Mid-Latitude Weather and Named Winds - ATPL 050 — Meteorology

Climatology comes home in this final lesson: the typical situations of the mid-latitude chart (the westerly parade, the settled highs, and the cold-air drop that hides from the surface analysis), followed by the winds famous enough to carry names: the warm Foehn and its Chinook cousin, the cold Mistral and Bora, and the dust-hazed Harmattan.

The westerly situation

The default weather map of the mid-latitudes is the westerly (zonal) situation: isobars and fronts strung west to east across the ocean, with a family of polar-front depressions travelling toward Europe a few days apart on the broad westerly flow. The weather it delivers is the air-masses module's sequence on repeat — warm front, warm sector, cold front, ridge day, and the next system's cirrus already on the horizon: changeable, system after system. On the chart it is recognised at a glance by the west-east alignment and the queue of frontal waves.

The highs of the mid-latitude chart

The interruptions to the westerly parade are the highs, and they are identified as easily as they behave: broad cells of closed isobars with wide spacing around a pressure maximum (wide spacing meaning weak gradients and light winds). The climatological cast: the Azores High and its ridges into Europe, the blocking highs of the Scandinavian pattern, and the winter continental high in the east. Their weather is the pressure-systems module's anticyclone package, settled: summer sunshine with haze below the inversion; winter gloom, radiation fog and frost for as long as the high holds.

The cold-air drop

One mid-latitude feature earns its own section by being invisible where forecasters usually look. A cold-air drop, also met as the cut-off low, is a pool of cold air aloft, cut off from the main polar westerlies and drifting on its own, usually with little or no signature on the surface chart. It forms when the upper flow meanders hard: a deep upper trough in strongly meridional flow pinches off, leaving a closed cold vortex aloft that drifts slowly and erratically while the westerlies reform poleward of it.

The identification rule follows from the anatomy: the drop is found on upper-air charts, as a closed low with a cold core on the 500 hPa chart, while the surface chart beneath may show only slack gradients and no fronts. And that is exactly its danger. Cold air aloft steepens the lapse rate, so heavy showers, thunderstorms, severe turbulence and icing can erupt from a situation the surface analysis calls harmless (the thermodynamics module's instability recipe assembled overhead) — and because the drop moves slowly and erratically, even its discovery does not say where tomorrow's outbreak will fire. The rule of thumb: when the sky misbehaves under an innocent surface chart, look up.

Foehn: the warm downslope wind

The named winds begin with the famous warm one. The Foehn (the Alps' own, with the Rockies' Chinook as the same machine abroad) works by an accounting trick with lapse rates: moist air forced up the windward slope cools at the saturated rate once cloud forms, raining out much of its moisture; descending the lee slope dry, it warms at the full dry rate all the way down, and arrives warmer and much drier than air at the same level on the windward side. The thermodynamics module's figure below tells it in one picture.

The lee-side weather is the Foehn's paradox package: warm, dry, strikingly clear air (the Foehn gap) beneath a wall of cloud standing on the crests (the Foehn wall), with strong gusty winds, mountain waves, severe turbulence and rapid snowmelt; the windward side, meanwhile, sits under cloud and prolonged precipitation. The mountain-wave turbulence riding the flow is the wind module's machinery, revisited in earnest in the flight-hazards module.

Mistral and Bora: the cold fall winds

Not every descending wind arrives warm. Two European winds descend and arrive violently cold — the anti-Foehn pair:

Mistral — a strong, cold, dry northerly channelled down the Rhone valley between the Alps and the Massif Central onto the Mediterranean, typically with high pressure over Biscay and a lee low in the Gulf of Genoa (the pressure-systems module's orographic low): gale force for days, with severe low-level turbulence.
Bora — a violently gusty, cold, dry northeasterly fall wind spilling over the coastal mountains onto the Adriatic: cold continental air pouring downslope. It warms adiabatically on the descent like any air, but its source is so much colder than the maritime air it displaces that it still arrives cold.

The Harmattan

The course's standing example of a local wind whose weapon is visibility: the Harmattan is the dry, dust-laden northeasterly trade off the Sahara over West Africa, dominant from about November to March — the winter airstream of the previous lesson's West African year. Its lifted fine dust reduces visibility severely, over areas the size of countries, and to heights that matter for descent and approach: an instrument approach flown through an orange murk with no cloud base to break out of, exactly as the previous lesson's Lagos example described.

What the exam asks

This lesson's questions are chart-reading and wind biographies: recognise the situation, name the wind, and know which arrives warm and which cold. The overview, following the learning objectives:

Identify the typical westerly situation with travelling polar-front waves on a weather chart

Describe the high-pressure zones and their weather, and identify them on a chart

Define the cold-air drop, describe its formation, identify it on charts, and explain its problems and dangers for aviation

Describe the Foehn mechanism (including the Chinook) and the associated weather

Describe the formation, characteristics and weather of the Mistral and the Bora

Describe the Harmattan and its visibility problems as an example of local winds affecting visibility