MET 15- Jet Stream & CAT

The Tropical Easterly Jet (TEJ), also known as the Equatorial Easterly Jet, is a summertime phenomenon in the Northern Hemisphere. It develops during the Northern Hemisphere summer when the strong solar heating of the Central Asian landmass (Asian plateau) creates an intense temperature gradient.
The TEJ runs from the South China Sea westwards across Southern India, Ethiopia, and the sub-Sahara regions of Central Africa.

⭐️ ⭐️ Key Data to Remember (Tropical Easterly Jet):
• Season: Northern Hemisphere Summer (Monsoon Season, generally June through September).
• Location (Latitude): Typically between 10∘N and 20 ∘N. Over India, the core is often near 9∘N to 13∘N (e.g., near Madras).
• Altitude/Pressure Level: High altitude, typically 15-16 km (45,000–55,000 ft), centered near the 100 hPa or 150 hPa level.
• Speed: Commonly 60-80 knots, but can reach up to 150 knots.
• Significance: It is closely linked to the Asian Monsoon circulation. It is one of the few global jets that blows strongly from the east.

in STJ wind shear is more. __because it is fastest and gradient will be more

A jet stream is a narrow, fast-moving current of air characterized by one or more velocity maxima. These segments of maximum wind speed, often referred to as jet streaks, vary in speed as they progress through upper air pressure systems.

Major wind maxima are typically located downstream from the region of strongest confluence (where streamlines converge). This implies that the jet flow strengthens (accelerates to its maximum speed) along its path after passing through regions favorable for pressure gradient intensification.

A jet stream, defined by the WMO, is a narrow current in the upper troposphere characterized by strong wind shear and featuring one or more velocity maxima. These high wind speed regions are often referred to as jet streaks or segments.

⭐️ ⭐️ Key Data to Remember (Jet Maxima):
• Structure: The jet maximum is not constant along the entire length of the stream but is often broken into segments.
• Location: Major wind maxima are typically situated downstream from the region of strongest confluence (convergence).
• Speed: Wind speeds must be 60 knots (30 m/sec) or greater to qualify as a jet stream. Speeds commonly exceed 100 knots, and segments up to 300 knots have been reported.
• It is not uncommon for a jet stream map to show multiple segments or maxima, sometimes two or three at one time.

The Tropical Easterly Jet (TEJ), or Equatorial Easterly Jet, is a prominent feature of the Northern Hemisphere summer circulation, specifically associated with the Asian Monsoon system. It occurs when intense solar heating of the large landmasses (like the Central Asian plateau) creates a reversal of the usual south-to-north temperature gradient at high altitudes.

This jet stream, which blows strongly from the east, is centered near the latitude of Madras. Since the monsoon season typically runs from June to September, the jet reaches its maximum strength and consistent position during the peak months of intense heating and monsoon activity, which is typically July (when the jet is centered at 100 hPa, e.g., near Madras).

⭐️ ⭐️ Key Data to Remember (TEJ):
• Season: Northern Hemisphere Summer.
• Location: Between 10∘N and 20∘N, running from the South China Sea westward across Southern India and Central Africa.
• Altitude: Typically centered around the 150 hPa to 100 hPa level (∼45,000 ft to 55,000 ft).
• Strength: Mean wind speeds are often reported around 70 knots (e.g., at Madras).

The layered structure of the Subtropical Jet Stream (STJ) is characterized by significant vertical and horizontal wind shear zones, which often contain maximum wind segments or turbulence layers. While the main core is the primary maximum, secondary areas of strong winds/shear exist flanking the jet stream, often described in terms of the polar (cold) and equatorial (warm) sides.

Assuming this question refers to the structure in the Northern Hemisphere:
1. Polar Side (North): The strongest Clear Air Turbulence (CAT), associated with maximum wind shear, is generally found near or just below the jet core on the cold air (poleward) side.
2. Equatorial Side (South): A secondary area of CAT is found above the core on the warm air (equatorward) side. Furthermore, specific characteristics of the Sub Tropical Westerly Jet (STWJ) over India indicate strong horizontal wind shear to the south of the core.
Therefore, the layered structure includes a significant maximum/shear zone extending to the equatorial side (South) above the core.

