MET 17- Tropical System & Cyclone

Tropical Revolving Storms (TRS), commonly known as hurricanes, typhoons, or cyclones, are thermal depressions that develop exclusively over warm tropical or subtropical oceans. Their formation depends on sea surface temperatures exceeding 26–26.5°C, which provide abundant water vapor and the latent heat energy required for intensification. These storms usually form between 5° and 25° latitude because the Coriolis force—necessary to initiate cyclonic rotation—is too weak within 5° of the Equator. TRS development also requires low vertical wind shear so that the storm’s vertical structure remains intact; high shear disrupts and weakens the system. Once a TRS moves over land, it rapidly loses access to its primary energy source—warm water and moisture—and therefore weakens and dissipates quickly.

In the North Indian Ocean, the average life period of storms and depressions in the post-monsoon months is about 3–5 days. The average life span of a monsoon depression is typically 2 to 5 days. Considering all storms and depressions in the North Indian Ocean, the life period works out to be about 3.5 days. This places the average lifespan squarely within the 3-4 day range.
⭐️ ⭐️ Key Data to Remember (TRS in Indian Seas)
• Average Lifespan (Indian Ocean/Monsoon Depression): 3 to 5 days.
• Definition: A tropical cyclone is an intense vortex originating over tropical waters.
• Structure: Has a diameter varying between 150 km and 1500 km at the surface, decreasing in size with height, and vertically extending up to 300/200 hPa levels.
• Eye Diameter (Indian Region): Typically about 20 to 30 km.
• Energy Loss: Storms rapidly lose intensity when they move over land, as the moisture supply is cut off and surface friction increases.

⭐️ ⭐️ Key Data to Remember
– Tropical Revolving Storms (TRS) form over tropical oceans between 5° and 25° latitude.
– The core development zone is 5°–15°.
– Formation is rare within 5° of the Equator because the Coriolis force is too weak to initiate rotation.
– Sea Surface Temperature must exceed 26°C, usually available only up to about 25° latitude.
– TRS require high SST, adequate Coriolis force, and latent heat from warm, moist air.
– Formation also requires very low vertical wind shear for proper storm development.

A well-developed cyclone, whether a temperate low or a Tropical Revolving Storm (TRS), is characterized by a region of low pressure where surface air converges and is forced to ascend. This lifting leads to extensive cloud formation (often cumulonimbus and stratiform layers), continuous or heavy showery precipitation, and typically strong, sometimes destructive, winds due to the steep pressure gradient. Consequently, cyclones are frequently referred to simply as storms. Aviation hazards include widespread turbulence, icing, and poor visibility.
⭐️ ⭐️ Key Data to Remember (Cyclone Weather)
• Pressure: Low central pressure, tightly packed isobars.
• Winds: Strong, spiraling inward (Northern Hemisphere: counterclockwise).
• Clouds: Extensive coverage, potentially reaching the tropopause, including Cumulonimbus (CB) in unstable air.
• Hazards: Heavy precipitation, strong winds, and widespread turbulence.

Monsoon Depressions (MDs) are intense low pressure systems occurring in regions like the Indian seas. While a westward-moving MD typically sees heavy rainfall concentrated in the southwest (SW) sector, the worst weather in a tropical cyclone system moving poleward (recurving to the North or Northeast in the Northern Hemisphere) is generally found in the right-front quadrant.
This dangerous sector, which corresponds to the Northeast (NE) sector during recurvature, experiences the combined effect of the storm’s forward translation speed and its cyclonic (counterclockwise) wind speed, resulting in the highest sustained winds and most destructive associated weather.
Key Data to Remember:
• Rotation: Cyclones/Depressions in the Northern Hemisphere exhibit counterclockwise circulation.
• Recurvature Direction: Recurvature typically means movement toward the North/Northeast.
• Dangerous Quadrant: The highest winds and most severe weather (including tornadoes in hurricanes) occur in the right-front quadrant relative to the storm’s movement.
• MD Specifics (Westward Movement): Heavy rainfall in a westward moving Monsoon Depression is often confined to the SW sector.

