Windshear & jetstream

050 MeteorologyChapter 7

Jet streams

Key fact

Definition: quasi-horizontal axis in upper troposphere/lower stratosphere; wind over 60 kt (SigWx charts often 80 kt). Dimensions (EASA): ~1000 NM long, 150 NM wide, 18 000 ft thick; horizontal:vertical ≈ 100:1. Core just below tropopause where no horizontal temperature gradient; slope of pressure surface maximum.

Key fact

Polar front jet: ~FL300 (winter 350 / summer 300), 50–60°N/S. Subtropical jet: ~FL400 / 200 hPa, 30–40°N/S, all year. Equatorial easterly jet: NH summer only, 10–20°N, up to FL500. Strongest winds: winter hemisphere; Japan July max; between trough and ridge.

Key fact

Jet located in warm air mass below tropopause (appears in polar air on charts). CAT most severe on cold-air (polar) side, 7000 ft below to 3000 ft above tropopause; layer avg 2000 ft (can be 60–100 ft). Long CI streaks on equatorial side = visual cue.

Clear air turbulence (CAT)

Key fact

No visual indicator. Associated with jet streams, troughs/ridges (troughs worse), isohypse convergence/divergence, vicinity of CB. Moderate CAT: definite seatbelt strain, objects dislodged, walking difficult. Severe: aircraft momentarily out of control, over 1 g on accelerometer.

Key fact

Response to CAT: maintain wings level, smooth pitch, reduce speed or climb/descend out of layer. Avoid by flight level change — usually descend (jet layer thickness).

Mountain waves & rotors

Key fact

Conditions: stable layer at/above ridge top, wind ≥15–20 kt (sources say 20–25 kt), within 30° of perpendicular, speed increasing with height to tropopause, inversion above crest often. Rotor/roll clouds downwind; cap cloud windward. Rotor: reversed flow near surface on lee side. Vertical speeds to 5000 ft/min; influence 50–100 NM downwind (up to 500 NM recorded).

Exam trap

Low-level upwind (into wind toward high ground from lee side) is more hazardous than tailwind approach from windward — out of phase with waves. No visible cloud does not mean no wave turbulence. Orographic cloud → moderate–severe mixed icing.

Windshear

Formula — Windshear intensity (vertical)

Light:     under 4 kt / 100 ft
Moderate:  4–8 kt / 100 ft
Strong:    8–12 kt / 100 ft
Severe:    over 12 kt / 100 ft

Key fact

Microburst/gust front worst near TS. Low-level windshear below 2000 ft; max at top of radiation inversion. Headwind increase → IAS ↑, lift ↑, ROD ↓. Tailwind increase → IAS ↓, lift ↓, ROD ↑. Horizontal gust (constant AoA) → load factor momentarily above 1.

Key fact

LLWAS/MBST reporting: microburst on final. Windshear alerts every 1 min where installed. On ILS: headwind → below glide path; headwind gust → above glide path.

Turbulence types

Key fact

Mechanical: friction over terrain/obstacles; ↑ with wind speed. Thermal: convection in unstable air; max early/mid afternoon. CAT: jet stream region. Reporting above FL150: light chop (no IAS change) vs light turbulence (5–15 kt IAS fluctuation).

Foehn & venturi

Key fact

Venturi in valleys: pressure drop, indicated altitude increases (actual altitude lower). Foehn net warming lee side ~1.2°C/1000 ft (DALR − SALR). Turbulence and good visibility leeward; clouds windward.