Ch 5 Quiz: Wind — Geostrophic, Gradient, Jet Streams & Surface

Wind is caused primarily by the pressure gradient force — air flows from high to low pressure. Coriolis, friction, and centrifugal forces modify direction and speed but do not initiate the flow.

The Coriolis force (due to Earth’s rotation) deflects wind to the RIGHT in the Northern Hemisphere and to the LEFT in the Southern Hemisphere. It is zero at the equator and maximum at the poles.

The friction layer (planetary boundary layer) extends to approximately 2,000–3,000 ft AGL. Above this level, friction is negligible and the wind approximates the geostrophic wind.

Geostrophic wind results from balance between pressure gradient force and Coriolis force. It flows PARALLEL to isobars with low pressure to the LEFT in the Northern Hemisphere (Buys Ballot’s Law).

Buys Ballot’s Law: in the NH, stand with back to the wind → low pressure is to your LEFT (slightly forward). Reversed in SH.

Gradient wind accounts for: pressure gradient force + Coriolis + centripetal acceleration. When isobars are curved, centripetal acceleration must be included. Gradient wind > geostrophic around lows, < geostrophic around highs.

Due to surface friction, surface wind is: (1) SLOWER than gradient wind, (2) BACKED (rotated anti-clockwise) in the NH — typically 15–30° over sea, 30–45° over land.

Veering: wind direction changes CLOCKWISE (e.g. S to SW to W to NW). Backing: changes anti-clockwise. In the NH, warm advection is associated with veering winds with height.

Wind shear = change in wind VELOCITY (speed and/or direction) over a distance (horizontal or vertical). It is a serious aviation hazard causing airspeed fluctuations, turbulence, and microburst encounters on approach.

LLWS below 2,000 ft AGL is most critical during take-off and landing. At low speed and altitude, there is insufficient energy to recover from sudden airspeed loss caused by microburst/LLWS encounter.

Microburst: intense small-scale (< 4 km) downdraft that hits the surface and spreads outward. Aircraft on approach first encounters a headwind increase (airspeed up), then downdraft, then tailwind (airspeed drops) — classic 'microburst encounter'.

Sea breeze: during the day, land heats faster than sea → low pressure over land → wind blows FROM SEA TO LAND. At night, land cools faster → land breeze (land to sea).

Katabatic (drainage) wind: cold, dense air flows downhill under gravity at night when slopes radiate heat. Examples: Mistral (France), Bora (Adriatic), Williwaw (Alaska). Can be very strong.

Foehn effect: moist air rises windward (cools at SALR ~6°C/1000m while precipitating), descends lee side (warms at DALR ~10°C/1000m) → arrives warm and DRY. Net temperature gain on lee side.

Anabatic (upslope) wind: during the day, slopes heat faster than the free atmosphere → low pressure near slope → air flows UPSLOPE. Associated with valley clouds and afternoon convection on mountains.

ICAO METAR standard: winds < 1 kt are reported as '00000KT' (calm). If the speed rounds to zero, it is reported as calm regardless of a variable direction.

Upper wind reports (TAF, SIGMET, SIGWX charts) use TRUE NORTH. Surface METAR winds are also true unless stated otherwise. Pilots convert to magnetic for cockpit instruments.

ICAO defines a jet stream as a narrow current of strong winds at high altitude with speeds ≥ 60 kt. Cores typically 100–200 kt. Located near the tropopause.

The ITCZ (Inter-Tropical Convergence Zone) is where northeast trade winds (NH) and southeast trade winds (SH) converge. This convergence drives the rising limb of the Hadley cell.

Thermal wind is the VECTOR DIFFERENCE between winds at two pressure levels. It blows parallel to thickness lines (warm air to the right in NH) and represents the effect of horizontal temperature gradient on wind shear with height.

Clear Air Turbulence (CAT) most commonly occurs near JET STREAM cores (especially the cyclonic shear side) and at upper-level fronts, where vertical and horizontal wind shear is strongest.

METAR wind format: DDDffGfmfmKT. 27018G35KT = direction 270°T, mean speed 18 kt, gusting 35 kt. Gusts imply rapid speed variations of ≥ 10 kt.

Meteorological wind direction is always in DEGREES TRUE, measured from North (000°) clockwise. The direction is FROM which the wind blows (e.g. 270° = westerly wind).