A Tropical Revolving Storm (TRS) is characterized by a deep depression where isobars are tightly packed together, resulting in a steep pressure gradient.
• The typical diameter of a TRS is approximately 350 nm (about 650 km), or approximately 300 km, much smaller than a mid-latitude depression (approximately 1000 nm). This compact size reflects the steep pressure gradient.
• The most extreme pressure gradient, leading to the strongest winds (50 to 100 kt), is found close to the center, specifically 10 to 20 nm from the eye.
• A radius of 100 km (approximately 54 nm) encompasses the core area immediately surrounding the central eye wall, where the pressure drop is most dramatic and the strongest weather is found.
Key Data to Remember:
• TRS Diameter: ∼350 nm (∼650 km) or approximately 300 km. steep pressure gradient radius of 100 km • Central Pressure: Typically 900 to 960 hPa, with 870 hPa being the lowest recorded value.
• Maximum Winds: Found 10 to 20 nm from the center (eye).

Tropical Cyclones derive their immense energy from the large transfer of latent heat and moisture from the warm sea surface. They rapidly lose intensity and dissipate when they move over land, as this energy source is cut off.
The Indian sub-continent is bordered by the relatively confined Bay of Bengal and Arabian Sea. This geographical constraint means that storms often have a short path of travel over the warm ocean waters before making landfall on the highly susceptible coasts of India, Bangladesh, or Myanmar. A short oceanic track significantly limits the time available for a storm to reach maximum potential intensity, particularly compared to storms in large open basins like the Western Pacific. The average lifespan of a tropical cyclone in this region is approximately 3–4 days.
Note on Atmospheric Constraints: Furthermore, strong vertical wind shear, especially during the monsoon season, is known to inhibit the formation of the intense central structure (wall cloud) necessary for extremely severe tropical cyclones in the North Indian Ocean.
⭐️ ⭐️ Key Data to Remember (TRS Intensity)
• Energy Source: Warm sea water (SST > 26∘C).
• Dissipation: Storms die quickly when moving over land, due to loss of moisture supply.
• Intensity Limitation (Indian Region): Short sea trajectories and strong vertical wind shear during monsoon.
• Average Lifespan (Indian Seas): Approximately 3–4 days.

Tropical Cyclones (Tropical Revolving Storms, Hurricanes, or Typhoons) are thermal depressions that derive their immense power and energy primarily from the release of latent heat from moisture supplied by warm sea surfaces.
When a tropical cyclone moves over a large landmass, it rapidly loses this critical energy source (influx of moisture is ceased). Furthermore, friction with the land surface increases, causing surface winds to decrease and blow more directly into the storm, which leads to a rise in central pressure. Consequently, the storm system weakens, or “dies out,” rapidly upon making landfall.
⭐️ ⭐️ Key Data to Remember (Cyclone Dissipation)
• Cause of Decay: Loss of moisture/heat source (warm ocean water) and increased surface friction over land.
• Mechanism: Loss of fuel causes the central pressure to rise and the strong cyclonic circulation to diminish.

A Tropical Revolving Storm (TRS) is characterized as a deep depression. The low central pressure creates an exceptionally steep pressure gradient force, which generates the high winds characteristic of a hurricane or typhoon. The lowest recorded mean sea level pressure ever observed in a tropical cyclone was 870 hPa (Typhoon Tip).
⭐️ ⭐️ Key Data to Remember (TRS Pressure)
• Typical Range (Deep Depression): 900 to 960 hPa.
• Lowest Recorded: 870 hPa.
• Location of Minimum Pressure: In the calm, central “eye” of the storm.
• Pressure Gradient: Very steep due to the low central pressure and small storm diameter (≈350 nm).

