Surface wind and friction layer
Surface wind is the wind in the near-ground layer (atmospheric boundary layer), where friction between air and surface breaks the geostrophic balance.
Source: WMO Guide; AMS Glossary; FAA-H-8083-25B PHAK Ch. 12.
Friction layer — up to 3000 ft AGL
The effect of surface friction on wind over land masses typically diminishes with increasing height but can usually be felt up to 3,000 ft above ground level:
- At the surface (0-100 ft): wind sharply slowed by friction, strongly deflected.
- 100-1000 ft: friction decreases, wind veers and accelerates.
- 1000-3000 ft: gradually approaches the geostrophic wind.
- Above 3000 ft AGL: essentially geostrophic (no friction).
Deflection across isobars
Surface friction in near-ground layers causes winds to slow and deflect across isobars towards low pressure, rather than flowing parallel to them as in higher altitude geostrophic balance:
- Over land (rough surface, high friction): deflection ~30-45° off the isobar direction.
- Over water (smooth surface, low friction): deflection ~10-20°.
The deflection of surface wind from the direction parallel to isobaric lines is smaller above the ocean than over continental areas.
Veering with height — NH
In the Northern Hemisphere wind veers (clockwise) and increases with height from the surface to the top of the friction layer (see "Geostrophic wind" lesson).
Friction turbulence
Friction turbulence is most significant in the altitude band between the surface and 3000 ft above ground:
- Caused by wind shear and eddies at obstacles and terrain features.
- VFR consequence: expect turbulence in low altitudes with strong winds.
Diurnal variation of surface wind
The surface wind speed usually becomes lower in the evening due to the formation of a surface inversion, where radiative cooling creates a stable layer of cooler air near the ground that decouples it from faster upper-level winds, reducing turbulence and speed:
- By day: sun warms ground → vertical mixing → higher surface winds via mixing with upper winds.
- Evening/night: ground cools → stable inversion → decoupling of near-ground air → weak surface wind.