PFJ (60°) & STJ (30°)
INDEX CYCLE
SUMMER MONSOON ONLY
Jet Streams
Permanent Westerlies (PFJ/STJ) · Temporary Easterlies (TEJ) · Rossby Waves · Monsoon Impact & Commercial Aviation (28 Micro-Realms)
High-Altitude Geostrophic Rivers & The Monsoon Mechanism
Jet Streams are fast-flowing, narrow, meandering air currents found near the tropopause (9–12 km). Driven by strong latitudinal thermal gradients (temperature contrasts) and balanced by the Coriolis force (geostrophic balance), the two primary permanent jets—the Polar Front Jet (60° N/S) and the Subtropical Jet (30° N/S)—blow from West to East. In winter, the STJ shifts south of the Himalayas, steering Western Disturbances into northern India. During summer, intense heating of the Tibetan Plateau generates a temporary, reversed jet: the Tropical Easterly Jet (TEJ), which blows from East to West, acting as the upper-air exhaust that triggers the burst of the Indian SW Monsoon.
I. Jet Stream Physics, Discovery & Rossby Waves
Jet Stream Definition
- Fast-flowing, narrow, meandering air currents
- Located near the Tropopause altitude breaks
- Speeds range from 100 to 400+ kmph
Discovery (WWII Bombers)
- Discovered by American B-29 bombers in WWII
- Flying Westward to Japan against massive headwinds
- Ground speed was reduced almost to zero!
Tropopause Gaps
- The Tropopause is not a continuous shell
- It breaks at cell boundaries (30° and 60°)
- Jet streams form exactly in these "breaks" or gaps
Geostrophic Balance
- High altitude = practically ZERO surface friction
- Pressure Gradient Force is balanced by Coriolis Force
- Wind flows parallel to the isobars (Geostrophic)
The Thermal Gradient Engine
- Speed is driven by horizontal temperature contrasts
- Steeper the temp difference = stronger the PGF aloft
- = Faster the Jet Stream!
Rossby Waves
- Jet streams do not flow in straight lines
- They form giant meanders called planetary Rossby Waves
- Triggered by mountain barriers (Rockies/Himalayas) & heating
Velocity: Winter vs. Summer
(FAST)
(SLOW)
- Jet streams are strongest during WINTER
- Because the temperature contrast between Equator and Pole
- Is maximum during the winter season
II. The Permanent Westerly Jets: PFJ & STJ
Permanent Westerly Jets
- Two primary permanent jets per hemisphere
- Both blow from WEST to EAST
- Due to Earth's rotation (Coriolis deflection)
Polar Front Jet (PFJ)
- Forms at the boundary of cold polar air and warm tropical air
- Highly variable path, meandering deeply
- Altitude is lower (~9 km) because polar air is dense
PFJ Weather Steering
- PFJ acts as the steering mechanism for Temperate Cyclones
- Upper-air divergence inside the jet "vacuums" air up
- Deepens and intensifies surface low-pressure systems
Subtropical Westerly Jet (STJ)
- Forms above the subtropical high-pressure belt
- More stable path, faster winds than PFJ
- Altitude is higher (~12 km) due to warmer air expansion
STJ & Western Disturbances
- In winter, STJ shifts south of the Himalayas
- Steers Mediterranean temperate cyclones into North India
- Bringing crucial winter rain for Rabi crops (Western Disturbances)!
Seasonal Migration (N/S)
- Like surface pressure belts, Jets follow the Sun
- Summer: STJ shifts north of Himalayas/Tibet
- Winter: STJ shifts south of Himalayas
The Jet Streak
- Wind speeds inside the jet are not uniform
- A localized core of maximum wind speed is a Jet Streak
- Can exceed 400 kmph, creating intense turbulence
III. Temporary Jets & The Indian Monsoon (TEJ & Somali Jet)
Tropical Easterly Jet (TEJ)
- A unique, temporary jet stream
- Exists ONLY during the Northern Hemisphere summer
- Plays a master role in the Indian SW Monsoon
TEJ Origin (Tibetan Plateau)
- Summer Sun intensely heats the high Tibetan Plateau
- Acts as an elevated heat pump, sending air aloft
- This rising air establishes the upper-air easterly flow
TEJ Direction (East to West)
W TO E
E TO W
- Unlike the permanent Westerly jets
- The TEJ blows from East to West across Peninsular India
- Driven by the high pressure aloft over Tibet
TEJ & Burst of Monsoon
- The TEJ subsides over the Mascarene High (Indian Ocean)
- Intensifies the high pressure there
- Which violently pushes the SW Monsoon winds into India!
Somali Jet
- A low-level (not upper troposphere) seasonal jet
- Flows off the coast of Somalia towards India
- Also known as the Findlater Jet
Somali Jet & Monsoon Intensity
SOMALI JET
INDIAN RAIN
- Acts as a super-charger for the SW Monsoon
- A strong Somali Jet = bountiful monsoon rains in India
- A weak Somali Jet = weak, deficit monsoon
Stratospheric Polar Night Jet
- Forms much higher (Stratosphere) during Polar Night
- Contains the Polar Vortex
- Disappears entirely during polar summer
IV. Aviation Impacts, Clear Air Turbulence & Weather Extremes
Aviation: Tailwind Advantage
- Flights traveling West to East (e.g., NY to London)
- Ride the jet stream as a massive tailwind
- Saves significant time and jet fuel
Aviation: Headwind Penalty
- Flights traveling East to West (e.g., London to NY)
- Face the jet stream as a severe headwind
- Airlines route planes north or south to avoid the core
Clear Air Turbulence (CAT)
- Caused by severe wind shear at the edges of jet streams
- Happens in perfectly clear, cloudless skies
- Cannot be detected by standard weather radar!
Polar Vortex & Cold Snaps
- When the Polar Jet Stream weakens, it meanders deeply
- Allows freezing Arctic air to spill southward
- Causes brutal winter cold snaps in USA & Europe
Blocking Highs & Heatwaves
- Extreme Rossby wave meanders can get "stuck"
- Traps a high-pressure dome (Blocking High)
- Causes devastating, prolonged summer heatwaves/droughts
Ozone Transport
- Tropopause gaps allow stratospheric ozone
- To mix down into the upper troposphere
- Affects high-altitude atmospheric chemistry
Master Jet Stream Matrix
- Completes the Upper Air Circulation engine
- Locks STJ (Winter) vs TEJ (Summer Monsoon) logic
- Transitions to Realm 81 (Weather Systems & Forecasting)
Core Distinctions
• Subtropical Westerly Jet (STJ): Present in WINTER. Blows from West to East. Shifts south of the Himalayas and steers Western Disturbances into North India (bringing winter rain).
• Tropical Easterly Jet (TEJ): Present ONLY in SUMMER. Blows from East to West. Driven by the intense heating of the Tibetan Plateau. Acts as the upper-air exhaust that intensifies the Mascarene High, triggering the burst of the SW Monsoon.
Both the Polar Front Jet (PFJ) and Subtropical Jet (STJ) are permanent features that exist year-round in both hemispheres, blowing West to East. The Tropical Easterly Jet (TEJ) is a temporary, seasonal jet that only exists during the Northern Hemisphere summer.
3. Clear Air Turbulence (CAT) in AviationCommercial aircraft fly at 9–12 km to save fuel by riding or avoiding the jet streams. The massive wind shear (speed differences) at the edges of the jet stream creates violent, invisible turbulence (CAT). Because there is no moisture to form clouds, CAT is completely invisible to standard weather radar, making it a sudden hazard.
Jet Stream Matrix
Upper Tropospheric Reference Guide