Tropical Cyclones (known as Cyclonic Storms in the Indian Ocean region) exhibit a bi-modal peak of activity over the Indian seas (Bay of Bengal and Arabian Sea). This bi-modal pattern means they primarily form during the transition periods before and after the strong Southwest (SW) Monsoon season (June to September).
1. Pre-Monsoon Season: Cyclones develop from April to mid-June. The secondary peak intensity occurs in May.
2. Post-Monsoon Season: Cyclones develop from October to mid-December. The primary peak intensity occurs in November.
These periods are associated with the advance and retreat of the monsoon, during which the Intertropical Convergence Zone (ITCZ) is positioned over warm seas, providing the necessary low-level convergence, instability, and warm sea surface temperatures (SST) necessary for cyclogenesis.
⭐️ ⭐️ Key Data to Remember (Indian Cyclones)
• Storm Season: April–mid-June (Pre-Monsoon) and October–mid-December (Post-Monsoon).
• Peak Months: May (Secondary Peak) and November (Primary Peak).
• Limitation During SW Monsoon: Development is often inhibited during the peak SW Monsoon season (July/August) due to strong vertical wind shear and the monsoon trough being confined too close to the continent, limiting the necessary oceanic heat supply.
• Formation Frequency: Annually, 5–7 storms develop in the Indian sea.

The eye wall is the ring of intense cumulonimbus (CB) clouds and strong convection that immediately encircles the calm, central eye of a Tropical Revolving Storm (TRS).
• Winds and Turbulence: The eye wall is the location of the strongest winds (sometimes exceeding 175 knots) and the most severe turbulence found within the entire storm system.
• Precipitation: It contains the heaviest precipitation and deluging rain.
• Hazard Rating: Due to its steep pressure gradient and concentration of all severe weather phenomena (winds, turbulence, icing, rain, hail), the inner storm area/eye wall is classified as the most hazardous part of the cyclone.
The eye, conversely, is characterized by light winds and broken clouds due to subsiding air, making it the least hazardous region of a mature TRS.

A wind surge or pulse is defined meteorologically as a sudden increase in air current velocity occurring within the same air mass. This phenomenon represents a type of wind shear known as velocity convergence. Surge and shear lines are recognized characteristics of the vigorous monsoon systems.
⭐️ ⭐️ Key Data to Remember (Wind Surge/Shear)
• Definition: Sudden increase in wind speed (velocity convergence).
• Air Mass Context: Occurs within the same air mass.
• Operational Context: Closely related to squalls (sudden wind increase of ≥16 kt to a peak of ≥22 kt lasting ≥1 minute).
• Turbulence Source: Wind shear, whether vertical or horizontal, is a major source of aviation turbulence.

Monsoon Depressions (MDs) are intense low pressure systems that typically form during the Southwest (SW) Monsoon season (June to September). During the peak monsoon months of July and August, these depressions normally form over the North Bay (or Head Bay) of Bengal, specifically north of 18∘N and west of 92 ∘E. This location corresponds to the position where the low-pressure monsoon trough dips into the Bay of Bengal. MDs formed here generally move west to west-northwest over north India.
⭐️ ⭐️ Key Data to Remember (Monsoon Depressions)
• Formation Area (Monsoon): North Bay of Bengal (Head Bay).
• Monsoon Trough Role: MD development occurs when the seasonal trough dips into the North Bay.
• Intensity Limitation: They rarely intensify into full tropical cyclones due to the confinement of the monsoon trough and the presence of strong vertical wind shear.
• Lifespan: Average life span is 3 to 5 days.

Tropical Revolving Storms (TRS), called cyclones in the Indian Ocean, form between 5° and 25° N/S.
Formation requires a sufficient Coriolis force; this force is too weak within 0°–5°, so storms rarely form near the Equator.
The 5°–15° band is the preferred zone because the Coriolis force becomes effective and SST > 26°C is consistently available.
The ITCZ often provides additional instability that supports development.
Key Data to Remember (TRS Formation Criteria):
Must be outside 5° latitude (adequate Coriolis force).
Usually develops below 25° latitude (warm SST zone).
Requires Sea Surface Temp > 26°C as the primary energy source.