The rotor zone is a region of intense turbulence and reversed flow beneath the standing wave crests on the lee side, roughly at mountain-top height. It is the most dangerous part of mountain wave for aviation.

As height increases through the friction layer, wind speed INCREASES (less surface drag) and direction VEERS clockwise (in NH) toward the geostrophic direction. This is Ekman spiral behaviour.

WMO definition of a squall: sudden increase of ≥ 16 kt, speed reaching ≥ 22 kt, and lasting ≥ 1 minute. Squalls are associated with thunderstorms, cold fronts, and convective lines.

The ageostrophic component (departure from geostrophic balance) drives secondary circulations: low-level inflow into lows and outflow from highs, responsible for mass convergence/divergence and associated rising/sinking motion.

Crosswind = wind speed × sin(angle). 20 × sin(30°) = 20 × 0.5 = 10 kt. Headwind component = 20 × cos(30°) = 20 × 0.866 = 17.3 kt.

Wind accelerates through a mountain pass or valley constriction due to the Venturi effect — the same mass of air must flow through a smaller cross-section, so speed increases. Responsible for locally strong mountain pass winds.

Beaufort Force 12 = Hurricane (≥ 64 kt, ≥ 32.7 m/s). The Beaufort scale runs 0 (calm) to 12 (hurricane). Force 7 = near gale (28–33 kt).

A microburst encounter on final approach causes: initial headwind (airspeed surge) → downdraft → sudden tailwind (airspeed loss of 30–40 kt). Without prompt go-around, the aircraft may not maintain VREF and impact short.

Coriolis force is proportional to sin(latitude). At the equator (latitude=0°), sin(0°)=0, so Coriolis=0. Maximum at the poles (sin 90°=1). This is why geostrophic balance does not apply near the equator.

Pressure gradient force: directed perpendicular to isobars, pointing from HIGH to LOW pressure. It is the primary initiator of wind. Magnitude = (1/density) × (ΔP/Δn), where Δn is the distance between isobars.

Closely spaced isobars = steep pressure gradient = strong winds. Widely spaced isobars = gentle gradient = light winds. Isobar spacing is used to estimate wind strength from surface analysis charts.

Backing: anticlockwise direction change. In the NH, backing with height indicates cold air advection (cold front approaching or cold air moving in). Veering with height = warm advection. Crucial for flight planning and wx forecasting.

Ekman spiral: idealized model of wind turning and decreasing in the friction layer. At the surface, wind is backed ~45° from geostrophic and slower. With increasing height, wind gradually veers and speeds up, approaching the geostrophic direction at ~2,000–3,000 ft.

Wind profiling: measurement of wind at multiple levels through the atmosphere. Methods: radiosonde (balloon), wind profiler radar (boundary layer), aircraft wind reports (ACARS), pilot balloon (PIBAL), and VAD (Velocity Azimuth Display) from Doppler weather radar.

ATIS, METAR, and ATC voice calls report winds in KNOTS TRUE unless there is a local agreement for magnetic. The standard ICAO format is DDDffKT for direction (°T) and speed (kt).

Upper wind forecasts (FD, WINTEM) always use TRUE NORTH and KNOTS. Surface METAR and ATC clearances may also use true north but pilots convert to magnetic using local variation for cockpit nav instruments.

Gust front: leading edge of thunderstorm cold outflow. Marked by sudden wind shift/increase (20–50 kt), temperature drop, and pressure jump. Can occur 10–20 nm ahead of the storm. Aviation hazard: windshear on approach/departure.

LLWS (Low Level Wind Shear) is most hazardous during take-off and landing when aircraft are slow, close to the ground, and have little energy reserve. A 20–30 kt airspeed loss at 200 ft AGL is potentially fatal without immediate full-power go-around.

LLWAS: multiple anemometers around the airport perimeter plus runway sensors. When wind speed/direction differences between sensors exceed defined thresholds, an alert is generated. Combined with TDWR for comprehensive coverage.

Wind gradient: rate of change of wind velocity over a distance (vertical or horizontal). Strong vertical wind gradient near the surface (speed increasing rapidly from near-zero at ground to geostrophic aloft) causes wind shear and mechanical turbulence.