⭐️ ⭐️ Key Data to Remember:
• Season: Dominant during the SW Monsoon, centered in July.
• Location: Occurs over Peninsular India, running from the South China Sea westward across Southern India and Central Africa. Over the Peninsula, the axis lies approximately between 10 ∘N and 15 ∘N.
• Altitude: Found high in the upper troposphere, centered near the 100 hPa level (approximately 15–16 km or 45,000 ft).
• Speed: Mean wind speeds range from 70-80 knots.
• Significance: Its presence over India corresponds with the high-altitude reversal of the temperature gradient caused by intense summer heating over the Asian landmass. The TEJ disappears from the Indian latitude during the SW monsoon period.

This scenario requires applying the modified version of Buys Ballot’s Law used for upper-level winds, assuming the flight is in the Northern Hemisphere (NH), where jet streams are predominantly Westerly.
1. Jet Stream Characteristics: In the NH, the wind flows so that it keeps the lower pressure and colder air mass to its left.
2. Falling Pressure: If an aircraft flying perpendicular to the jet stream experiences falling pressure, it means the aircraft is tracking toward the region of lower pressure.
3. Crosswind Direction: Since the jet stream (Westerly wind) keeps the Low Pressure area (cold air) on its left side, flying perpendicularly toward that Low Pressure (falling pressure) means the wind component is crossing the aircraft’s track from the left.

⭐️ ⭐️ Key Data to Remember (Upper Winds, NH):
• Rule: When flying with your back to the upper wind, Low Pressure (cold air) is on your left.
• Action: If flying toward the cold air/low pressure region (falling pressure), the upper-level wind component will produce left drift (wind from the left).
• Operational Context: Flying toward colder air or lower pressure at a constant indicated altitude results in a loss of True Altitude (“High to Low, Look out Below”).

A jet stream is defined by strong vertical and lateral wind shear. Vertical wind shear is the change in wind speed or direction with altitude. The strongest winds are concentrated at the jet core, typically just below the tropopause.
Significant vertical wind shear is experienced both above and below the jet core due to the rapid decrease in wind speed away from the axis.
• Below the Core: The area just below the core on the cold air (polar) side typically experiences the most severe Clear Air Turbulence (CAT), highly correlated with strong wind shear.
• Above the Core: A strong vertical wind shear zone, sometimes described as a secondary area of severe CAT, exists above the core. This occurs as the jet stream winds rapidly decrease in strength as they transition into the lower stability (and typically warmer air) of the Stratosphere.
Therefore, strong vertical wind shear is pronounced in the layers immediately surrounding the core, including the region above the core.

⭐️ ⭐️ Key Data to Remember:
• Definition: Jet streams feature strong vertical and lateral wind shear.
• Primary CAT/Shear location: Just below the core, on the cold air side (polar side).
• Secondary CAT/Shear location: Above the core, extending toward the warm air side (equatorial side) tropopause.

The wind speed along the axis of a jet stream is not uniform. A jet stream is characterized by strong winds concentrated along a quasi-horizontal axis featuring one or more velocity maxima.
These regions of maximum wind speed are known as jet streaks or jet maxima. The jet maximum is not constant but is broken into segments. The speed of the wind varies as these segments move through upper-air pressure systems. Speeds commonly exceed 100 knots, and on occasion, speeds of 300 knots or more have been reported.

⭐️ ⭐️ Key Data to Remember:
• Structure: Jet streams are discontinuous, featuring segments of maximum speed (jet streaks).
• Definition: A jet stream must have wind speeds of 60 knots (30 m/sec) or greater.

The vertical wind shear (change in wind speed over a change in height) in a jet stream is significantly stronger than the horizontal (lateral) wind shear (change in speed over a horizontal distance).
A jet stream is defined by its strong vertical and lateral wind shear. Quantitatively, the vertical wind shear is typically on the order of 5 to 10 m/sec per km, while the lateral shear is approximately 5 m/sec per 100 km. This immense gradient in the vertical dimension is a primary cause of Clear Air Turbulence (CAT).

⭐️ ⭐️ Key Data to Remember:
• Vertical Shear Location: The strongest CAT and shear are located near or just below the jet core on the cold air (poleward) side.
• Magnitude: Vertical shear values can reach 6 knots per 1,000 feet or more, indicating severe turbulence potential.
• Definition: Vertical wind shear is measured in units like knots per 100 feet. Lateral shear is measured in knots per 1000 feet or other large horizontal distances.

Combined Summary of Subtropical Westerly Jet (STWJ)

The Subtropical Westerly Jet (STWJ) is a semi-permanent upper-air jet stream strongest during the colder season, dominating the subtropical latitudes of both hemispheres. Over India, it is typically observed from October to May, with its core strongest near the 200 hPa level (10.5–12 km).

