MET 18- Climatology Of India

he wind experienced by an aircraft flying from Mumbai to Ahmedabad at 03 km during the Monsoon season is typically:
SWly (*)

Explanation
During the Southwest (SW) Monsoon season (June to September), the lower troposphere over the Indian region is characterized by a flow of warm, moist, unstable maritime air drawn from the southwest due to a seasonal low-pressure area over Asia.
At 03 km (approximately 10,000 ft), which is within the lower troposphere (below 6 km), the winds over South India, including the Mumbai to Ahmedabad route, are predominantly Westerly (Wly) or Southwesterly (SWly).
This SWly flow is part of the monsoon circulation, where the winds have crossed the Equator and veered, carrying high temperatures and high humidity.

Key Data to Remember:
• Season: SW Monsoon (June to September).
• Altitude Context: 03 km is in the lower troposphere, below the mean monsoon trough axis. Trade winds (monsoon flow originates from SE trades crossing the Equator) are considered to exist up to 10,000 ft.
• Wind Direction: Below 6 km during the SW monsoon, winds over South India are W-SWly.

The monsoon advances with: Bay of Bengal and Arabian sea currents (*)

Explanation
The Indian Summer (SW) Monsoon season, which typically runs from June to September, advances over the subcontinent through two distinct branches:
1. The Arabian Sea Branch: This branch normally brings monsoon rainfall over Kerala around June 1 and advances rapidly northward along the west coast, ultimately bringing monsoon rainfall over the entire Peninsula and central India.
2. The Bay of Bengal Branch: This branch advances northward, entering Bangladesh, Assam, and adjacent states, and subsequently deflects westward to enter the Gangetic plains as a South Easterly/Easterly current.
Both branches originate from the Southern Hemisphere’s Southeast Trade Winds, which cross the Equator and veer right to become the South Westerly monsoon winds due to the Coriolis effect.

Key Data to Remember:
• SW Monsoon Season: June to September.
• Advance: In two branches: Arabian Sea and Bay of Bengal.
• Origin: Veered Southeast Trade Winds (SWly flow).

A “Break in Monsoon” occurs when the axis of the monsoon trough shifts northward, typically resting near the foothills of the Himalayas.
This synoptic reorganization causes a drastic decrease of rainfall over the central parts of the country. Consequently, the monsoon pressure rises over most parts of the country. The pressure departures from normal over the central parts of India, extending from Gujarat to the North Bay, become positive, often ranging from 2 to 4 hPa or even up to 8 hPa above normal.

Key Data to Remember:
• Synoptic Cause: Monsoon trough shifts to the foothills of the Himalayas.
• Pressure Change: Pressure rises over most parts of the country.
• Magnitude: Positive pressure departures of 2 to 8 hPa are observed over central India.
• Weather Effect: Low rainfall over central India.

A “Break in Monsoon” occurs when the axis of the monsoon trough shifts significantly northward toward the foothills of the Himalayas. While this causes a drastic decrease in rainfall over central India, it leads to a concentration of heavy rainfall over the foot hills, including areas of West Bengal, Nepal, Bhutan, Northeast India, Bihar, and Uttar Pradesh.
In this specific region of concentrated heavy rainfall and floods, the intense weather conditions are accompanied by surface wind that becomes strong.

Key Data to Remember:
• Synoptic Position: Monsoon trough is near the foothills of the Himalayas.
• Weather Effect (East UP/Bihar): Heavy rainfall, leading to floods.
• Wind Speed: Surface wind becomes strong in this region.

During the winter season in India, the passage of a Western Disturbance (WD)—a synoptic system moving across North India in mid-latitude westerlies—creates conditions conducive to radiation fog formation in the plains.
1. Rear Sector Conditions: Conditions favorable for fog occur mainly in the rear sector of the Western Disturbance.
2. Timing: Radiation fog typically occurs one or two days after the cessation of rainfall associated with the WD.
3. Mechanism: The WD passage often results in clear skies (increasing terrestrial radiation loss), stabilization of the air mass, and an ample supply of low-level moisture (due to previous rainfall or advection), which are the necessary conditions for radiation fog formation. This fog often occurs over large areas of North India, including Punjab, Uttar Pradesh, and Bihar.

