Wind: Local, Gradient and Upper-Level Winds
What the exam tests
This domain tests the forces that produce wind, surface friction effects, local circulations and upper-level flow.
Core rules
- Pressure gradient force drives air from high to low pressure; Coriolis deflects it (to the left in the Southern Hemisphere) and friction slows surface wind and turns it across the isobars toward low pressure.
- Gradient wind is the balanced flow around curved isobars; geostrophic wind applies to straight isobars aloft with negligible friction.
- Surface wind typically veers with height in the Southern Hemisphere as friction decreases; always distinguish veer (clockwise) from back (counter-clockwise).
- Local winds include sea breezes (day, onshore), land breezes (night, offshore), anabatic upslope winds and katabatic downslope winds.
Worked example
On a sunny afternoon, the land heats faster than the sea, pressure falls over the land and a sea breeze develops. A surface wind of 120°/20 kt with higher-level wind 150°/30 kt shows both veer and strengthening with height. The reasoning combines thermal contrast, pressure gradient and reduced friction aloft.
Traps
Applying Northern Hemisphere deflection; mixing veer and back; assuming surface wind equals gradient wind; overlooking local terrain effects.