How wind forms, forces and surface friction
Wind = air in motion relative to Earth's surface (AWH 10.1). Moves from high toward low pressure.
Naming: direction blown from, not toward.
| Wind | Blows from |
|---|---|
| North | 360° |
| East | 090° |
| South | 180° |
| West | 270° |
Three forces:
| Force | Effect |
|---|---|
| Pressure-gradient | Starts air moving, high to low, perpendicular to isobars |
| Coriolis | Deflects moving air to right (Northern Hemisphere). Greatest at poles, zero at equator, grows with wind speed |
| Friction | Drags on bottom of wind, within about 2,000 ft AGL (FAA AWH 10.3.3; AIM MET 10.0 item 4) |
| Layer | Wind vs isobars | Around high | Around low |
|---|---|---|---|
| Aloft (above friction layer) | Parallel (geostrophic wind) | Clockwise | Counter-clockwise |
| Near surface | Backed toward low by friction | Clockwise and outward (TC AIM MET 10.0) | Counterclockwise and inward (TC AIM MET 10.0) |
- Aloft: pressure gradient and Coriolis balance.
- Near surface: friction backs wind (turns toward low) and slows it.
- Near surface: larger turn over rough terrain than water (AWH 10.4).
- Near surface: both circulation directions reverse in Southern Hemisphere.
Trap: surface wind is not geostrophic: friction backs it toward the low, off parallel to isobars.
References
- FAA Aviation Weather Handbook (FAA-H-8083-28) 10.1, 10.3, 10.3.3, 10.4
- TC AIM MET 10.0 item 4
- TC AIM MET 10.0