Key Data to Remember:
• WD Effect: Increased radiation fog likelihood in the rear sector.
• Location: Primarily affects the plains of North India during winter (December to March).
• Time of Day: Normally occurs in the early morning and may prevail till late forenoon

The Southwest (SW) Monsoon season, which typically prevails over the Indian subcontinent from June to September, is synonymous with the period of greatest precipitation and is thus often referred to as the Rainy Season.
In tropical meteorology, regions influenced by the monsoon circulation experience a distinct wet season (or rainy season) when moisture-laden winds blow from sea to land (summer monsoon), contrasting sharply with the dry season.

Key Data to Remember:
• SW Monsoon Period: June to September (Summer Monsoon).
• Alternative Name: Rainy Season or Wet Season.
• Other Seasons: Pre-monsoon/Hot weather season (March to May) and Post monsoon/Transition season (October to December) are distinct periods.

During the Southwest (SW) Monsoon season (June to September), the general circulation over the Indian subcontinent is driven by an intense thermal low-pressure area (Heat Low) over Northwest India/Pakistan, coupled with a high-pressure system (Mascarene High) over the South Indian Ocean.
This large pressure contrast generates the strong inflow of maritime, moisture-laden air, resulting in the maximum pressure gradient for the season.
A “vigorous monsoon” implies strong surface winds (Southwesterly/Westerly flow) along the west coast, and strong winds are directly proportional to a steep pressure gradient (closely spaced isobars or contours).

Key Data to Remember:
• SW Monsoon Period: June to September.
• Cause: Strong Pressure Gradient Force (PGF) between the Mascarene High and the Heat Low.
• Result: Strong Southwesterly flow, often associated with the Low-Level Jet (LLJ) in the Arabian Sea (40–60 kt).
• Operational Context: Strong winds and associated heavy rainfall along the coast are directly linked to the steep pressure gradient.

During the Southwest (SW) Monsoon season (June to September), the upper tropospheric wind pattern over Peninsular and Eastern India is characterized by a strong reversal of flow, resulting in the presence of the Tropical Easterly Jet (TEJ).
• Altitude Context: 14 km is approximately 46,000 ft, which is near the core of the TEJ, typically found between the 150 hPa and 100 hPa levels.
• Direction and Speed: This jet stream blows strongly from the East. The axis of the TEJ is centered between 10 ∘N and 15∘N (the latitude band covering the route). Mean speeds often reach 70 to 80 knots at this level over Southern India, such as Madras (Chennai).
• Mechanism: The TEJ is a result of the extreme thermal gradient from north to south over Asia during summer, causing the subtropical ridge to shift northward, and moderately strong lower-level westerlies to reverse direction at higher elevations.
Key Data to Remember:
• Season: SW Monsoon (Northern Hemisphere Summer).
• Altitude: 14 km (150 hPa to 100 hPa level).
• Feature: Tropical Easterly Jet (TEJ).
• Wind Direction: Easterly (E) flow.
• Typical Speed: 60 to 80 knots.

The axis of the Monsoon Trough, which extends from the intense Seasonal Low over Pakistan across the Indo-Gangetic Plain to the Bay of Bengal, is the primary factor dictating the distribution of rainfall across India during the Southwest (SW) Monsoon season.
The daily variations in the trough’s position have a vital bearing on monsoon rains. When the axis shifts northward towards the foothills of the Himalayas, it results in a “Break in Monsoon” condition, causing a drastic decrease of rainfall over the central parts of the country while concentrating heavy rain along the foothills.

During the winter season (December to February) over North India, the upper air circulation is dominated by the Sub-Tropical Westerly Jet (STWJ) system.

1. Altitude and Location: At 06 km (approximately 20,000 ft or 500 hPa), the air masses north of the subtropical ridge line (which is typically around 12 ∘N to 18∘N at this level) are subject to predominantly Westerly (Wly) winds.

2. Wind Speed: Over the North Indian belt (25 ∘N to 30 ∘N), the westerly winds strengthen with height, reaching speeds around 40 kts at the 500 hPa level.

3. Operational Context: The flight path from Delhi to Kolkata is generally South Eastward. An aircraft flying Eastward encountering a strong Westerly wind will experience wind from right and tail, benefit from a significant Tail wind component, reducing flight time and fuel consumption.

