Wind: Forces, Global Circulation and Local Effects
This domain tests the forces that create wind, the resulting patterns from planetary to local scale, and the rules that let you infer wind from the pressure field.
Forces and balance
The pressure gradient force drives air from high to low pressure, perpendicular to the isobars. Coriolis force deflects it to the right in the Northern Hemisphere and left in the Southern, proportional to wind speed and increasing with latitude, and is zero at the equator. Friction slows surface wind and turns it across the isobars toward low pressure; at about 2,000 ft and above the flow is nearly geostrophic and parallel to straight isobars. Around curved isobars the gradient wind is subgeostrophic around a low and supergeostrophic around a high.
Rules
Buys Ballot in the Northern Hemisphere: with your back to the wind, low pressure is on your left. Wind veers (turns clockwise) with height as friction decreases. The thermal wind blows parallel to thickness lines, with cold air on the left in the Northern Hemisphere.
Local effects
Sea breeze by day (onshore), land breeze at night (offshore), anabatic upslope winds by day, katabatic drainage at night, mountain waves and Foehn-type downslope winds. In Pakistan the summer south-westerly monsoon and the hot dry Loo are the dominant low-level signatures.
Worked example
At 30°N with straight isobars 4 hPa apart per 100 km, the geostrophic wind that balances Coriolis against the pressure gradient blows parallel to the isobars, low pressure on the left. Near the surface, friction turns the actual wind 20-30° toward the low and reduces its speed by around 30%.
Traps
Reversing the Coriolis direction, treating surface wind as parallel to the isobars, and forgetting that wind veers rather than backs with height in the Northern Hemisphere.