CYCLONES

CYCLONES

Cyclones are rapid inward air circulation around a low-pressure area, often accompanied by violent storms and destructive air circulation. In the Northern Hemisphere, cyclones move counter-clockwise, while in the Southern Hemisphere, they move clockwise.

Classification of Cyclones:

1. Tropical Cyclones:
   • Occur between the Tropics of Capricorn and Cancer.
   • Develop in tropical oceans within latitudes of 5° to 20°.
2. Extra-Tropical Cyclones (Temperate or Wave Cyclones):
   • Develop in temperate zones and high-latitude regions.
   • Common in polar regions.

TROPICAL CYCLONES

General Characteristics:

• These involve circular air motion towards a low-pressure center.
• The direction of wind circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Characteristics of Tropical Cyclones:

1. Location: Found in well-defined areas and seasons in tropical oceans (5° to 20° latitude).
2. Size: Smaller than temperate cyclones, with a diameter ranging between 500 to 600 km. In some cases, the diameter can be restricted to 50 km or even less.
3. Air Pressure:
   • High intensity due to steep pressure gradients.
   • Central pressure ranges between 50-60 mb.
   • Associated with extreme wind velocity.
4. Frequency: Tropical cyclones form over oceans but weaken after reaching land due to a loss of moisture and energy from condensation.
5. Mobility: Not always mobile; can become stationary for days, causing heavy rainfall and resulting in destruction along their paths.

Further Characteristics of Tropical Cyclones:

• Identification: Tropical cyclones are identified based on their shape, size, and associated weather.
• Circular Center (Eye): A distinctive feature of tropical cyclones. The eye’s diameter ranges from 8 to 50 km.
• Direction of Movement: Cyclones move east to west under the influence of prevailing trade winds.
• Velocity: Tropical cyclones exhibit high wind speeds, typically ranging between 100 to 250 km/h (and can even exceed 200-1200 km/h in the upper troposphere).
• Weather Conditions: Cyclones bring heavy rainfall, strong winds, and storm surges, causing destruction in localized areas.
• Direction: Cyclones generally move westward due to trade winds.

Factors Responsible for the Formation of Cyclones:

Six key factors necessary for the formation of tropical cyclones:

1. Water Temperature (> 27°C): Warm water is necessary to cause instability in the atmosphere, sustaining convection and thunderstorms.
2. Rapid Cooling with Height: This promotes the release of latent heat, which powers the cyclone.
3. High Humidity: Moisture in the lower-to-mid troposphere favors disturbance development.
4. Low Wind Shear: Low wind shear helps sustain the cyclone’s convective feedback loop.
5. Distance from the Equator: Cyclones typically form at least 5° (500 km) away from the equator to allow the Coriolis force to initiate circulation.
6. Pre-existing Weather System: A low-pressure center or disturbed weather system is required for cyclones to form.

Why There Are Fewer Cyclones Over the Arabian Sea than the Bay of Bengal:

• Annual Frequency: The Indian Ocean (Bay of Bengal and Arabian Sea) experiences about 5-6% of the global cyclone frequency.
• Ratio of Frequency: There are more cyclones in the Bay of Bengal than in the Arabian Sea, with a frequency ratio of 4:1.

Reasons Behind the Trend (Why the Bay of Bengal Sees More Cyclones Than the Arabian Sea):

• Water Temperature: The Bay of Bengal (BoB) maintains a warm temperature of about 24-27°C, which is ideal for cyclonic activity. Factors like underwater volcanic activities and geographical features contribute to this constant temperature.
• Influx of Freshwater: Freshwater from the Ganga and Brahmaputra rivers prevents warm water from mixing with cooler layers below, maintaining a high temperature in BoB.
• Geographical Features: The BoB is surrounded by land that continuously transfers heat, making it conducive to cyclonic formations. This feature is absent in the Arabian Sea.
• Occurrence of ITCZ: The Intertropical Convergence Zone (ITCZ) forms over BoB, especially during the withdrawal of the monsoon from the Indian subcontinent, leading to cyclonic conditions.
• Link with Pacific Ocean: The BoB has a passage that facilitates the movement of wind systems from the Pacific Ocean, which can gain power and become cyclones.
• Higher Salinity in Arabian Sea: The Arabian Sea has higher surface salinity, which causes more evaporation, preventing the surface from maintaining the required moisture for cyclone formation.
• Monsoon Winds Drive Away Moisture: Monsoon winds in the Arabian Sea blow the moisture toward India, limiting its potential for cyclone formation.

