Local winds

Local winds

Local winds arise from regional effects — topography, land-sea contrast, diurnal cycle — and superimpose on the large-scale geostrophic wind.

Source: WMO Guide; AMS Glossary; DWD Klima-Atlas; NASA Earth Observatory.

Main categories

1. Thermal winds: from temperature differences (sea breeze, mountain-valley wind).
2. Orographic winds: from terrain shape (Foehn, Bora, Mistral).
3. Convective winds: from local convection cells (thunderstorm outflow).

Mountain-valley wind

The mountain/valley wind circulation in a mountain range is primarily caused by the diurnal variation of temperature due to solar radiation — leading to upslope (anabatic) winds during the day when slopes heat faster than valleys and downslope (katabatic) winds at night as slopes cool more rapidly.

Valley wind (anabatic)

Valley wind is encountered during day time:

• Sun warms slopes → warm air rises along slopes → from valley up.
• Convergence in the valley → air flows out of the valley, then upward.

Mountain wind (katabatic)

The mountain wind is encountered at night:

• Slopes cool faster than valley air → cold air sinks along slopes into the valley.
• Cold pool builds in the valley → flows out.

Foehn — orographic wind

Warm, dry catabatic wind is called Foehn:

• Foehn forms when moist air crosses a mountain range.
• Windward side: ascending air cools wet adiabatically (-0.6 °C/100 m) → clouds and precipitation.
• Lee side: descending air warms dry adiabatically (+1.0 °C/100 m) → warmer, drier than at the same altitude on the windward side.

The higher temperature on the Foehn (lee) side of a mountain range results from stronger dry adiabatic heating during air descent compared to the wet adiabatic cooling during ascent on the windward side, as moisture condenses out on the ascent, leaving drier air to warm more rapidly on descent.

Characteristic Foehn-side weather: few clouds, unusual temperature rise, low humidity, often gusty winds.

Lee waves and lenticularis

Altocumulus lenticularis clouds close to a mountain range indicate Foehn with generation of atmospheric waves, possibly with rotors and severe turbulence:

• Lenticular clouds (lens-shaped) form on the leeward side over the ridge.
• Lee waves are atmospheric waves in the lee of mountain ranges.
• Rotor: cylindrical turbulence zone below the lee wave — extremely dangerous, when crossing a rotor, severe turbulence must be expected.
• Foehn and lee waves usually occur only in stable conditions.

Northern Foehn in the Alps

Northern Foehn in the Alps refers to low clouds often lying on mountain ridges when moist air is advected from northerly directions, leading to persistent overcast conditions.

Bora — orographic Adriatic wind

Bora is an orographic wind, encountered at the Adriatic Sea:

• Cold, dry air from the Karawanken/Velebit mountains flows down to the Mediterranean.
• Typically very strong, gusty — up to 200 km/h.
• Dangerous for small aircraft.

Mistral — orographic Rhone wind

Mistral is an orographic wind encountered inside the Rhone Valley:

• Cold, dry wind from the north/northwest.
• Channelled by the Rhone valley → very high speeds.
• Typical in winter, can last for days.

Wind conditions during Foehn approach

During strong Foehn winds on the northern side of the Alps, where conditions are clear and cloud-free due to descending dry air, VFR flight to Venice on the southern side is predominantly not possible because moist air ascends and forms accumulating clouds that obscure the mountains and reduce visibility:

• North side: clear, dry, excellent visibility.
• South side (windward): dense clouds, poor visibility.
• Cumuliform clouds extending down to the surface are typical on the southern (windward) side of the Alps during Foehn.

Wind angled to a mountain

When moist air with strong winds flows at an angle to a mountain chain, the windward side typically develops a dense cover of low clouds due to orographic lifting and condensation, while the leeward side experiences good visibility as descending air warms and dries.

Front crossing a mountain

When a front crosses a mountain ridge, its effect on the weather decreases on the leeward side and increases on the windward side:

• Windward: more precipitation, clouds.
• Lee: weather partly "dried out".

Klima-Atlas Deutschland*; NASA Earth Observatory Local Winds; FAA-H-8083-25B PHAK Ch. 12.*