Cyclones: Formation, Effects and Characteristics

Cyclones are big, swirling masses of air rotating around a centre of low pressure. This is a powerful natural phenomenon characterized by strong winds and heavy rainfall. It is a low-pressure area in the atmosphere in which winds spiral upward. Cyclones can vary in size and strength. Understanding the formation, behaviour, and impact of cyclones is crucial for mitigating their effects and protecting vulnerable communities in their path.

Cyclones and Circulation Patterns

• The wind around a low-pressure system in a cyclone is called cyclonic circulation, while around a high-pressure system, it’s called anticyclonic circulation.
• A cyclone is a large-scale air mass rotating around a strong centre of low atmospheric pressure.
• These can be classified on the basis of their origin into tropical and extratropical cyclones.
• Wind circulation around a low-pressure system is called cyclonic circulation, while around a high-pressure system is called anticyclonic circulation.  
• The table given below shows Pressure Systems and Wind Circulation Patterns

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Characteristics of Cyclones

• Direction: In the northern hemisphere, cyclonic winds blow counter-clockwise, while in the southern hemisphere, they blow clockwise.
• Origin: It originates from thunderstorms, requiring warm ocean waters above 28°C for their formation.
• Source: Heat and moisture from warm ocean water serve as the energy source for cyclones.
  • Cyclones weaken rapidly over land or colder ocean waters lacking sufficient heat and moisture.
• High Relative Humidities: in the lower and middle tropospheres is essential for cyclone development.
• Vertical Wind Shear: Wind shear, the change in wind direction or speed with altitude, affects cyclone development.
  • Weak Wind Shear: allows storms to grow vertically, aiding in cyclone development, \
  • Stronger Wind Shear: leads to slanted storms and disperses latent heat over a larger area.
• Types of Cyclones: Cyclones fall into two main categories: tropical cyclones and extra-tropical cyclones.
  • In India: Cyclones are classified based on the strength of associated winds, storm surges, and exceptional rainfall.
  • Extra-tropical cyclones (Temprate cyclones): typically occur in temperate and high-latitude regions, originating even from the Polar Regions.
  • Tropical Cyclones: form between the Tropics of Capricorn and Cancer, developing over tropical or subtropical waters, where they organize into surface wind circulation patterns.
  • Cyclones also include tornadoes, hurricanes, and typhoons, which are, characterized by high winds, heavy rains, thunder, and lightning.
  • Hurricanes: form over the North Atlantic or eastern North Pacific oceans, while 
  • Typhoons form in the Western Pacific.
  • In the north Indian Ocean, these storms are referred to as tropical cyclones.
• Pressure: Sea level atmospheric pressure falls continuously from the periphery to the center of the cyclone.
• Inward Movement of Moisture-rich Winds: Moisture-rich low-level winds possess strong tangential and radial components, spiralling air parcels inwards towards the cyclone’s centre.
• Velocity (tangential wind): increases rapidly due to partial conservation of angular momentum.
• Warm Anomaly: in the central regions keeps them warmer than the surrounding tropical atmosphere, reducing radial pressure gradients at higher altitudes.
• Outward Movement of Air Parcels: Cyclically rotating air parcels rising in the central regions move outward in the upper troposphere under the action of unbalanced centrifugal forces.
  • Outward-moving cirrus clouds at upper levels and inward-spiraling low-level clouds are observed in satellite pictures of tropical cyclones in the northern hemisphere.

Movement of Cyclones

• Role of Winds in Guiding Cyclones: Winds guide cyclones, influenced by the global wind pattern known as the “general circulation,” which divides surface winds into three main belts.
  • These include the Polar Easterlies (60-90 degrees latitude), Prevailing Westerlies (30-60 degrees latitude), and Tropical Easterlies (0-30 degrees latitude).
• Formation of the Intertropical Convergence Zone: The convergence of easterly trade winds from both hemispheres forms the Intertropical Convergence Zone (ITCZ), characterized by a band of clouds and thunderstorms encircling the globe near the equator.
• Influence of Wind Belts: A cyclone’s path is determined largely by the wind belt it is situated in. 
• Example: Cyclones originating in the eastern tropical Pacific are driven westward by easterly trade winds before turning northwestward around the subtropical high and moving into higher latitudes. 
  • However, local steering winds may sometimes deviate from this pattern.

Special Cyclones

• Twin Cyclones:
  • Cause: Equatorial Rossby Waves.
  • Characteristics: Positive vorticity, counterclockwise and clockwise rotation.
  • Favourable Conditions: Positive vorticity, airlifting, warm ocean (>27°C), low wind shear.
  • Development: Starts as tall vortices, intensifies and moves westward.
• Fujiwhara Effect:
  • Definition: Interaction between simultaneous tropical storms within 1,400 km.
  • Impact: Alters track and intensity; can lead to merging.
• Bomb Cyclone:
• • Characteristics: Midlatitude storm, rapid intensification (≥24 mb in 24 hours).
  • Genesis: Clash of warm and cold air masses.
  • Areas: Common on the US coast, colder months.

Thunderstorms and Tornadoes

• Thunderstorms and tornadoes are brief but highly destructive, typically limited to a small area.
• Thunderstorms: A well-grown cumulonimbus cloud producing thunder and lightning. 
  • Caused by intense convection during hot, humid conditions. 
  • Short duration occurring over a small area; Violent.
  • When these clouds reach altitudes with sub-zero temperatures, hailstones form and fall as hailstorms. In cases of limited moisture, dust storms are created.
• Tornadoes: Sometimes severe thunderstorms generate a spiralling wind with very low pressure at the center that descends like the trunk of an elephant, causing massive destruction on its way; generally occurring in middle latitudes;  tornadoes over the sea are called waterspouts.

Difference Between Tropical Cyclone and Temperate Cyclone

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Conclusion

• Cyclones, whether tropical or temperate, are formidable weather phenomena driven by distinct mechanisms. Both types bring about significant impacts, from torrential rains and violent winds to snowfall and lower temperatures. 
• Understanding their characteristics and formation processes is crucial for effective disaster preparedness and mitigation efforts in vulnerable regions. 

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