Wind: global and local circulation, geostrophic and gradient wind, wind shear
This domain tests the forces that set wind direction and speed, the difference between geostrophic and gradient flow, local circulations and the identification of shear.
Forces and balances
Pressure-gradient force acts from high to low pressure. Coriolis force acts to the right in the Northern Hemisphere and is proportional to wind speed and sin(latitude). Geostrophic wind is a straight, frictionless balance: Vg = (1/(ρf)) Δp/Δn, with f = 2Ω sin φ. Gradient wind adds centripetal acceleration: around a low, flow is cyclonic and subgeostrophic; around a high, anticyclonic and supergeostrophic. Friction slows wind and turns it toward low pressure. Buys Ballot: in the Northern Hemisphere, stand with your back to the wind and low pressure is on your left.
Local circulation and shear
Sea breeze blows from sea to land by day; land breeze reverses at night. Anabatic and katabatic flows follow heating and cooling slopes. Global circulation includes trade winds, westerlies and the ITCZ. Wind shear is vector change with height or distance; low-level shear matters for take-off and landing, while vertical shear organizes thunderstorms.
Worked example
At 30°N, Δp/Δn = 2 hPa/100 km, ρ = 1.2 kg/m³. Convert: 200 Pa/100,000 m = 0.002 Pa/m; f = 2 × 7.292 × 10⁻⁵ × sin 30° = 7.29 × 10⁻⁵ s⁻¹. Vg = 0.002/(1.2 × 7.29 × 10⁻⁵) ≈ 22.9 m/s ≈ 44 kt. This is the geostrophic estimate; friction would reduce it and back it toward low pressure.
Traps
Geostrophic wind cannot be used near the equator where f approaches zero, or in strongly curved flow. Backing is anticlockwise; veering is clockwise. Wind direction is where the wind comes from.