The eye is the roughly circular central area of a well-developed tropical cyclone. It is characterized by subsiding (sinking) air, which warms by compression, causing clouds to evaporate.
• Winds: Winds are light or calm.
• Clouds/Precipitation: Skies are often clear or broken clouds and rainfall ceases.
• Pressure: The eye contains the lowest surface pressure of the storm.
The air in the eye is typically warmer than the surrounding air in the eye wall. The most violent weather, including hurricane-force winds and torrential rain, is concentrated in the towering cloud ring, the eye wall, immediately surrounding the calm eye.
⭐️ ⭐️ Key Data to Remember (TRS Eye)
• Winds: Light/Calm.
• Weather: Clear or broken clouds; little to no precipitation.
• Mechanism: Subsidence (sinking air) causes adiabatic warming, leading to cloud evaporation.
• Size: Diameter typically 20 to 50 km (10 to 20 nm radius).

A well-developed extratropical cyclonic storm, often referred to as a wave cyclone, is modeled according to the Polar Front Theory.
• Structure: The characteristic configuration consists of a central low pressure system (cyclone). Radiating from this low are two principal frontal boundaries: a warm front and a cold front, which enclose a region of warmer air known as the warm sector.
• Northern Hemisphere Movement: In the Northern Hemisphere, winds circulate counterclockwise around the low. The system typically moves eastward or northeastward.
• Occlusion: The warm front generally moves slower than the cold front. As the storm matures, the faster-moving cold front eventually overtakes the warm front, leading to the formation of an occluded front (occlusion), which progressively increases in extent from the low pressure center. The system is most intense at the time of maximum occlusion.
Key Data to Remember:
• System Type: Extratropical cyclone (mid-latitude cyclone).
• Core Components (Open Wave/Peak Development): Low pressure center, Warm Front, Cold Front, Warm Sector.
• Evolution: The occlusion forms when the Cold Front catches the Warm Front.
• Movement: Generally east or northeastward, steered by upper-level westerlies.

Extratropical cyclone families are a series or succession of wave cyclones that form along the Polar Front. These systems generally move in an easterly or northeastward direction.
These depressions form one behind the other along the polar front, typically running from southwest (where new waves often form, usually along the trailing cold front) to northeast (where older waves are dissipating). They are found in sequence (or “family”) along this boundary, progressing across the temperate latitudes. The sequential spacing and movement result in the systems being arranged diagonally relative to each other along the frontal boundary.
⭐️ ⭐️ Key Data to Remember (Cyclone Families)
• Definition: A succession of mid-latitude (extratropical) cyclones forming along the Polar Front.
• Movement: Systems typically move East or Northeast.
• Formation Location: New members often form as secondary depressions on the trailing cold front, generally located to the SW of the preceding low.
• Arrangement: They are lined up sequentially along the front.

Tropical Revolving Storms (TRS), including cyclonic storms, obtain their energy from latent heat released by warm ocean surfaces.
For development, sea surface temperature (SST) must exceed 26–26.5°C, and evidence shows that temperatures above 27°C significantly enhance formation. Warmer SST leads to greater pressure drops in the storm core, increasing storm intensity. If SST is too low, a TRS cannot form.
Key Data to Remember (SST Requirement):
Minimum SST: > 26–26.5°C (often > 27°C for strong development).
Energy Source: Warm water providing moisture and latent heat.
Depth Requirement: Warm water must extend to a depth of 200–300 ft.
Latitude Constraint: Warm-water availability typically restricts TRS formation to 5°–25° latitude.

In the Northern Hemisphere:
1. Wind Direction and Drift: Strong wind coming from the port (left) side of the aircraft will induce starboard drift (drift to the right).
2. Buys Ballot’s Law: In the Northern Hemisphere, if an observer stands with their back to the wind, the low pressure (the center of the cyclone) is on their left.
3. Aviation Avoidance Rule: Pilots are instructed that if they experience strong wind from the port resulting in starboard drift, they are flying toward the cyclone. Consequently, if the aircraft is tracking toward the cyclone center, the center is located Ahead of the aircraft’s current position.
To avoid the cyclone, a pilot experiencing starboard drift (wind from port) must alter course until port drift occurs, which indicates the aircraft is then heading away from the storm.
Key Data to Remember (NH):
• Wind Source: Port side (Left).
• Resultant Drift: Starboard drift (Right).
• Indication: Tracking toward the cyclone center.
• Center Location: Ahead.