Key Data to Remember:
• Season: Winter (Dec–Feb).
• Altitude: 06 km (approx. 500 hPa) is within the westerly regime over North India.
• Dominant Feature: Sub-Tropical Westerly Jet influence.
• Wind Direction: Prevailing Westerly (Wly) flow.

In Indian Climatology, the primary monsoon period is formally known as the Southwest (SW) or Summer Monsoon season. This season is characterized by a seasonal reversal of winds bringing moisture-bearing air from the sea to the land.
The duration of this major circulation pattern is consistently defined as commencing in June and concluding in September. This period represents the wet season over most of the Indian subcontinent.

Key Data to Remember:
• Season Name: SW or Summer Monsoon.
• Duration: June to September.
• Operational Context: This is the primary period of copious rain, convective activity (Cumulonimbus), and high humidity over the region.

A “Break in Monsoon” is a synoptic situation characterized by a major northward shift of the axis of the monsoon trough, moving it close to the foothills of the Himalayas.
This shift causes a drastic decrease in rainfall over the central mainland part of the country. Consequently, the heavy rainfall concentrates over the foothills, including areas of Northeast India, resulting in floods over regions like Assam, Bihar, and Uttar Pradesh.

Key Data to Remember:
• Cause of Break: Monsoon trough axis shifts to the foothills of the Himalayas.
• Maximum Rainfall Area: Along the foothills of the Himalayas and Northeastern parts of India.
• Impact: Heavy rainfall causes floods in this region (e.g., Assam, Bihar, UP)

The Hot weather period is officially defined in Indian Climatology as the Pre-monsoon season, typically spanning March to May. It serves as the transition phase between the winter circulation and the onset of the Southwest (SW) Summer Monsoon.

Key Data to Remember:
• Period: March to May.
• Synoptic Features: Increased land heating leads to the development of surface low pressure (thermal lows) over Northwest India.
• Aviation Relevance: The season is known for very high temperatures, heat wave conditions, and dust/sand storms over Northwest India. Intense convective activity is common, resulting in severe wind squalls and thunderstorms. Tropical cyclones also have a secondary peak activity period during April to mid-June.

The Southwest (SW) Monsoon season (June to September) is characterized by an intense thermal low-pressure area, known as the Seasonal Low or Heat Low, centered over Pakistan and adjoining Rajasthan.
The strong inflow of maritime, moisture-laden air into this low from the South Indian Ocean (Mascarene High) creates a strong pressure gradient across the North Arabian Sea and the West Coast of India. A steep pressure gradient is associated with closely spaced isobars, leading to strong surface winds (SWly/Wly flow) characteristic of a vigorous monsoon period [110, conversation history].

Key Data to Remember:
• Season: SW Monsoon (June to September).
• Driving Force: Intense Seasonal Low over NW India, replacing the winter high.
• Gradient Effect: Steep pressure gradient force drives the strong Southwesterly flow required for the monsoon and vigorous wind conditions along the West Coast [110, conversation history].
• Contrast: Pressure gradient is weak during the pre-monsoon and post-monsoon seasons

The term SW monsoon (Southwest Monsoon) is used to identify the primary rainy season over the Indian subcontinent and Southeast Asia. This period is also known as the Summer Monsoon season.

Key Data to Remember:
• Period: June to September.
• Weather: Characterized by warm, moist, unstable air resulting in copious rainfall.
• Nomenclature: Refers to the prevailing wind direction of the current, which blows from the southwest (SW). (The NE monsoon refers to the winter circulation, primarily during December to February).

The Pre-monsoon season typically covers the months of March through May.
At 10 km (approximately 33,000 ft or near the 300 hPa pressure level), the upper-level wind pattern over North India is largely influenced by the Sub-Tropical Westerly Jet (STWJ) system.

• Dominant Flow: During the period from October to May (encompassing winter and pre-monsoon), the STWJ is generally observed over North India. The upper winds over the northern part of India are overall Westerly (Wly).
• Speed: Over North India between 25 ∘N and 30 ∘ N, the westerly winds strengthen with height. At the 300 hPa level (10 km), wind speeds are typically strong, around 75 kts.
• Direction: The STWJ is a predominant feature of the mid-latitude circulation, blowing strongly from the West.
Since the flight path from Delhi to Kolkata is generally Eastward, the aircraft will experience a substantial Tail wind due to the prevailing Westerly flow at this cruising altitude.

