is concentrated in the small inside area.
• As the air subsides, adiabatic warming takes place, and the air dries.
• The pressure falls across the band since warm air is lighter than cold air, causing a sharp contrast in pressure to fall across the band.
• The pressure falls on the inside, and the tangential winds around the tropical cyclone increase due to the increased pressure gradient.
• The band eventually moves toward the center, encircles it, and the eye and eye wall form.
• The cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eyewall and to a forced descent caused by the moist convection of the eyewall.

Vertical Structure of a Tropical Cyclone

• There are three divisions in the vertical structure of tropical cyclones.
  • The lowest layer, extending up to 3 km and known as the inflow layer, is responsible for driving the storm.
  • The middle layer, extending from 3 km to 7 km, is where the main cyclonic storm takes place.
  • The outflow layer lies above 7 km. The maximum outflow is found at 12 km and above. The movement of air is anticyclonic in nature.

Categories of Tropical Cyclones

This is the tropical cyclone category system as used by the Bureau of Meteorology:

• Category 1 (tropical cyclone): Strongest winds are GALES with typical gusts over open flat land of 90-125kph
• Category 2 (tropical cyclone): Strongest winds are DESTRUCTIVE with typical gusts over open flat land of 125-164kph
• Category 3 (severe tropical cyclone): Strongest winds are VERY DESTRUCTIVE with typical gusts over open flat land of 165-224kph
• Category 4 (severe tropical cyclone): Strongest winds are VERY DESTRUCTIVE with typical gusts over open flat land of 225-279kph
• Category 5 (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

• South-east Caribbean region where they are called hurricanes.
• Philippines islands, eastern China, and Japan where they are called typhoons.
• The Bay of Bengal and the Arabian Sea where they are called cyclones.
• Around the south-east African coast and Madagascar-Mauritius islands.
• North-west Australia.
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Regional names for Tropical Cyclones

• Indian Ocean – Cyclones
• Atlantic – Hurricanes
• Western Pacific and South China Sea – Typhoons
• Western Australia – Willy-willies

Characteristics of Tropical Cyclones

• Tropical cyclones have symmetrical elliptical shapes with a 2:3 ratio of length and breadth.
• They have steep pressure gradients and are compact in size, ranging from 80 km to 1500 km.
• Wind velocity is higher in poleward margins than at the center and is higher over oceans than over landmasses.
• Wind velocity can range from nil to 1200 km per hour.
• Tropical cyclones start with a westward movement, then turn northwards around 20° latitude, further north-eastwards around 25° latitude, and then eastwards around 30° latitude before losing energy and subsiding.
• Their path follows a parabolic shape, parallel to the isobars and influenced by the Coriolis force, easterly and westerly winds.
• Tropical cyclones die at 30° latitude due to cool ocean waters and increasing wind shear caused by westerlies.

Warning of Tropical Cyclones

• Detection of cyclones is based on three parameters: fall in pressure, increase in wind velocity, and track of the storm.
• Weather stations worldwide monitor pressure fall and wind velocities, including in the Arctic and Antarctic regions.
• Detection radars and aircraft are used in India to monitor cyclones.
• High-resolution radiometers in satellites help detect cyclones and their structure.
• Remote sensing by radars, aircraft, and satellites helps predict where a cyclone will strike, allowing for advanced preparation such as closing ports and harbors, suspending fishing activities, evacuating the population, and stocking food and drinking water.
• Safety homes are provided for shelter and sanitation facilities during cyclones.
• Cyclones can be detected from their genesis in the high seas and tracked to give a warning at least 48 hours in advance.
• Predictions made only 12 hours in advance are not highly precise.

Tornado

• A tornado is a rotating column of air that extends from a thunderstorm to the ground.
• Tornadoes form due to changes in wind speed and direction within a storm cell.
• The spinning effect is tipped vertically by rising air moving up through the thunderclouds.
• Winds within the tornado funnel can exceed 500kmph.
• High-velocity winds cause most of the damage associated with tornadoes.
• Tornadoes can also cause damage through air pressure reductions.
• The air pressure at the tornado center is approximately 800 millibars.
• Human-made structures may collapse outward when subject to pressure drops of this magnitude.
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Origin of Tornado

• Tornado formation requires four things: shear, lift, instability, and moisture.
• Wind shear is the most important factor for creating tornadoes.
• Horizontal winds may start to roll into a column of air due to wind shear.
• A strong updraft can transport the column of air from the ground to the atmosphere, causing it to become vertical, and a storm usually develops.
• Tornadoes are often spawned by supercell thunderstorms, which are rotating and spinning storms.
• Tornadoes are most common in spring and least common in winter, with peaks of activity in spring and fall due to stronger winds, wind shear, and atmospheric instability.
• The occurrence of tornadoes is highly dependent on the time of day due to solar heating.
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Distribution of tornadoes

• Rare in polar regions and infrequent at latitudes higher than 50° N and 50° S.
• The temperate and tropical regions are the most prone to thunderstorms.
• Tornadoes have been reported on all continents except Antarctica.
• The United States has the most violent tornadoes.
• Canada reports the second largest number of tornadoes.
• In the Indian sub-continent, Bangladesh is the most prone country to tornadoes.
• At any moment there are approximately 1,800 thunderstorms in progress throughout the world.
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Differences between Tornado and Cyclone

	Tornado	Cyclone
Definition	A tornado is a rotating column of air ranging in width from a few yards to more than a mile and whirling at destructively high speeds, usually accompanied by a funnel-shaped downward extension of a cumulonimbus cloud. Winds 40-300+ mph.	A cyclone is an atmospheric system of rapidly circulating air massed about a low-pressure centre, usually accompanied by stormy often destructive weather. Storms that begin in the Southern Pacific are called cyclones
Rotation	Clockwise in the southern hemisphere and counter clockwise in the northern hemisphere	Clockwise in the southern hemisphere and counter clockwise in the northern hemisphere.
Forms of precipitation	Rain	Rain, sleet, and hail
Frequency	The United States records about 1200 tornadoes per year, whereas the Netherlands records the highest number of tornadoes per area compared to other countries. Tornadoes occur commonly in spring and the fall season and are less common in winters	10-14 per year
Location	Tornados have been spotted in all continents except Antarctica	Southern Pacific Ocean, Indian Ocean. Cyclones in the northwest Pacific that reach (exceed) 74 mph are “typhoons”.
Occurrence	Places where cold and warm fronts converge. Can be just almost anywhere.	warm areas

• Tornadoes and cyclones occur in India
• Cyclones are more frequent than tornado outbreaks in India
• Cyclones originate in the Bay of Bengal region and Arabian Sea region
• Tornadoes of weak strength occur in the north-western and north-eastern region of India
• Tornadoes can cause significant damage to people and property in affected areas.