The Peninsular Discontinuity, also known as the dry-line, forms over central peninsular India (south of latitude 23∘N) during the pre-monsoon season (April and May). This boundary separates drier air flowing from the west/northwest from warm, moist air drawn in from the sea. The discontinuity causes instability and often results in afternoon thunderstorms.
⭐️ ⭐️ Key Data to Remember
• Location: Central peninsular India (south of 23∘N).
• Alternative Name: Dry-line.
• Time of Occurrence: Summer/Pre-monsoon season (April and May).
• Associated Weather: Afternoon thunderstorms.

A wind surge (or pulse) is defined as a sudden increase in the air current velocity occurring within the same air mass. It is also known explicitly as velocity convergence. Surges and shear lines are recognized characteristics of the Vigorous Monsoon.
⭐️ ⭐️ Key Data to Remember (Surge)
• Definition: Sudden increase in wind speed.
• Alternative Name: Velocity convergence.
• Operational Context: Characteristic of a Vigorous Monsoon.
• Air Mass: Occurs within the same air mass.

A Cyclonic Storm (CS), which falls under the broader category of a Tropical Cyclone (TC) or Tropical Revolving Storm (TRS), is defined by its minimum sustained surface wind speed.
1. Threshold for CS: Monsoon Depressions (MDs) have prevailing surface winds in the range of 17 to 33 kts. When the maximum sustained surface winds exceed 33 kts/hr, the system is upgraded and termed a Cyclonic Storm (CS). ICAO standards define a Tropical Cyclone/Storm Advisory to be issued when the 10-minute mean surface wind speed is expected to reach or exceed 34 kts.
2. Tropical Storm Range: In the international context (FAA/WMO), a Tropical Storm (which includes the CS category) generally carries sustained winds from 35 through 64 knots.
3. Range Selection: The option 34−47 kt correctly captures the minimum speed necessary for a system to be classified as a Cyclonic Storm (starting at 34 kt) and covers the lower to moderate end of the gale/storm spectrum (Beaufort 8: 34−40 kts; Beaufort 9: 41−47 kts).
⭐️ ⭐️ Key Data to Remember (Tropical Cyclone Classification)
• Tropical Depression/Monsoon Depression: Winds 17 to 33 kts.
• Tropical/Cyclonic Storm (CS): Winds ≥34 kts and usually up to 64 kts.
• Hurricane/Typhoon: Winds ≥65 kts.

A monsoon depression Slopes *SW with height.
Explanation (Based on Observed Tilt)
Monsoon Depressions (MDs) in the Indian region are deep low-pressure systems. The axis of a depression or the associated monsoon trough is generally not vertical but tilts with altitude.
• Direction of Tilt: The depression axis in a Monsoon Depression has a small southern tilt in the vertical from the surface up to the 500 hPa level. The monsoon trough itself also extends up to 500 hPa tilting southwards with height.
• Structure: This means that the center of the depression in the middle troposphere is located slightly to the south (or southwest, consistent with the typical WNW movement of MDs) of the surface center.
• Thermal Structure: The MD has a cold core in the lower troposphere (up to about 600 hPa) and a warm core above this level.

Tropical Revolving Storms (TRS), which include Cyclonic Storms, require a significant Coriolis force to initiate and sustain the cyclonic rotation necessary for development.
• Coriolis Force: The Coriolis force is virtually zero at the Equator (0 ∘ latitude).
• Formation Limit: Consequently, it is rare for a TRS to form within 5∘ of the Equator because the Coriolis force is too small to turn the winds sufficiently to flow around a low-pressure area. Instead, air tends to flow directly into the low, rapidly dissipating it.
• Operational Context: While TRS also do not form at higher latitudes like 40∘ or the Poles (due to insufficient sea surface temperature, among other factors), the lack of Coriolis force at the Equator provides the fundamental physical limit preventing cyclogenesis in that specific region.