Key Characteristics Over India (Winter Season)

Mean Winter Position: Near 26°N (Jodhpur, Allahabad, Guwahati) at 200 hPa (~12 km) with core speeds around 100 kt.
Seasonal Shift: The jet axis moves southward as winter progresses, reaching its lowest latitude (~22°N) in February, then shifts northward and weakens by May.
Twin Maxima: Two wind maxima occur—one over western India (Jodhpur) and one over eastern India (Guwahati).

Observed Speeds:
Jodhpur: ~95 kt at 12 km
Guwahati: 95 kt at 10.5 km; 90 kt at 12 km
Allahabad: ~80 kt at 10.5 and 12 km

Wind Structure and Vertical/Horizontal Shear
Winds of 60 kt or more occur north of 20°N, concentrated in a thin layer.
Jet-level winds begin near 9 km at 23°N and as low as 7 km over Assam, extending up to 14 km.
Horizontal wind shear: > 5 km/hr per 100 km south of the core.
Vertical shear: ~20 km/hr per km below the core.
Position Relative to Tropopause

The STWJ lies entirely within the troposphere, about 3–4 km below the tropopause, located near the break between the tropical tropopause (~100 hPa) and the mid-latitude tropopause (200–250 hPa).

Seasonal Behavior
Strongest: In February along the Delhi–Allahabad–Guwahati belt.
Shift: Moves to ~30°N by May and disappears during the SW monsoon.
Reappearance: Returns near 30°N in October at 12 km.
Monthly Speeds: ~70 kt in November, ~80 kt in December.

A jet stream is a narrow current concentrated along a quasi-horizontal axis featuring one or more velocity maxima (jet streaks). When the jet stream flow is wavy (meridional flow), it follows large-scale atmospheric troughs (elongated areas of low pressure) and ridges (elongated areas of high pressure).
The segments of maximum wind speed, or jet streaks, are typically located over or near the crests of the ridges. In general, where the horizontal temperature contrasts are large, the contour lines on upper-level charts are close together, resulting in strong winds.
Pilot Operational Context:

The movement of jet streaks through troughs and ridges is dynamically linked to changes in divergence and convergence aloft, which dictates the intensification of surface weather systems (cyclogenesis). A strong jet core swinging over a developing mid-latitude cyclone creates areas of convergence and divergence along its flanks. The strongest Clear Air Turbulence (CAT) is often found near these jet streaks, particularly in sharp upper troughs or near the jet core where wind speed accelerates or decelerates.

Summary (Crisp):
The Subtropical Jet Stream (STJ) is a semipermanent upper-air westerly jet that forms above subtropical anticyclones.

Mean Position: Around 26–27°N in winter (near 200 hPa) over regions like Jodhpur, Allahabad, and Guwahati.

Altitude & Speed: Core near 200 hPa (~45,000 ft) with speeds 100–200 kt, sometimes 300 kt near Japan.

Direction: Blows from the west.

Seasonality: Strongest in winter (25–40°N). In summer, it shifts north to 40–45°N and weakens.

Jet streams, including the Subtropical Jet Stream (STJ), are fundamentally thermal wind systems caused by large horizontal temperature differences (thermal gradients). These large temperature differences create corresponding strong horizontal pressure gradients aloft, which result in the concentrated stream of high-speed wind. The jet stream typically occurs in an area of intensified temperature gradients characteristic of tropopause breaks.

⭐️ ⭐️ Key Data to Remember:
• Cause: Large horizontal temperature difference or extreme thermal wind component.
• Effect: Leads to strong pressure gradients aloft and high wind speeds.
• Location: The STJ is found where air from the Hadley cells meets air from higher latitudes, creating a large thermal contrast.

A jet stream is usually impossible to identify visually unless there is sufficient high-level moisture present. When moisture is available, the primary visual indicator from the ground or in flight is the presence of streaks of Cirrus (CI) cloud.
• These streaks are caused by the very high velocity wind flow around the jet core, which lowers pressure and temperature, causing water vapor to sublimate into ice crystals, forming the cloud.
• When viewed from the ground, these wind-blown wisps of cirrus may be seen moving at right angles to lower-level clouds.
• Cirrus clouds are composed entirely of ice crystals and are found at high altitudes.