Changing Trend (Reasons Behind Increasing Cyclonic Activities in the Arabian Sea):

1. Evidences:

• Annual Average: Five cyclones form annually between the BoB and Arabian Sea, with four in the BoB and one in the Arabian Sea.
• Spreading to Lower States: Tropical cyclones, previously restricted to Gujarat, are now affecting Kerala and Karnataka.
  • Example: Cyclone Oc Khi.
• Increasing Frequency: Cyclone Tauke in 2021 was the fourth consecutive cyclone to form in the Arabian Sea, following Mekanu (2018), Vāyu (2019), and Nisarga (2020).
• Rise in Intensification Rate: Cyclones have been intensifying more rapidly since 2018.
  • Example: Cyclone Take developed into a Very Severe Cyclonic Storm (VSCS) in just 2 days, whereas Mekanu and Nisarga took 4 to 5 days.

2. Reasons Behind Changing Trends:

• Increasing Surface Temperature: Due to global warming, sea surface temperatures in the Arabian Sea have risen by 1.2-1.4°C compared to four decades ago. This rise supports stronger convection, heavier rainfall, and more intense cyclones.

Energy for Intensification:

• Rising Temperature: The Arabian Sea’s rising sea surface temperature, with water temperatures up to 50 meters deep allowing intensification. For instance, Cyclone Take became a Very Severe Cyclonic Storm (VSCS) in just 2 days.
• Wind Shear: Favorable wind conditions, like easterly winds, contributed to the movement of Cyclone Ochi from the Bay of Bengal to the Arabian Sea.
• Greater Occurrence of El Niño Modoki: This climate phenomenon, a pseudo El Niño, does not favor cyclone formation in the Bay of Bengal, but instead supports cyclone formation in the Arabian Sea.

Positive Effects of Cyclones:

1. Bringing Rainfall: Tropical cyclones bring much-needed rainfall to rain shadow regions and parched areas like Telangana, Rayalaseema, Vidarbha, and others in South India.
2. Temperature Moderation: Cyclones transport heat and energy away from the tropics to temperate regions, maintaining a temperature balance in the atmosphere.
3. Red Tide Dilution: Cyclones can help dissipate red tides—a phenomenon where algal blooms discolor coastal waters and release toxins. The cyclones dilute these blooms and mitigate their harmful effects.
4. Replenishing Barrier Islands: Cyclones carry sediments from the ocean bed and deposit them on barrier islands, aiding their formation or relocation.
5. Seed Dispersal: Cyclone winds can spread seeds inland for long distances, sometimes as far as 1000 miles, aiding post-fire or post-urbanization regrowth.
6. Clearing Pollution: The fast-moving winds of cyclones clear pollutants from the atmosphere, making the air cleaner and fresher.

Nomenclature of Tropical Cyclones:

1. Reason for Naming:
   • Avoiding Confusion: Cyclones often occur simultaneously in a region; names help distinguish them.
   • Quick Information Sharing: Assigning names (often after flowers, animals, or people) allows for faster communication during weather events.
2. Process for Naming:
   • The World Meteorological Organization (WMO) assigns the responsibility for naming cyclones in different ocean basins to regional bodies. Each region has specific rules for naming, with alternating male and female names often used.