Valley (diurnal thermally driven) circulation: daytime solar heating of valley slopes drives anabatic (upslope, upvalley) flow. At night, cooling causes katabatic (downslope, downvalley) drainage winds. Can be significant (20–30 kt) in alpine terrain.

Sirocco: hot, dry, dusty southerly wind from the Sahara, blowing across the Mediterranean into Europe (especially Italy, Sicily). As it crosses the sea it picks up moisture, becoming hot and humid. A Foehn-type wind in its dry form over North Africa.

Upper-level divergence: air leaving the upper levels faster than it can be replaced from below. This creates a ‘suction’ pulling surface air upward, lowering surface pressure, intensifying the low. QG theory links upper-level divergence to surface cyclogenesis.

Mistral: cold, dry, strong northerly wind that accelerates through the Rhône-Saône corridor in France into the Gulf of Lion. Caused by cold air spilling south from a high over central Europe through the mountain gap. Gusts to 60–100+ kt in extreme cases.

Subtropical high pressure belt: ~25–35°N/S. The descending limb of the Hadley cell (sinking, diverging, stable air). Source of the trade winds (equatorward) and westerlies (poleward). Associated with desert climates (Sahara, Arabian Peninsula, Australian Outback).

Surface convergence: air flowing together → cannot stay at the surface → rises. Rising motion → cooling → cloud and precipitation potential. The ITCZ is the zone of maximum tropical convergence. Cold fronts and troughs create mid-latitude convergence.

Over land: rougher surface → more friction → slower, more backed surface wind. Over sea: smoother surface → less friction → surface wind closer to gradient wind in speed and direction.

Haboob: intense wall of sand/dust raised by thunderstorm cold outflow winds. Common in Sudan (Arabic: haboob = blowing sand), Arabia, Sahel, and the American Southwest. Can reduce visibility to near zero over a wide area.

VRB: wind direction variable (wind speed too low or variable direction). VRB03KT = wind from variable direction at 3 kt. When wind speed ≥ ~6 kt, a dominant direction is given; when < 3 kt, often reported as VRB or 00000KT.

Aircraft wind reports (ACARS, PIREPs) are a primary source of upper-level wind information. Modern commercial aircraft automatically transmit ACARS wind/temperature data, which is assimilated into NWP models and used for other flight planning.

Bernoulli/Venturi effect: as wind flows through a mountain pass (reduced cross-section), speed increases to maintain mass continuity (mass flux = ρ × A × V = constant). This explains locally intense winds in mountain passes and gaps.

Trade winds: persistent easterly winds from ~30° latitude toward the equator, deflected to the right (NE direction in NH) by Coriolis. Drive surface ocean circulation, evaporation, and moisture transport. The energy source for tropical cyclones.

Wind vane: instrument that aligns with and indicates the wind DIRECTION. The arrow points into the wind (from which direction the wind blows). Combined with an anemometer (cup or sonic) in an AWS to give full wind speed + direction.

Real-time wind: ATIS (airport surface wind), ATC position reports, ACARS uplink from airline operations, and weather datalinks. In-flight: own aircraft GPS-derived wind is the most current at aircraft position.

Anemometer: instrument for measuring wind speed. Types: (1) cup anemometer — 3 cups rotate, speed from rotation rate; (2) sonic anemometer — ultrasound travel time; (3) propeller anemometer. Typical accuracy: ±0.1 m/s, ±3° direction.

Maximum demonstrated crosswind component: the highest crosswind at which the manufacturer has demonstrated satisfactory take-off/landing control during flight testing. Many aircraft have higher structural capability; the demonstrated figure is the normal operational limit.

Bora: cold, dry, strong to violent NE wind flowing from the interior mountains of the Balkans/Croatia into the Adriatic Sea. Temperature can be −20°C, gusts > 50 kt. Caused by deep cold air pooling behind mountains spilling over the top.

Within the friction layer, wind speed INCREASES with height as the effect of surface drag decreases. The wind approaches the geostrophic speed at the top of the friction layer (~2,000–3,000 ft). Above this level, wind speed depends on the pressure gradient.

Wind rose: circular frequency diagram showing the distribution of wind direction and speed at a station over a period (month, season, year). Petal length = frequency from that direction. Used for runway planning, wind turbine siting, and climate analysis.