⭐️ ⭐️ Key Data to Remember (Other Recognition Methods):
• In Flight: Jet streams are recognized by increasing wind speed, changes in temperature, and the onset of Clear Air Turbulence (CAT).
• Charts: On meteorological charts (such as upper wind charts or Significant Weather Charts), jet streams are delineated by isotachs (lines of equal wind speed).
• Lenticular clouds indicate mountain wave activity, not jet streams, although mountain waves often intensify turbulence near jet streams.

By definition, a jet stream is a narrow current of air in the upper troposphere characterized by strong vertical and lateral wind shear, featuring one or more velocity maxima. The WMO assigns an arbitrary lower limit of 30 m/sec (approximately 60 knots) to the wind speed along the axis of the jet stream.

⭐️ ⭐️ Key Data to Remember:
• Definition Threshold: A jet stream is a wind greater than 60 kt in speed.
• Metric Equivalent: 30 m/sec.
• Typical Speeds: Speeds in excess of 100 kt are common.

The Subtropical Westerly Jet (STWJ) is a semipermanent feature that dominates the upper air circulation over North India during the colder half of the year. It is generally observed over North India from October to May.
The STWJ is strongest during the winter months. As the season advances, the axis of the jet stream shifts southward, reaching its southernmost position toward the end of winter or early spring.

⭐️ ⭐️ Key Data to Remember (STJ over India):
• Period of Prevalence: October to May. This period covers the winter, pre-monsoon, and post-monsoon seasons.
• Absence: It disappears from the Indian latitude during the Southwest (SW) monsoon period (June to September), shifting north of the Himalayas.
• Strength: Strongest during the winter.
• Weather: Associated Clear Air Turbulence (CAT) can be expected over North India during the winter season.

The Tropical Easterly Jet (TEJ) over Peninsular India develops in summer due to a strong north–south temperature gradient, which reverses lower-level westerlies into powerful easterlies aloft. The jet’s core lies between 14–16 km (≈ 150–100 hPa), centered around 9°–13°N.

Madras: Peak near 16 km (100 hPa) with mean winds around 085°/70 kt.

Trivandrum: Peak lower at 14 km (150 hPa) with 080°/65 kt, stronger than Madras at that level.

Bombay: Mean winds around 55 kt.

Colombo & Trivandrum: Max winds near 14 km (150 hPa).

Madras, Vishakhapatnam & Nagpur: Max winds near 16 km (100 hPa).

Overall, the jet stream’s maximum height is lower in the southern stations and higher toward central India.

The high-altitude jet stream found in the tropical region, specifically the Tropical Easterly Jet (TEJ) or Equatorial Easterly Jet, is characterized by strong winds blowing from the east.
This direction is an exception to the predominantly westerly flow found in the upper atmosphere outside the tropical and polar regions.
⭐️ ⭐️ Key Data to Remember:
• Direction: Easterly.
• Location: Northern Hemisphere summer (Monsoon Season) between 10
∘
and 20
∘
North. It runs from the South China Sea westward across southern India and Central Africa.
• Altitude: Typically found at high levels, circa 100 hPa or 150 hPa (45,000 ft to 55,000 ft or 13 km to 16 km).
• Speed: Commonly reported around 70-80 knots over regions like India during peak season.

A jet stream is characterized by strong vertical and lateral wind shear. The typical order of magnitude for the vertical wind shear in a jet stream is 5 to 10 m/sec per km.
⭐️ ⭐️ Key Data to Remember:
• Vertical Shear Magnitude: 5 to 10 m/sec per km.
• Lateral Shear Magnitude: Approximately 5 m/sec per 100 km.
• Operational Context (Hazard): Vertical wind shear is a primary cause of Clear Air Turbulence (CAT). Pilots should avoid areas where vertical shear exceeds 6 knots per 1,000 feet.
• Location: Strong vertical wind shears are concentrated in stable layers just above and below the jet core. The greatest wind shear typically occurs just below the core, on the cold air (low pressure) side.

The Subtropical Jet Stream (STJ) is predominantly a Westerly wind current that exists throughout the year. It is a semipermanent feature that occurs above the subtropical anti-cyclones.
The STJ forms where air flowing poleward from the Hadley cells is deflected due to the Coriolis force, resulting in a strong westerly flow in the upper troposphere.
⭐️ ⭐️ Key Data to Remember (Subtropical Jet Stream):
• Direction: Westerly.
• Cause: Caused by the circulation of the Hadley cells. Air moving poleward from the equator is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, resulting in a westerly jet.
• Altitude/Pressure Level: Typically found at the 200 hPa level (approximately 45,000 ft).
• Location (Latitude): Generally between 20
∘
to 40
∘
N/S. In the Northern Hemisphere, it is located in the latitude band 25
∘
to 40
∘
in winter and 40
∘
to 45
∘
in summer.

