foehn wind

Barrier-to-lee connection

Record the barrier, candidate crossing route and evidence connecting descending air to the lee observation.

1. Which mountain barrier and lee receiving area define this candidate episode? definition
2. Which observations or trajectory results support a descending connection between them? provenance

Diagnostic discrimination

Record the applied diagnostic, its local validity and evidence for drainage flow, warm advection or mixed conditions.

1. What evidence distinguishes this episode from radiatively driven drainage or warm-air advection? boundary
2. Which locally evaluated recognition method is being used, and what evidence would leave the classification ambiguous? definition

Comparable thermodynamic observations

Record temperature, potential temperature where appropriate, and moisture contrasts with their reference sites, levels and times.

1. Relative to which upstream air, elevation-adjusted reference or pre-onset interval is warming assessed? measurement
2. Does the humidity change represent lower relative humidity, lower specific humidity, or both? measurement

Mechanism evidence

Record evidence for drawdown, precipitation-related heating, mixing and radiative contributions; several mechanisms may contribute. [Met Office](https://weather.metoffice.gov.uk/learn-about/weather/types-of-weather/wind/foehn-effect)

1. What source-altitude, precipitation, cloud or turbulence evidence supports each proposed contribution? provenance
2. Can the contributions be distinguished with available evidence, or must their relative importance remain unresolved? boundary

Crossing support

Record measured or forecast profiles and pressure differences, including whether their spatial resolution resolves the relevant barrier or pass.

1. What do upstream and crest wind and stability profiles indicate about the air able to cross the barrier? measurement
2. How was the cross-barrier pressure difference calculated, and are elevation effects treated consistently? provenance

Cold pool and penetration

Record the vertical relationship between candidate foehn, any valley cold pool and surface observations.

1. At what heights and sites is foehn supported, and which valley-floor sites remain outside it? boundary
2. Which profile or station changes support intermittent or sustained surface breakthrough? measurement

Episode continuity

Record local onset and cessation intervals, temporary interruptions and the rule used to group them into one episode.

1. What persistence and gap rules distinguish an onset, a temporary interruption and a new episode? definition
2. Which observations bracket onset and cessation, and how wide are the timing uncertainties? measurement

Local exposure envelope

Record the evolving surface footprint, vertical reach, sustained winds and gusts without extrapolating a single exposed station across the valley.

1. Which areas have direct evidence of foehn, which are inferred, and where is the boundary unconstrained? boundary
2. What sustained winds and gusts occur at each relevant height and averaging interval? measurement

Atmospheric input package

Provide located and time-bounded wind, temperature and moisture evidence, keeping inferred impacts distinct from measured atmospheric conditions.

1. Which linked exposure model needs the episode's winds, drying, warming or evidence of turbulence? action
2. Which additional fuel, snowpack, asset or operating conditions must that model establish before inferring consequences? boundary

Threshold and evidence gate

Record the applicable exposure threshold, forecast validity, evidence sufficiency and authority for any proposed response.

1. Which configured threshold is met, at what location and time, and what response is authorized? action
2. What new observation, forecast expiry or change in surface penetration requires reassessment? action

Kinds and varieties

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• classical föhn (moisture-stripped windward ascent, condensation/precipitation, then dry adiabatic descent on the lee)
• anticyclonic or inversion föhn (subsiding air drawn from above a lee inversion, with little or no windward precipitation)
• shallow föhn (cold-air-pool drainage through gaps and saddles, often without a deep cross-barrier flow)
• deep föhn (cross-barrier flow that scours the lee cold pool and reaches the valley floor)
• south föhn / north föhn (Alpine directional types: southerly föhn into the northern Alps, northerly föhn into the southern Alps)
• chinook (Rocky Mountain lee föhn, often with a chinook arch)
• berg wind (South African plateau-to-coast föhn-type downslope wind)
• zonda (Andean lee föhn on the Argentine side)

1. Which of these kinds and varieties hold for the sense of foehn wind this model covers, and on what evidence? provenance

Identifiers and schemes

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Wikidata - Q184766 - Item for föhn / foehn wind.

