Geostrophic wind
The geostrophic wind is the theoretical wind speed where the pressure-gradient force and Coriolis force are in balance. Found approximately in the free atmosphere above the friction layer (above ~3000 ft AGL).
Source: WMO Guide; AMS Glossary of Meteorology; FAA-H-8083-25B PHAK.
Force balance
- Pressure gradient force (PGF): from high to low pressure, perpendicular to isobars.
- Coriolis force: perpendicular to motion, to the right in the Northern Hemisphere (NH).
In balance: wind flows parallel to isobars, low to the left (NH).
Formula (simplified)
V_geostrophic = (1 / ρ × f) × (Δp / Δn)
- ρ = air density
- f = Coriolis parameter (= 2 Ω sin(latitude))
- Δp/Δn = pressure gradient perpendicular to isobars
→ Stronger pressure gradient = stronger geostrophic wind. → Higher latitude = stronger Coriolis effect = weaker geostrophic wind at same gradient.
Veering vs backing
- Veering: wind turns clockwise (e.g. south → SW → west → NW).
- Backing: wind turns counter-clockwise.
Veering with height — friction layer
In the Northern Hemisphere, wind direction veers (turns clockwise) and speed increases with height from the surface up to the top of the friction layer due to decreasing frictional influence allowing closer alignment with geostrophic flow:
- At the surface: wind slowed by friction and deflected toward low (over land up to 45° off the isobar direction).
- At ~3000 ft AGL: friction minimal → nearly geostrophic (parallel to isobars).
- Transition: gradual turn + speed increase.
Practical application
- WINTEM chart gives the geostrophic wind at various pressure levels.
- The pilot derives the surface-wind component from the geostrophic wind considering the friction layer.