A jet stream is defined as a narrow current concentrated along a quasi-horizontal axis in the upper troposphere characterized by strong wind shear, featuring one or more velocity maxima. These centers of high-speed winds are specifically referred to as jet streaks or jet maximums.
The wind speed along the axis is not uniform; instead, it is broken into segments, or jet streaks. These jet streaks move through upper pressure systems (troughs and ridges) and are associated with regions of strong convergence and divergence along the flanks of the jet.

⭐️ ⭐️ Key Data to Remember (Jet Streaks):
• Synonym: Also called a jet maximum.
• WMO Definition: Represents the regions of “velocity maxima” within the jet stream structure.
• Location: Major wind maxima are typically located downstream from the region of strongest confluence.
• Significance: The jet streak plays a critical role in the development and intensification of surface low- and high-pressure areas by creating necessary patterns of upper-level divergence (above lows) and convergence (above highs).

The Tropical Easterly Jet (TEJ) is a prominent feature of the Northern Hemisphere summer. Globally, the equatorial jet stream latitude band is between 10∘N and 15∘N. Over the Indian Peninsula:
• The location is generally cited between 9 ∘N and 13 ∘N.
• During its maximum strength (July/August), the jet stream core is centered near the latitude of Madras (Chennai), often specified as 13 ∘ N or lying between 10∘N and 15∘N.
• The jet stream runs from the South China Sea westward across Southern India.

⭐️ ⭐️ Key Data to Remember (TEJ):
• Direction: Easterly.
• Altitude: 15-16 km (100 hPa).
• Season: Northern Hemisphere Summer (Monsoon season).

The WMO defines a jet stream as a narrow current concentrated along a quasi horizontal axis in the upper troposphere. This axis is the path or centerline that denotes the maximum wind speeds of the jet stream.
• Axis/Core: The wind speed is fastest at the core. The core lies along this quasi-horizontal axis. The axis of the jet stream is often indicated on weather charts with wind speed and height information.
• Jet Streak: The specific centers of high-speed winds that move through the axis of a jet stream are known as jet streaks or velocity maxima.
Therefore, the axis refers to the centerline or path of the maximum speed concentration.

⭐️ ⭐️ Key Data to Remember:
• Axis: The quasi-horizontal path or centerline along which the strongest winds are found.
• Core: The region of highest velocity on the axis.
• Jet Streak: A segment or center of maximum wind speed within the core/axis.

The presence of a jet stream, while generally difficult to identify visually, can sometimes be indicated by trails of cirrus clouds if the air is moist enough. The high velocity of the air around the jet stream causes a lowering of pressure and temperature, cooling the air to its dewpoint, which leads to water vapor sublimating out as ice crystals, forming cirrus clouds. These cirriform clouds, which are also known as cirrus streaks or wind-blown wisps, can be seen along the path of the jet stream.

⭐️ ⭐️ Key Data to Remember:
• Visual Indicator: Cirrus clouds are the key visual signpost for high-altitude jet streams.
• Cloud Type: Cirrus (CI), Cirrostratus (CS), and Cirrocumulus (CC) clouds are often found near the tropopause where jet streams exist.
• Mechanism: Pressure and temperature decrease near the high-velocity jet core, causing water vapor to sublime into ice crystals.

When penetrating Clear Air Turbulence (CAT), the primary objective, after establishing the recommended turbulence penetration airspeed, is to minimize structural stress on the aircraft while maintaining positive control. Therefore, the procedure requires concentrating on maintaining a constant pitch attitude and wings level by referencing the attitude indicators, while carefully avoiding harsh or excessive control movements. The aircraft should be allowed to “ride the waves,” disregarding fluctuations in altitude or airspeed, as excessive maneuvering increases stress on the airframe. Large attitude excursions in the rolling plane should not be permitted, as these may lead to undesirable nose-down pitch changes.

⭐️ ⭐️ Key Data to Remember (FAA/ICAO Operational Context):
• Control Input: Avoid harsh or excessive control movements; use smooth inputs.
• Attitude: Maintain a constant pitch attitude and wings level.
• Airspeed/Altitude: Do not chase airspeed or altitude; allow the aircraft to “ride the waves”. Significant changes in altitude/attitude may occur, but positive control must be maintained (Moderate Turbulence definition).
• Automation: If using autopilot, disengage altitude hold and speed hold modes, as these can increase structural stresses through greater aircraft maneuvering.
• Hazard: Loss of control resulting from incorrect recovery maneuvers is a greater hazard than structural failure due directly to turbulence.