Extratropical storms, also known as middle-latitude cyclonic storms or depressions, are migratory frontal cyclones that form outside of the tropics. Their development begins along the Polar Front, the semicontinuous global boundary that separates cold polar air masses from warmer tropical air masses.
The mechanism explaining the formation and life cycle of these systems is known as the Polar Front Theory. These frontal depressions are typically found in the disturbed temperate climatic zone between approximately 35∘ and 65∘ latitude in both hemispheres.
⭐️ ⭐️ Key Data to Remember
• Origin: The Polar Front.
• Type: Migratory frontal cyclones (Extratropical or Mid-latitude).
• Mechanism: Polar Front Theory/Cyclogenesis.
• Location: Temperate latitudes, generally 35 ∘to 65 ∘ N/S.
• Structure: Characterized as cold-core lows.

Cyclonic Storms (CS) affecting the Indian seas exhibit a bi-modal frequency, meaning there are two distinct seasons of maximum activity: the Pre-Monsoon (March–May) and the Post-Monsoon (October–December).
The primary peak season for the formation of severe storms, and particularly those that impact the southeast coast of India, including Tamil Nadu, is during the Post-Monsoon period. This period corresponds to the onset and retreat of the Northeast (NE) monsoon.
• October marks the beginning of the retreating monsoon season, and cyclonic storms frequently affect the Bay of Bengal, often moving toward the Tamil Nadu and Andhra Pradesh coasts.
• November contains the primary peak for cyclonic activity in the Indian seas. Storm activity shifts southwards as the season advances, making the coastal regions of Tamil Nadu highly vulnerable in November and early December.
⭐️ ⭐️ Key Data to Remember (Indian Cyclones)
• Storm Seasons: Pre-Monsoon (April–mid-June) and Post-Monsoon (October–mid-December).
• Primary Peak: November.
• Tamil Nadu Activity: Concentrated during the Post-Monsoon season, when the NE monsoon brings significant rainfall.
• Track: Storms tend to move WNW/NW during October and November.

Monsoon Depressions (MDs) are intense low-pressure systems occurring primarily during the Southwest (SW) Monsoon season over the Indian subcontinent.
The typical lifespan for a monsoon depression is 3–5 days. Other sources confirm a similar range, noting that the average life span is 2 to 5 days, though they can occasionally persist longer (e.g., beyond 6 to 7 days). Low-pressure areas forming during the monsoon, which often precede or include MDs, have an average life span of 4 to 6 days. The average life period for depressions and storms in the North Indian Ocean during the post-monsoon months is also around 4–5 days.
MDs dissipate when they move over land or merge with the seasonal low pressure area.
⭐️ ⭐️ Key Data to Remember (Monsoon Depression Life Cycle)
• Average Duration: 3 to 5 days.
• Monsoon Low Pressure Systems: Average life span is 4 to 6 days.
• Intensity: MDs are intense low-pressure systems but are generally limited in duration compared to mature mid-latitude depressions (which can last 10 to 14 days).

The intensity classification of Tropical Revolving Storms (TRS) varies by region. Under the Indian Ocean / IMD scale, a Severe Cyclonic Storm (Severe Cyclone) corresponds to a defined wind-speed range.
A Severe Cyclonic Storm has 48–63 kt maximum sustained surface winds.
Wind speeds of 64 kt or more classify the system as a Very Severe Cyclonic Storm, Hurricane, or Typhoon depending on the ocean basin.
Key Data to Remember (Tropical Cyclone Classification):
Tropical Depression: Up to 34 kt
Tropical Storm: 35–64 kt
Cyclonic Storm: 34–47 kt
Severe Cyclonic Storm: 48–63 kt
Hurricane / Typhoon: 65 kt or more

Tropical Cyclones (Hurricanes, Typhoons, Cyclones) are purely tropical systems because they require conditions found only in tropical regions.

Latitude Range:
Form mainly between 5° and 25° North or South. This lies within the tropical belt between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S).

Coriolis Requirement:
Rare within 5° of the Equator because the Coriolis force is too weak to start the rotational circulation needed for cyclone development.
Energy Source: Powered by latent heat released from moist, warm ocean air.

Requires SST ≥ 26°C, a condition found in tropical seas, not in cooler mid-latitude or polar waters.

Mid-latitude storms (Extratropical Cyclones): Form outside the tropics, typically between 35° and 65°, along the Polar Front.
Tropical cyclones that move into mid-latitudes usually weaken or transition into extratropical lows and lose their tropical structure.