1. Which of these identifiers and schemes hold for the sense of foehn wind this model covers, and on what evidence? provenance

Standards and regulation

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• WMO International Cloud Atlas and meteorological observing practice: föhn is a named local wind, not a separately coded WMO phenomenon class; reported as a local wind with associated temperature/humidity change.
• ICAO / national aviation weather services: föhn and similar downslope winds are treated under mountain-wave, turbulence, and wind-shear hazard reporting rather than a unique legal instrument.

1. Which of these standards and regulation hold for the sense of foehn wind this model covers, and on what evidence? provenance

Real-world use

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Operational forecasting in the Alps, Rockies, Andes, New Zealand, and other ranges as a local wind that can raise lee temperatures by 10-20 °C in hours.
• Wildfire and avalanche management: dry, gusty lee winds raise fire danger and can load or trigger snow slopes.
• Agriculture and viticulture: sudden warming and drying on lee slopes (e.g. Alpine föhn, Argentine zonda).
• Aviation: mountain-wave rotor and severe turbulence on the lee of the barrier.
• Public health and infrastructure: föhn-related heat, dust, and wind damage to roofs, trees, and power lines; historically linked (weakly) to föhnkrankheit complaints.

1. Which of these real-world use hold for the sense of foehn wind this model covers, and on what evidence? provenance

Typical measurements

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• temperature rise at a lee station during onset - 5-20 (extreme cases >25) - °C
• relative humidity on the lee during established föhn - 10-40 - %
• gust speed in valleys and at crest - 15-40 typical; damaging gusts 40-70+ - m s⁻¹
• potential-temperature contrast across the barrier (windward vs lee) - 5-15 - K

1. Which of these typical measurements hold for the sense of foehn wind this model covers, and on what evidence? provenance

Failure modes and hazards

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Rapid wildfire spread and extreme fire weather on the dry lee.
• Severe clear-air and rotor turbulence, including aviation accidents in mountain waves.
• Structural wind damage, fallen trees, and power outages from föhn storms.
• Snowmelt floods and ice-fall / avalanche risk from sudden warming.
• Dust, reduced visibility, and respiratory irritation (e.g. zonda, berg wind).
• Human thermal stress from abrupt warming, especially in winter chinook/föhn events.

1. Which of these failure modes and hazards hold for the sense of foehn wind this model covers, and on what evidence? provenance

Regional variation

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• Alps: föhn/föhnwind; south föhn vs north föhn is the operational distinction in Switzerland, Austria, Germany, and northern Italy.
• North America: chinook on the east slopes of the Rockies; Santa Ana and sundowner on Southern California ranges are föhn-like but often more katabatic/gap-driven.
• South America: zonda in western Argentina; puelche in Chile.
• New Zealand: Canterbury northwester / nor'wester as a föhn across the Southern Alps.
• South Africa: berg wind off the plateau toward the coast.
• Adriatic/Dinaric: bora is a cold downslope/gap wind and is not a föhn, despite similar orography.

1. Which of these regional variation hold for the sense of foehn wind this model covers, and on what evidence? provenance

Neighbouring kinds and how to tell them apart

Reported by the breadth pass; each item needs checking against its source before it becomes normative.

• bora (and other cold downslope / gap winds) - Bora is a cold, dense-air overflow, often with a hydraulic jump; föhn is a warm, dry lee wind with adiabatic warming. Separate by temperature change at onset: föhn warms, bora cools or stays cold.
• katabatic wind (drainage of radiatively cooled slope air) - Katabatic flow is nocturnal, shallow, and cold; föhn is typically synoptically forced, deeper, and warm. Separate by diurnal timing, inversion depth, and temperature anomaly relative to the free atmosphere.
• mountain wave / hydraulic-jump windstorm without föhn warming - Strong lee winds can occur with little warming if the descending air is not potentially warmer than the valley air. Separate by potential-temperature increase and humidity drop at the surface, not by wind speed alone.
• sea breeze or valley wind - Thermally driven diurnal circulations reverse with the sun and do not require a cross-barrier synoptic flow or a dry-adiabatic lee signature.

1. Which of these neighbouring kinds and how to tell them apart hold for the sense of foehn wind this model covers, and on what evidence? provenance