Why In News?

Cyclonic Storm Arnab is moving west-northwestwards at 20 kmph over the Bay of Bengal toward the Andhra Pradesh and Yanam coasts, with landfall expected around October 28, 2026.

What Is Cyclone Arnab?

It is a low-pressure system developed over the east-central Bay of Bengal on October 23, 2026, rapidly escalating into a depression, and attaining Very Severe Cyclonic Storm (VSCS) status by October 27, 2026.

What is a Tropical Cyclone?

  • A tropical cyclone is an intense, warm-core, non-frontal low-pressure system that develops over warm tropical and subtropical oceans, between 5° and 30° latitude.  

  • Favourable Conditions for Formation

  • Warm Sea Surface Temperature (SST): Temperatures greater than 27°C across a deep layer (at least 60 meters) to provide continuous latent heat of condensation.  

  • Coriolis Force: Must be present to deflect winds into a spin; absent near the equator (0° to 5° latitude), which is why cyclones do not form there.  

  • Low Vertical Wind Shear: Minimal change in wind speed or direction with height, allowing vertical towering clouds (cumulonimbus) to stay intact.  

    • Free Mentorship Need Guidance for UPSC / WBCS / State PSC?
    100% Free & Confidential • Quick Callback

  • Pre-existing Low-Pressure Disturbance: A weak low-level cyclonic circulation or trough that acts as an initial trigger. 

  • Upper-Level Divergence: Outflow of air in the upper troposphere above the storm center to continuously remove rising air.  

  • A mature tropical cyclone consists of three main structural parts:  

  • The Eye: The calm, roughly circular center (8 to 65+ km across) with light winds and broken or clear skies, characterized by descending air and the lowest barometric pressure.

  • The Eyewall: A dense ring of towering cumulonimbus clouds surrounding the eye, featuring the most violent winds and heaviest rainfall.

  • Spiral Rainbands: Curved bands of clouds and thunderstorms spiraling outward from the center, producing heavy intermittent squalls.

  • Rotation: Counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere due to the Coriolis force.  

  • Tropical cyclones are known by different names depending on the ocean basin: 

  • Cyclone: Indian Ocean and South Pacific

  • Hurricane: Atlantic Ocean and Eastern North Pacific

  • Typhoon: Western North Pacific

  • Willy-Willies: Australia  

The name Arnab was officially contributed by Bangladesh under the regional naming protocol established by the WMO/ESCAP Panel on Tropical Cyclones. 

WMO/ESCAP Panel on Tropical Cyclones (PTC)

It is an intergovernmental regional body established jointly by the World Meteorological Organization (WMO) and the United Nations Economic and Social Commission for Asia and the Pacific (ESCAP).

  • Origin: Created in 1972 under WMO’s Tropical Cyclone Programme (TCP), following the devastating 1970 Bhola cyclone in Bangladesh. Its first session took place in 1973.  

  • Mandate: Focuses on the Bay of Bengal and the Arabian Sea. It aims to improve tropical cyclone warning systems, disaster risk reduction, hydrology, training, and research to minimize socio-economic losses. 

  • Member Countries (13 Nations): Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, United Arab Emirates (UAE), and Yemen. 

  • Cyclones Naming Convention:

    • The panel manages the naming of tropical cyclones in the North Indian Ocean.

    • A pool of 169 names (13 names contributed alphabetically by each of the 13 member countries) is used sequentially by the India Meteorological Department (IMD)'s Regional Specialized Meteorological Centre (RSMC) New Delhi, which assigns names to storms reaching 34+ knots.

    • Names must be politically, religiously, culturally, and gender-neutral, non-offensive, and short (maximum 8 letters). 

  • Secretariat: Hosted by India (via IMD).  

How Are Tropical Cyclones Classified Under IMD Standards?

Under IMD Pune standards, cyclonic disturbances over the North Indian Ocean are categorized into Low Pressure Area (<31 kmph), Depression (31–49 kmph), Deep Depression (50–61 kmph), Cyclonic Storm (62–88 kmph), Severe Cyclonic Storm (89–117 kmph), Very Severe Cyclonic Storm (118–167 kmph), Extremely Severe Cyclonic Storm (168–221 kmph), and Super Cyclonic Storm (≥222 kmph). 

Cyclone Category

Sustained Winds (kmph)

Operational Advisory & Impact

Cyclonic Storm (CS)

62 – 88 kmph (34–47 knots)

Total suspension of fishing; thatched roof damage.

Severe Cyclonic Storm (SCS)

89 – 117 kmph (48–63 knots)

Coastal hutment evacuation; severe tree & power disruption.

Very Severe Cyclonic Storm (VSCS)

118 – 167 kmph (64–90 knots)

Mobilize evacuation; extensive kutcha house destruction.

Extremely Severe Cyclonic Storm (ESCS)

168 – 221 kmph (91–119 knots)

Large-scale evacuation; massive power & rail grid damage.

Satellite-Based Dvorak Technique (SDT): Tropical cyclone intensity estimation utilizes the Dvorak Technique, assigning T-numbers from T1.0 to T8.0 based on cloud pattern analysis, central dense overcast characteristics, and infrared imagery.  

Color-Coded Weather Alert Mechanism: The operational protocol employs four distinct warning stages: Green (No Warning), Yellow (Watch / Be Updated), Orange (Alert / Be Prepared for 12–20 cm heavy rain), and Red (Take Action for >20.4 cm extremely heavy rain). 

Source: DOWNTOEARTH

PRACTICE QUESTION

Q. With reference to the structure of a mature tropical cyclone, consider the following statements:

  1. The "eye" of the storm is a region of violent descending air, high pressure, and maximum surface wind speeds.

  2. The "eyewall" immediately surrounds the eye and is characterized by towering cumulonimbus clouds and heaviest rainfall.

  3. The release of latent heat of condensation in the rising moist air is the main engine driving the storm's circulation.  

Which of the statements given above is/are correct?

(a) 1 and 2 only

(b) 2 and 3 only

(c) 1 and 3 only

(d) 1, 2, and 3

Answer: b

Explanation:

Statement 1 is incorrect: The "eye" of a mature tropical cyclone is a region of calm, light winds, low atmospheric pressure, and gently descending air. The highest wind speeds and lowest pressures at the surface do not coincide in this calm center; rather, the maximum wind speeds are found in the eyewall.

Statement 2 is correct: The eyewall is a dense ring of towering cumulonimbus clouds immediately surrounding the eye. It is the most destructive part of the storm, hosting the fastest winds and heaviest rainfall.

Statement 3 is correct: Tropical cyclones are fueled by the release of latent heat of condensation. As warm, moist air over tropical oceans rises and cools, water vapor condenses into water droplets, releasing massive amounts of heat. This warming causes the air to rise even faster, lowering the surface pressure and driving the storm's intense cyclonic circulation.