Clear Air Turbulence (CAT) is high-level turbulence usually associated with wind shear in narrow zones near the jet stream core. Because CAT is often concentrated in relatively narrow vertical layers, changing altitude is typically the most effective method to exit the turbulent region and avoid its effects. A change of just a few thousand feet (climb or descent) is often sufficient to escape the severe shear layer.

⭐️ ⭐️ Key Data to Remember (ICAO/Operational Context):
• CAT Location: Strong vertical wind shears are often concentrated in stable layers just above and below the core of the jet stream. CAT is strongest near or just below the jet axis on the cold air side.
• Avoidance: If CAT is encountered near the jet core, the pilot should climb or descend a few thousand feet or move farther from the jet core.
• Penetration Speed: If penetration is unavoidable, reducing speed to the recommended turbulence penetration airspeed is necessary to minimize structural stresses, but this is a penetration technique, not an avoidance technique.
• Patchy Nature: CAT is patchy, meaning moving vertically often leads to smooth air quickly.

The combined meteorological phenomenon of a jet stream interacting with mountain waves causes severe CAT.
Clear Air Turbulence (CAT) normally occurs around jet stream boundaries due to large vertical and horizontal wind shear. This turbulence is greatly increased in extent and intensity when the jet stream flows over high ground, particularly when mountain waves are present. The distortion of the stable layers near the jet core by the mountain wave system enhances the wind shears, leading to a breakdown of the airflow into turbulence.

⭐️ ⭐️ Key Data to Remember (Operational Context):
• Jet Stream Turbulence: CAT is strongest near or just below the jet axis on the cold air side.
• Mountain Influence: Normal jet stream turbulence is significantly increased when the jet passes over mountainous areas.
• Resultant Intensity: The combination leads directly to severe CAT.

Severe Clear Air Turbulence (CAT) over North, Central, and Northeast India is most frequent during the winter season (December to February). This is because the Sub-Tropical Westerly Jet (STWJ), which is the primary cause of high-level CAT in this region, is at its peak strength during the winter months. The intensity of CAT correlates directly with the magnitude of the horizontal and vertical wind shear generated by the powerful jet core.

⭐️ ⭐️ Key Data to Remember (Operational/Climatological Context):
• Period: Winter season (December to February).
• Cause: Sub-Tropical Westerly Jet (STJ) operating over North India.
• Intensity: Severe CAT occurs during this period because the STJ is strongest compared to the Tropical Easterly Jet (TEJ) which causes CAT in South India during summer.
• CAT location (STJ): Near and below the jet core on the cold (pole-ward) side, where wind shear is greatest.

Clear Air Turbulence (CAT) over Peninsular India is anticipated in association with the Tropical Easterly Jet (TEJ). This jet stream experiences its most frequent activity during the summer months of July and August, which corresponds to the Indian monsoon season. The TEJ is generally stronger during the summer compared to winter.

⭐️ ⭐️ Key Data to Remember (Operational/Climatological Context):
• Cause: Tropical Easterly Jet (TEJ).
• Peak Period: Summer (July–August).
• TEJ Location: Typically found over South/Peninsular India at heights around 15–16 km (100 hPa).
• Contrast: CAT over North India, associated with the Sub-Tropical Westerly Jet (STWJ), is strongest and considered severe during the winter season (December to February).

Severe turbulence is defined as turbulence that causes large, abrupt changes in altitude and/or attitude, where the aircraft may be momentarily out of control. Inside the aircraft, the intensity is such that occupants are forced violently against seat belts or shoulder straps, and unsecured objects are tossed about.

⭐️ ⭐️ Key Data to Remember:
• Aircraft Control: May be momentarily out of control.
• IAS Fluctuation: Fluctuates more than 25 knots.
• G-Force: Greater than 1.0 g at the aircraft’s center of gravity.
• Internal Reaction: Occupants forced violently against restraints; walking and food service impossible.
• Action Required: Immediate diversion is necessary if severe icing/turbulence is encountered, as anti-icing equipment may fail to control the h

Clear Air Turbulence (CAT) is high-level turbulence encountered in cloudless air, primarily associated with jet streams and strong circulation. The core meteorological cause is wind shear, which is the abrupt change in wind speed and/or direction (both vertically and horizontally).
The degree of CAT experienced by an aircraft is directly proportional to the intensity of the vertical and horizontal wind shear present in the atmosphere. Large vertical and horizontal wind shears create the conditions for severe CAT.

