Ch 14 Quiz: Jet Streams & High-Altitude Weather

PFJS: narrow, fast wind current at ~30,000–40,000 ft near the polar tropopause. Located at the convergence of polar and subtropical air masses. Strength: 100–200 kt. Migrates equatorward in winter (stronger) and poleward in summer (weaker).

STJS: relatively steady westerly jet at ~25–30°N/S, near FL350–400, at the poleward edge of the Hadley cell. Less variable than the PFJS. Strongest in winter. Important for trans-oceanic flight planning (Asia to North America routing).

Jet stream core winds: typically 100–200 kt but can exceed 300 kt in very strong winter polar jets. The jet stream axis is the core of maximum wind. Wind speeds decrease rapidly away from the core.

Jet stream tailwind provides a significant fuel and time saving. East-bound trans-Atlantic/trans-Pacific flights are typically routed to use the jet. West-bound flights avoid the jet headwind. A 100 kt tailwind at FL350 effectively adds 100 kt to groundspeed.

Jet stream CAT: Kelvin-Helmholtz instability in zones of strong wind shear adjacent to the jet core. Most intense on the cyclonic (low pressure/poleward) side of the jet and below the jet core. No visible warning. Detected by PIREP and specialized CAT forecasts.

Tropical Easterly Jet (TEJ): upper-level (150–100 hPa) easterly jet over South Asia and Africa during the NH summer monsoon (June–September). Important in sustaining the South Asian monsoon circulation. Strength ~50–100 kt.

Vertical shear is greatest BELOW the jet axis (where wind speeds increase rapidly upward to the jet core) and on the poleward (cyclonic) side. CAT encounters are therefore most frequent below and poleward of the jet core.

Winter PFJS: greater polar-subtropical temperature contrast → stronger thermal wind → stronger, more equatorward jet (over North Africa/Mediterranean/Japan). Summer PFJS: weaker gradient → weaker, more poleward jet. Trans-Atlantic winter headwinds are much stronger westbound.

Tropopause height: Tropics ~FL500–560 (16–18 km), Mid-latitudes ~FL350–400 (10–12 km), Poles ~FL250–300 (7–9 km). The lower polar tropopause means Cb tops and Ci clouds occur at lower altitudes over polar regions.

Above and equatorward of the jet core: anticyclonic shear, relatively smooth. Poleward/below the jet core: cyclonic shear — greater CAT probability. Pilots can often find smooth air by offsetting slightly equatorward of the jet axis.

Contrails form when hot/humid exhaust mixes with cold/dry ambient air, usually above ~FL260 (−40°C or colder) where exhaust moisture rapidly condenses and freezes. They persist (become ‘persistent contrails’) in ice-supersaturated air, potentially evolving into cirrus.

SIGWX high-level charts: jet streams shown by ARROWS on the jet axis with barbs indicating speed at the axis, and dashed ISOTACH lines (60, 80, 100, 120, 150 kt…) showing the speed field. Typically colour-coded (amber, red) for speed intensity.

Jet stream wind gradient: typically 5–10 kt/100 km across (meridional direction), with the maximum gradient on the cyclonic side. This steep shear is the primary source of jet-related CAT via Kelvin-Helmholtz instability.

The Hadley cell: rising near ITCZ, poleward flow aloft, descending at ~30°N/S (horse latitudes → subtropical highs at surface). The STJS forms where this descending branch meets the polar side of the Hadley cell at the tropopause.

Jet stream winds in aviation are always reported in KNOTS (kt) on SIGWX charts, TAFs, upper-wind forecasts, and PIREPs. Scientific publications may use m/s. The ICAO standard for aviation wind reporting is knots.

SIGWX jet stream notation: long arrow on jet axis with wind barbs (speed encoded like surface chart barbs: full barb=10kt, half barb=5kt, pennant=50kt). Label: direction/FL/speed or just axis altitude and maximum speed. Isotachs may be added.