Key Data to Remember:
• Season: Pre-monsoon (March–May).
• Altitude Context: 10 km (300 hPa) is within the core influence of the STWJ over North India

During the winter season (October to May), the upper air circulation over Northern and Peninsular India is characterized by the dominance of the Sub-Tropical Westerly Jet (STWJ).
• Altitude Context: 12 km (approximately 40,000 ft) corresponds closely to the core region of the STWJ, typically found near the 200 hPa level.
• Direction and Location: Over North India (including the region between Chennai and Kolkata, north of 8 ∘N where the ridge lies), the winds at this height are predominantly Westerly (Wly), strengthening with height and potentially reaching speeds around 85 kts at the 200 hPa level.
• Operational Context: Since the flight path from Chennai to Kolkata is generally Eastward, a prevailing Westerly wind provides a significant Tail wind component.

Key Data to Remember:
• Season: Winter (Dec–Feb), part of the prevailing Westerly regime (Oct–May).
• Altitude: 12 km (approx. 200 hPa).
• Feature: Sub-Tropical Westerly Jet (STWJ).
• Wind Direction: Westerly (Wly)

A “Break in Monsoon” is a critical synoptic condition characterized by the significant northward shift of the axis of the monsoon trough, moving it close to the foothills of the Himalayas.
This shift results in a drastic decrease of rainfall over the central parts of the country (the mainland plain). Simultaneously, the heavy rainfall concentrates along the foothills of the Himalayas, often causing floods in areas such as Assam, Bihar, and Uttar Pradesh.

Key Data to Remember:
• Synoptic Position: Monsoon trough axis shifts to the foothills of the Himalayas.
• Rainfall Impact: Rainfall decreases over the central plains but concentrates heavily along the foothills.
• Pressure Impact: Pressure departures over the central parts of India become positive (pressure rises).
• Duration: The average duration of a break in monsoon is typically 4 to 6 days

The Mid Tropospheric Cyclone (MTC), officially referred to as Mid Tropospheric Circulation, is a significant synoptic feature observed over South Asia during the Southwest (SW) or Summer Monsoon season.
These cyclonic vortices are found in the middle layers of the troposphere, primarily between 3 km and 6 km altitude, with their maximum amplitude near the 600 hPa level. MTCs are responsible for causing heavy rainfall in their vicinity, particularly in the western and southern quadrants, due to intense convection. They are a phenomenon distinct from monsoon depressions but are common during the wet season.

Key Data to Remember:
• Season: SW or Summer Monsoon months (June–September).
• Occurrence: More common in the first half of the monsoon season.
• Vertical Extent: Typically found between 3 km and 6 km (near 600 hPa).
• Location: Develop over the Northeastern Arabian Sea and adjoining land areas.
• Duration: Generally lasts for 3 to 7 days

The flight path from Kolkata (East India) to Nagpur (Central India) involves a general trajectory toward the West or West-Southwest. The altitude of 02 km (approximately 6,500 ft) is within the lower troposphere, often below the maximum influence of the upper-level westerly flow, placing it within the domain of the surface/low-level circulation.
1. Winter Circulation (North India): The winter season (December to February, falling within the October to May regime) over North and Central India is characterized by the influence of the Siberian High, resulting in continental outflow. Surface winds in North India (North of 25 ∘ N and West of 88 ∘E) are predominantly Northwesterly (NWly). East of this area, over regions like Assam, winds are light easterlies. Since the flight traverses a path from East India toward Central India, NWly or NEly winds are probable at low levels.
2. Wind Direction Relative to Track: Whether the prevailing low-level wind is Northwesterly or Northeasterly, the wind direction will come from the aircraft’s left side (Port side) on a westward track.
◦ If the wind is NWly (e.g., 315 ∘T) on a westbound track (approx. 250 ∘T), the wind has a significant left crosswind component.
◦ A prevailing wind blowing from the left of the aircraft causes Port drift.

Key Data to Remember:
• Season/Altitude: Winter, low level (02 km).
• Prevailing Wind: NWly flow predominates over North/Central India below 3 km in winter.
• Operational Effect: On a westward track, a Northwesterly wind results in a significant left crosswind component, causing Port drift.