Winds and Wind-Related Concepts Notes

Wind: Direction and Speed

Wind is the horizontal movement of air, characterized by direction (from which it blows, measured from True North in compass points or degrees) and speed (measured in knots, kt).

Unit conversion: 1 \text{ km/h} \approx 0.278 \text{ m/s} \approx 0.540 \text{ kt}.

Cross Wind and Runways

Runways align with prevailing winds; a cross wind is 90° to the runway and can be hazardous, especially for lighter aircraft.

Instruments for Measuring Wind

  • Surface wind: Anemometer (speed) and Wind Vane (direction).

  • Upper winds: RAWIN (radiosonde/balloon) and Pilot Balloon equipment (tracked by RADAR/Optical Theodolite).

Exposure of Wind Instruments

Surface instruments are at 10m height, averaged for 10 minutes for observations, and 2 minutes for take-off/landing.

Gusts, Lulls, and Squalls

  • Gust: irregular, rapid increase in wind speed due to turbulence.

  • Lull: temporary decrease in wind speed.

  • Squall: sudden increase of at least 32 km/h (16 \text{ kt}) lasting one minute or more, reaching 44 \text{ km/h} (22 \text{ kt}) or more. Often linked to convective activity (CB clouds, thunderstorms).

  • Difference: Gusts are transient (seconds), squalls last several minutes and are convective.

Gale

Persistent mean wind speed of \ge 34 \text{ kt}, often with depressions/cyclonic storms.

Backing and Veering of Wind

  • Backing: Wind direction changes anti-clockwise (e.g., 090\degree to 060\degree).

  • Veering: Wind direction changes clockwise (e.g., 060\degree to 090\degree).

  • In NH, winds back in low pressure and veer in high pressure areas.

Buys Ballot’s Law (Pressure and Wind)

In NH, with back to wind, low pressure is to the left and high pressure to the right; wind blows anticlockwise around lows (cyclone) and clockwise around highs (anticyclone).

Effect of Rotation of the Earth: The Coriolis Force

Deflects moving air to the right in NH and left in SH. Magnitude F_C = f V where f = 2 \Omega \sin \varphi (Earth's angular velocity \Omega, latitude \varphi, wind speed V).

Geostrophic Wind (GF) and Isobaric Balance

Idealized wind parallel to straight isobars where Pressure Gradient Force (PGF) balances Coriolis Force (CF). Low pressure to its left in NH. V_g = \frac{|\nabla p|}{\rho f}.

Stronger at lower latitudes; good approximation in mid-latitudes, away from surface friction.

Limitations of the Geostrophic Rule

Only applies when PGF and CF balance, isobars are straight/parallel, and flow has no significant curvature or time variation. Not valid for gusts, squalls, or local winds.

Cyclostrophic Wind

Balance between PGF and Centripetal Force (V^2/r) for flow along curved isobars where Coriolis force is negligible. Applicable near tropical cyclone or tornado centers. V_{cyclo} = \sqrt{\frac{r}{\rho} ; \frac{\partial p}{\partial n}}.

Gradient Wind

Wind parallel to curved isobars under balance of PGF, Centripetal Force, and Coriolis Force. More general than geostrophic wind.

Reduces to geostrophic if isobars are straight, and to cyclostrophic if Coriolis is negligible.

Inertial Wind and Isallobaric Wind

  • Inertial wind: Steady circular motion when pressure gradients are weak, V_{inertial} = f R.

  • Isallobaric wind: Occurs when pressure changes rapidly; an additional isallobaric force (from higher to lower isallobar pressure) is considered.

Effect of Surface Friction

Creates a friction layer (~1km) where friction reduces Coriolis force, deflecting wind toward low pressure (cross-isobaric flow) and reducing speed.

  • Over sea: ~15° deflection, ~2/3 geostrophic speed.

  • Over land: ~30° deflection, ~1/3 to 1/2 geostrophic speed.

Turbulence, Gustiness, and Atmospheric Turbulence Types

Irregular wind fluctuations (gusts/lulls) from frictional (surface interaction) or thermal (buoyancy/convection) sources.

Enhanced by rugged terrain, steep lapse rates. Reduced over smooth surfaces, stable lapse rates.

Diurnal variation: Daytime (ground heating, convection, stronger/gustier winds), Night (radiative cooling, weaker surface winds, wind shear).

Wind Shear (WS) and Its Effects

Sudden change in wind speed/direction with height or distance.

  • Causes: Thunderstorms (gust fronts, microbursts), jet streams, fronts, topography.

  • Aviation hazard: Microbursts are especially dangerous during takeoff/landing, causing abrupt wind shifts and potential loss of height/IAS changes.

Local Winds: Topography-Driven Winds

  • Anabatic: Daytime upslope winds due to heating.

  • Katabatic: Nocturnal downslope winds from dense cold air.

  • Föhn (Foehn): Warm, dry downslope wind on leeward side of mountains from adiabatic warming.

  • Bora: Strong, cold offshore wind (e.g., Adriatic coast).

  • Ravine winds: Strong flows through ravines due to pressure differences.

Sea Breeze and Land Breeze

  • Sea breeze: Daytime flow from cooler sea to warmer land due to pressure differences.

  • Land breeze: Nighttime reversal, flow from land to sea due to land cooling faster.

Thermal Wind: Wind Change with Height due to Temperature Gradients

Vector difference between upper and lower geostrophic winds (\mathbf{V}t = \mathbf{V}g(upper) - \mathbf{V}_g(lower)).

Perpendicular to temperature gradient, implying stronger winds aloft where temperature gradients are large (e.g., jet streams).

Variation of Wind with Height and Large-Scale Circulation

In troposphere, westerlies generally increase with height (equator to poles temperature gradient). Regional variations exist (e.g., strong easterlies over India during monsoon; tropical Easterly Jet).

Vorticity: Rotation in the Atmosphere

Measure of rotation in flow, crucial for weather systems.

  • Sources: Horizontal wind shear, flow curvature, Earth's rotation (planetary vorticity).

  • Absolute vorticity: Sum of relative vorticity (due to flow) and planetary vorticity (due to Earth's rotation).

Names of Winds Around the World

Examples include Bora, Chinook, Doldrums, Harmattan, Mistral, Monsoon, Roaring Forties, etc.