Wind
Wind questions test the balance of forces, the difference between geostrophic and surface wind, and the local wind systems that affect take-off, landing and terrain clearance.
Forces and balances
Pressure gradient force, Coriolis force, friction and, in curved flow, centripetal force. Straight isobars without friction give the geostrophic wind, parallel to the isobars. Curved flow gives the gradient wind: sub-geostrophic around a low, super-geostrophic around a high.
Surface wind
Friction slows the wind and turns it towards low pressure. Over land the surface wind is typically backed about 30–45° from the geostrophic direction and reduced to roughly half its speed; over the sea, backed about 10–20° and about two-thirds of the speed. Wind veers with height through the friction layer.
Local systems
- Sea breeze by day (sea to land), land breeze by night.
- Anabatic upslope flow by day, katabatic downslope flow by night.
- Föhn: warm, dry descending wind on the lee side.
- Mountain waves and rotors in strong cross-mountain flow.
Buys Ballot
In the Northern Hemisphere, stand with your back to the wind and low pressure is on your left; in the Southern Hemisphere, on your right.
Worked example
A geostrophic wind of 040°/40 kt over land at 2000 ft. Applying friction: back about 30° to 010°, speed about half to 20 kt. The surface wind is therefore roughly 010°/20 kt, so a runway oriented 010/190 is the most favourable.
Traps
- Confusing backing (anticlockwise) with veering (clockwise).
- Quoting wind direction as where the wind goes rather than where it comes from.
- Applying Northern Hemisphere rotation rules in the Southern Hemisphere.
- Forgetting that friction backs the surface wind and reduces its speed relative to the geostrophic wind.