Trans-Pacific jet effect: eastbound is with jet → faster. LAX→NRT is WESTBOUND against polar jet → ~11–12 hrs. NRT→LAX EASTBOUND with polar jet → ~9–10 hrs. The difference is 1–3+ hours depending on jet strength.

CAT products: GTG (Graphical Turbulence Guidance) — objective NWP-based turbulence probability chart. WAFC turbulence severity index. Route SIGMET for SEV CAT. Automated aircraft EDR (eddy dissipation rate) reports via ACARS. Best practice: combine all sources.

STJ characteristics: more zonally consistent, less meandering than PFJS. Located at ~25–35°N near FL350–400. Most pronounced in winter NH when the Hadley cell is strongest. The PFJS meanders widely N-S with Rossby wave pattern, driving weather systems.

Westbound routing to avoid jet: staying equatorward (east in NH, or south of the jet axis) puts the aircraft on the anticyclonic shear side where westerly wind speeds are lower. Route planning examines wind fields to find the optimal trade-off between distance and wind benefit.

Tropopause break: at ~25–35°N/S, the tropopause steps from tropical (high ~FL550) to extratropical (FL350–400). The temperature difference across this break drives the STJ. The step can be abrupt, associated with stratospheric intrusions of dry ozone-rich air into the upper troposphere.

Jet stream icing: Cirrus in the jet environment provides light icing from ice crystals (low accretion rate). More significant icing occurs in the lower jet flank where temperature is 0 to −20°C and supercooled water exists in mid-level cloud associated with fronts.

NAT (North Atlantic) Tracks: a daily system of organised tracks (Organised Track System, OTS) issued by NATS and Nav Canada. Designed to take maximum advantage of jet stream forecast: eastbound tracks in the core of the jet; westbound tracks detour to minimise headwind.

CATEDR: CAT Eddy Dissipation Rate. EDR is aircraft-independent (unlike vertical acceleration which varies with aircraft size/speed). WAFS now issues gridded CAT EDR forecasts. Airlines’ ACARS turbulence reports also use EDR for standardised reporting. Threshold: MOD = EDR 0.2–0.4 m²/³ s⁻¹.

Stratospheric polar vortex: strong westerly circulation in the winter stratosphere. Sudden Stratospheric Warming (SSW): rapid warming/reversal of the vortex → downward propagation → displacement of tropospheric polar vortex → cold arctic air spills equatorward → extreme cold outbreaks in NH winter.

CAT statistics: approximately 1–5% of flights encounter moderate or greater turbulence during cruise. Severe encounters are rare but result in injuries when they occur. The number of turbulence-related injuries is ~500–1,000/year globally in commercial aviation.

Climate change and CAT: research (e.g. Williams et al., 2023) indicates that CAT has increased ~55% in frequency since 1979. Proposed mechanism: reduced equator-to-pole temperature gradient at surface but increased gradient in the upper troposphere due to differential warming patterns.

NAT OTS publication: eastbound tracks issued ~22:30 UTC for overnight transits (arriving Europe morning); westbound tracks issued ~14:30 UTC for afternoon transits (arriving North America afternoon). Track positions shift daily with forecast jet stream location.

Tropopause fold: occurs on the cyclonic side of the jet stream where the tropopause folds down to lower levels. Stratospheric (dry, high PV) air intrudes into the upper troposphere. Associated with clear-air turbulence, upper-level fronts, and cut-off lows when the fold becomes very deep.

Jet stream fuel benefit: a 150 kt tailwind on a 3,000 nm oceanic leg increases groundspeed from, say, 470 kt to 620 kt — a 32% increase. Block time reduces proportionally. For a 3,000 nm leg at 470 kt (6.4 hrs) vs. 620 kt (4.8 hrs) — saving 1.6 hrs of fuel burn at cruise fuel flow ~7,000 kg/hr = ~11,000 kg savings.