⭐️ ⭐️ Key Data to Remember (ICAO/FAA Operational Context):
• Cause: CAT is produced in regions of marked wind shear.
• Location: CAT is strongest near or just below the jet axis on the cold air (low pressure) side, where the greatest wind shear occurs.
• Intensity Measurement: Severe turbulence (which includes severe CAT) is characterized by Indicated Airspeed (IAS) fluctuations exceeding 25 knots and causes large, abrupt changes in attitude and/or altitude.
• Forecast Criteria: Moderate CAT may be forecast if the jet core speed exceeds 100 knots and the vertical wind shear is 4 knots per 1,000 feet.

CAT, particularly when associated with a jet stream, is concentrated in relatively narrow vertical layers due to large vertical and horizontal wind shear. Since the jet stream core is at FL310, the aircraft at FL270 is 4,000 feet below the core. The strongest CAT is typically found near or just below the jet axis on the cold air side (the low-pressure side).
To exit the turbulence quickly, the most effective measure is a vertical displacement. While climbing is also an option, descending moves the aircraft rapidly out of the highly sheared layer associated directly with the jet stream core and its accompanying turbulence, which is often vertically localized.

⭐️ ⭐️ Key Data to Remember (ICAO/Operational Context):
• Avoidance Technique: Change of flight level is the most effective measure to reduce or avoid CAT effects, as the turbulence is vertically patchy.
• CAT Location (Jet Stream): Strongest CAT occurs near or just below the jet axis.
• Action: If CAT is encountered near the jet core, the pilot should climb or descend a few thousand feet to escape the shear layer.

When encountering turbulence associated with strong convective currents (which indicate unstable air), the primary procedure involves minimizing stress on the airframe and exiting the turbulent layer.

1. Speed Control (Decrease Speed): The initial action upon entering turbulence is to reduce airspeed to the aircraft’s recommended turbulence penetration speed (V RA or similar speed specified in the flight manual). Flying at reduced speed limits the aircraft’s abrupt changes in acceleration, thereby minimizing structural stresses.

2. Vertical Maneuver (Climb): Convective turbulence exists primarily within the unstable atmospheric layer. The tops of convective clouds (cumulus or towering cumulus) usually mark the approximate upper limit of the strong currents. If aircraft performance parameters allow, climbing above the unstable layer often places the aircraft in smooth air, which is typically found above the cloud tops.

⭐️ ⭐️ Key Data to Remember:
• Speed: Reduce to turbulence penetration speed (V RA).
• Altitude: Turbulence resulting from convection (thermals) is vertical in nature and height-limited; ascending to cruise above the instability is the most effective technique for finding smooth air, provided performance and severe weather separation criteria (e.g., 5,000 ft clearance above storm tops) are met.
• Control: Maintain a constant attitude and allow the aircraft to “ride the waves,” avoiding abrupt maneuvers.

This definition corresponds to Moderate Turbulence, as categorized by ICAO criteria. Moderate turbulence involves noticeable changes in aircraft attitude and/or altitude, but critically, the aircraft remains in positive control at all times. The acceleration forces range from 0.5 to 1.0 g at the aircraft’s center of gravity, leading to occupants feeling definite strains against restraints and finding walking or food service difficult.

⭐️ ⭐️ Key Data to Remember (ICAO/FAA Criteria):
• Aircraft Control: Remains in positive control at all times.
• IAS Fluctuation: Typically 15 to 25 knots.
• G-Force: 0.5 to 1.0 g at the aircraft’s center of gravity.
• Internal Reaction: Occupants feel definite strains; walking/food service is difficult; loose objects move about.

Clear Air Turbulence (CAT) at high altitudes (above FL 150) is primarily caused by marked horizontal and vertical wind shear, often associated with the jet stream or mountain waves. Mountain waves, which can intensify CAT, create a standing, or wave-like, pattern that may extend into the lower stratosphere.
While the aircraft should be allowed to “ride the waves” and structural stresses are minimized by maintaining a constant attitude, the effects are classified by severity:

• Moderate Turbulence: Causes moderate changes in aircraft attitude and/or altitude, but the aircraft remains in positive control at all times. Occupants feel definite strains against seat belts, and walking/food service is difficult.