For the Indian Ocean Region:

• The naming of cyclones in the Indian Seas is not in alphabetical order, but based on names submitted by countries contributing to the naming system.
• A storm is named when it reaches tropical storm strength with a wind speed of at least 63 km/h.
• The Indian Meteorological Department (IMD) issues cyclone advisories and is responsible for naming cyclones in the region, which includes contributions from eight countries: Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka, and Thailand.
• A formula for naming cyclones was established in 2004. Each country submits a set of names that are used sequentially whenever a cyclonic storm develops. Every country in the group has a chance to name a cyclone, after which the list rotates.

TEMPERATE CYCLONES

Temperate cyclones, also known as extra-tropical cyclones, frontal depressions, or wave cyclones, form in mid and high latitudes beyond the tropics. These cyclones develop between 30° to 60° latitudes in both hemispheres.

Characteristics of Temperate Cyclones:

1. Location: Formed between 35° and 65° latitudes in both hemispheres due to the interaction between cold polar air and warm tropical air.
   • These cyclones are more pronounced in the Northern Hemisphere due to greater temperature contrast.
2. Velocity: The pressure gradient is smaller, so the wind speeds are moderate, around 30 to 40 km/h.
3. Frequency: They occur throughout the year, with higher frequency in winter than in summer.
4. Area Covered: Temperate cyclones can span 1500 to 2000 km in diameter and can reach up to the tropopause.
5. Air Pressure: The pressure gradient in these cyclones is gentle, with isobaric differences between 10 to 15 Mb.
6. Formation: Formed due to the interaction of fronts—the thermal contrast between the warm and cold air masses is crucial for the formation of the low-pressure center.
7. Direction of Movement: In the Northern Hemisphere, temperate cyclones move from the southwest to northeast. In the Southern Hemisphere, they move from northwest to southeast, aligned with the westerly wind belts.
8. Weather Conditions: Associated weather conditions include cloudy skies, moderate to heavy rainfall, and flooding. The rainfall may persist over large areas for long periods.

Formation of Temperate Cyclones

1. Polar Front Theory

• This theory explains that temperate cyclones form when warm, humid air masses from the tropics meet the cold, dry air masses from the poles, creating a polar front as a surface of discontinuity.
• This typically occurs in subtropical high and subpolar low-pressure belts and along the tropopause.
• Cold air pushes the warm air upwards, causing the warm air to expand, and as a result, a cyclone is formed, advancing with the westerlies and jet streams.

2. Stages of Formation

• Stage I (Initial Stage): Involves the convergence of two air masses with contrasting properties moving parallel to each other, forming a stationary front.
• Stage II (Incipient Stage): Warm and cold air masses penetrate into each other’s territories, forming a wave-like front.
• Stage III (Mature Stage): The cyclone fully develops, and the isobars become almost circular.
• Stage IV: The warm sector narrows as the cold front advances, causing the warm front to retreat.
• Stage V: Occlusion of the cyclone occurs as the advancing cold front overtakes the warm front, forming an occluded front.
• Stage VI: The warm sector completely disappears, and the occluded front diminishes, leading to the death of the cyclone.

Role of Temperate Cyclones in Precipitation Across India

• Cloudy Weather: These cyclones bring highly variable and cloudy weather, forming polar and tropical air masses that meet in temperate zones.
• Western Disturbances: These disturbances often affect India, particularly in the northwest during winter, causing rain and snow.
• Rain in Western India: Temperate cyclones are a common source of winter rain in the northwestern part of India, such as Punjab, Haryana, and Delhi.
• Intensity of Rain: These cyclones can bring moderate to heavy rain, especially in low-lying areas and mountains.
• Foggy Conditions: Over the Indo-Gangetic plains, these cyclones sometimes lead to cold waves and fog.
• Less Effect in Southern India: Western disturbances generally don’t cause precipitation in peninsular India.
• Agricultural Importance: Winter rains from temperate cyclones are critical for rabi crops, such as wheat. Around 5-10% of India’s total annual rainfall comes from temperate cyclones, making them crucial for agriculture.
• Saffron and Tea Cultivation: Saffron and tea crops, especially in the Himalayan foothills, depend on rainfall from temperate cyclones.

Comparison of Tropical and Temperate Cyclones