Wind: Global Circulation, Local Winds, Föhn and Turbulence
This domain tests the general circulation, the modification of wind by friction and terrain, and the named local wind systems, including the föhn and its thermodynamics. Expect a föhn temperature calculation or a question on sea-breeze timing.
Global circulation
- Hadley cells lift moist air at the ITCZ and subside in the subtropical highs, giving the trade winds (north-easterly and south-easterly).
- Ferrel cells give the mid-latitude westerlies; polar cells give polar easterlies; the polar front separates the two.
- Coriolis force turns air to the right in the northern hemisphere and to the left in the southern.
Wind balance and friction
- Geostrophic wind flows parallel to straight isobars with pressure gradient balanced by Coriolis - valid above the friction layer.
- Gradient wind around curved isobars: cyclonic flow is subgeostrophic, anticyclonic flow is supergeostrophic.
- Surface friction reduces speed to roughly two-thirds of the geostrophic value and backs the wind 30-45° over land, 10-20° over sea. Wind veers with height through the friction layer.
Local winds
- Sea breeze: daytime, onshore, typically 10-20 kt, depth 1,000-2,000 ft, with a distinct sea-breeze front; it strengthens against a light offshore gradient wind and cancels it against a strong one.
- Land breeze: night, offshore, weaker and shallower.
- Anabatic (upslope, daytime) and katabatic (downslope, night, cold and dense, drainage into valleys).
- Föhn: air forced over a ridge loses moisture on the windward side and descends dry-adiabatically on the lee side, arriving warmer and drier than at the same level on the windward side; it brings turbulence, mountain waves, lenticular cloud and a marked temperature jump.
Turbulence
- Mechanical (terrain, buildings, wind shear), thermal (convection), frontal shear, clear air turbulence and wake turbulence.
- Rotor streaming and venturi acceleration through mountain passes are the classic terrain hazards.
Worked example
Air at 20°C with dew point 10°C is lifted over a 2,500 m ridge. It saturates at about 1,000 m (temperature 10°C). From 1,000 m to the summit it cools at 0.6°C/100 m: 10 − 9 = 1°C at the top. Descending 2,500 m at 1°C/100 m adds 25°C, giving 26°C on the lee side - 6°C warmer and much drier than the upwind surface value.
Traps that cost marks
- Putting the sea breeze offshore, or forgetting it is a daytime phenomenon.
- Forgetting that surface wind backs relative to the geostrophic wind (northern hemisphere) and veers with height.
- Computing föhn descent with the saturated rate, which understates the warming.