• Severe Turbulence: Causes large, abrupt changes in attitude and/or altitude, and the aircraft may be momentarily out of control. Occupants are forced violently against seat belts, making flight extremely hazardous.
Therefore, while the underlying motion may be wave-like, Moderate CAT does not essentially affect maneuvering (positive control maintained), but Severe CAT significantly affects control and maneuvering.

⭐️ ⭐️ Key Data to Remember (CAT Intensity/Effects):
Moderate – 0.5 to 1.0 g at the aircraft’s center of gravity. — 15–25 kt IAS Change – Remains in positive control – Definite strain, walking difficult

Severe – Greater than 1.0 g at the aircraft’s center of gravity. — > 25 kt IAS Change — May be momentarily out of control- Forced violently against seat belts, walking impossible

Clear Air Turbulence (CAT) intensity is directly proportional to the intensity of vertical and horizontal wind shear. Jet streams form in regions characterized by large differences in horizontal mean temperature, which result in steep pressure gradients aloft.
The fringes of the jet stream, often coinciding with the upper-level frontal zone (such as the Polar Front), represent areas where the temperature contrast is strong. This sharp temperature contrast leads to maximized wind shear (both vertical and lateral), creating the atmospheric breakdown known as CAT.

⭐️ ⭐️ Key Data to Remember (Operational Context):
• Intensity is proportional to the intensity of wind shear.
• Condition: Strong horizontal temperature gradients produce strong winds and intense wind shear.
• Location: Most CAT occurs on the fringes of the jet where temperature contrast is strong.
• Intensity: The combined effect of strong winds and sharp curvature (or presence of mountain waves) further intensifies CAT.

Moderate turbulence is defined as atmospheric disturbance causing changes in aircraft attitude and/or altitude, but the aircraft must remain in positive control at all times. Although attitude and height changes may be moderate, airspeed variations are usually small. Occupants will feel definite strains against seat belts, and walking or food service becomes difficult.

⭐️ ⭐️ Key Data to Remember:
• Control: Aircraft remains in positive control at all times.
• IAS Fluctuation: Typically 15 to 25 knots.
• G-Force: Changes in accelerometer readings of 0.5 to 1.0 g at the aircraft’s center of gravity.

The ICAO definition describing turbulence that causes large, abrupt changes in altitude and/or attitude, where the aircraft may be momentarily out of control, corresponds to Severe Turbulence. This intensity is characterized by large variations in airspeed and forces greater than 1.0 g at the aircraft’s center of gravity, making flight control extremely difficult and forcing occupants violently against their restraints.

⭐️ ⭐️ Key Data to Remember (ICAO Criteria):
• Aircraft Control: May be momentarily out of control.
• IAS Fluctuation: Fluctuates more than 25 knots.
• G-Force: Greater than 1.0 g at the aircraft’s center of gravity.
• Internal Reaction: Occupants are forced violently against seat belts; walking and food service are impossible.

Clear Air Turbulence (CAT) is primarily generated in regions of marked wind shear associated with jet streams.
This preferred location, typically near and just below the jet core, is where the vertical and horizontal wind shear is greatest, leading to the turbulent exchange of energy between contrasting air masses.
The strongest and most frequently reported CAT is found on the cold air side (pole-ward side) of the jet stream axis.
A secondary area of CAT can also be found above the core on the warm (equator-ward) side.

⭐️ ⭐️ Key Data to Remember (ICAO/Operational Context):
• Primary Location: Cold (polar) side, near and below the jet core.
• Cause: Turbulence intensity is proportional to the magnitude of vertical and horizontal wind shear.
• Shear: The rate of wind speed decrease is generally greater on the polar (cold) side than on the equatorial (warm) side.
• Curvature: CAT is more severe with curved jets and in sharp upper troughs.

Clear Air Turbulence (CAT) frequency in North, Central, and Northeast India is highest during the period when the Sub-Tropical Westerly Jet (STWJ) is active. The STWJ is generally observed over North India from October to May. This jet stream is the primary cause of high-level turbulence in this region due to large vertical and horizontal wind shear, which is proportional to CAT intensity. Although the CAT is generally present throughout this period, the peak intensity (severe CAT) occurs during the winter months (December to February) when the STWJ is at its peak strength.

⭐️ ⭐️ Key Data to Remember (Climatological Context):
• Cause: Sub-Tropical Westerly Jet (STWJ).
• Frequency Period: Most frequent in North, Central, and Northeast India (from October to May.)
• Intensity Peak: Severe CAT occurs during the winter season (December to February) when the STJ reaches maximum strength.