Extratropical storms, often referred to as mid-latitude cyclonic storms or Polar Front Depressions, are migratory low-pressure systems. Their movement is governed by the prevailing winds aloft, which are generally westerly in the middle and high latitudes.
• Steering Flow: The entire storm system moves in an easterly direction, typically following the direction of the upper-level westerly winds, particularly the wind flow at the 500 hPa level.
• Direction: Extratropical cyclones generally move eastward or northeastward across the continents and oceans. The system’s track is known as a westerly wave.

⭐️ ⭐️ Key Data to Remember (Extratropical Movement)
• Driving Force: Prevailing Westerlies aloft.
• Direction: Generally Eastward or Northeastward.
• Speed: Surface systems move at approximately half the speed of the winds at the 500 mb (18,000 ft) level.

A Cyclonic Storm (CS), by international classification (Tropical Storm), must have maximum sustained surface winds between 35 through 64 knots. This range covers moderate to strong gale force winds. While some Indian cyclones intensify into Severe Cyclonic Storms (SCS) or Very Severe Cyclonic Storms (reaching 65 knots or more), the sources indicate that the incidence of such severe storms in the Indian seas is relatively very small compared to other storm basins. Therefore, the systems that achieve the classification threshold of a Cyclonic Storm (CS) but do not reach the higher intensity thresholds (Severe/Hurricane force) are most commonly classified as Moderate. ⭐️ ⭐️ Key Data to Remember • CS Wind Range (Tropical Storm): 35 through 64 knots (65 to 119 km/h). • Severe Storm Incidence: Relatively very small in the Indian seas compared to other basins. • High Intensity Threshold: Hurricane/Typhoon force winds start at 65 knots or more.

Fronts are the defining feature of Extra-tropical Depressions, also known as mid-latitude cyclonic storms or lows. • Origin: Extra-tropical depressions originate along the Polar Front, which is the boundary between cold polar air and warm tropical air. The development and structure of these systems are explained by the Polar Front Theory. • Structure: They contain distinct frontal surfaces: warm fronts, cold fronts, and often occluded fronts. • Contrast: Conversely, a Tropical Cyclone (TRS/Hurricane/Typhoon) is defined as a non-frontal synoptic-scale cyclone, and a hurricane is specifically characterized by having no fronts. ⭐️ ⭐️ Key Data to Remember • Extra-tropical Depressions: Frontal storms. Originate on the Polar Front. Move West to East (W to E). • Tropical Depressions/Cyclones: Non-frontal storms. Warm-core lows.

A Tropical Cyclonic Storm (TRS) is an intense vortex system with an extensive horizontal dimension. • Size Range: The overall diameter of a Cyclonic Storm, often defined by its outermost closed isobar, typically varies between 150 km and 1500 km. • Typical Diameter: The average diameter of a TRS is approximately 500 km (310 mi). Some sources indicate the depression associated with a TRS can have a diameter of ≈350 nm, which is roughly 648 km. • Indian Ocean Context: Cyclonic storms in the Indian Ocean basin show varying sizes, with pre-monsoon storms averaging 800 km and post-monsoon storms averaging about 1200 km in diameter. Therefore, selecting a size greater than 400 km captures the typical scale of a well-developed tropical storm system. ⭐️ ⭐️ Key Data to Remember (TRS Dimensions) • Total Diameter: 150 km to 1500 km. • Typical Diameter: ≈500 km (310 mi). • Eye Diameter (Calm Center): 20 to 50 km. • Winds: Strongest winds are found in the eyewall, just outside the eye.

Monsoon Depressions (MDs) are intense low-pressure systems with considerable asymmetry in their structure. For a typical westward-moving MD, the maximum weather—defined by heavy rainfall and cloud cover—is concentrated in the southwest sector. • Rainfall: Heavy rainfall is mostly confined to the SW sector within a radius of about 400 km from the center. The Southern sector generally experiences overcast skies, thick clouds (AS, NS, CB, and stratus), and almost continuous rain. • Winds: The highest wind speeds also occur in the southern sector of the depression, while the lowest speeds are typically in the northwest sector.

A Surge (or pulse) is characterized as a sudden increase in air current velocity within the same air mass, also known as velocity convergence. Shear lines are lines where wind speed decreases within the same air mass. Both Surge and shear lines are characteristics of Vigorous Monsoon. These features are indicators of strong, active flow within the monsoon system.

A Tropical Depression or Monsoon Depression (MD) is characterized by sustained wind speeds that are significant enough to warrant classification but remain below gale force. • Monsoon Depressions (MDs) have prevailing surface winds in the range of 18 to 33 kts. • The international standard classifies a Tropical Depression as having sustained winds up to 34 knots (64 km/h). The selected range of 17−27 kt fits accurately within the maximum wind range of a meteorological depression. Once sustained winds exceed 34 knots (33 kts in some regional contexts), the system is upgraded to a Cyclonic Storm (Tropical Storm). ⭐️ ⭐️ Key Data to Remember • Depression (MD/Tropical Depression): Maximum sustained winds are typically defined as ≤34 knots. • Monsoon Depression Range (India): 18 to 33 kts. • Upgrade Threshold: Systems become Cyclonic Storms when winds exceed 33 kts.

The Peninsular Discontinuity is a significant synoptic feature observed over central peninsular India (south of latitude 23 ∘ N) during the Pre-Monsoon (or Summer) season, specifically in April and May. It is also known as the dry-line and represents a boundary that frequently provides the necessary convergence for the development of atmospheric instability, giving rise to afternoon thunderstorms. The discontinuity is linked to the development and position of lower tropospheric anticyclones over the Arabian Sea and the Bay of Bengal.

A Cyclonic Storm (CS), which is a type of Tropical Revolving Storm (TRS), features a central, relatively calm area known as the eye. This eye is surrounded by an intense ring of convection referred to as the eyewall or wall cloud. • Structure: The wall cloud is a well-defined bank of vertically developed clouds, often reaching great heights (e.g., above 50,000 ft). • Composition: This feature consists of highly active Cumulonimbus (CB) and Cumulus (CU) clouds, formed in spiraling patterns. • Hazards: The eyewall is the most hazardous part of the storm, containing the strongest winds (maximum sustained winds are normally observed just beyond the eye region), heaviest precipitation (deluging rain), and most severe turbulence.

Monsoon Depressions (MDs) form primarily over the Bay of Bengal and typically move in a West to West-Northwest direction across North India. The speed of movement for these systems varies depending on their location along their track: • Eastern Track (Bay/Coastal Region): The average speed of a depression is approximately 5 to 10 Km per hour to the east of Longitude 85 ∘ E. • Western Track (Inland Plains): The average speed increases to 10 to 20 Km per hour to the west of Longitude 85 ∘ E. Since the region west of 85 ∘ E encompasses the Gangetic plains and central parts of India, the MD accelerates as it moves over the Plains of the country. ⭐️ ⭐️ Key Data to Remember • Speed (East/Bay): 5−10 km/hr (East of 85 ∘ E). • Speed (West/Plains): 10−20 Km/hr (West of 85 ∘ E). • Movement Direction: Mainly West to West-Northwest in July and August.

Tropical Revolving Storms (TRS), including Cyclonic Storms, generally originate in the tropics and are steered primarily by the easterly trade winds in low latitudes. • Speed in Tropics: The initial movement speed is characteristically slow, typically cited in the range of 10 to 20 knots. • Daily Distance: Using the lower end of this typical speed range (e.g., 10 knots): ◦ 10 knots×24 hours=240 nautical miles (NM). ◦ 240 NM≈444 km. • Classification: This result falls directly into the range of 300−500 km, representing the slow, characteristic drift of the system during its initial tropical stage. Systems may move faster (up to 50 knots) after recurving into the prevailing westerlies at higher latitudes. ⭐️ ⭐️ Key Data to Remember • Tropical Speed: 10 to 20 knots. • Movement Direction (Initial): West or Northwestward. • Recurvature: Movement accelerates (sometimes >50 kts) when caught in the westerly flow at higher latitudes.