Local & regional winds

Why does air move up a sunny slope and down a cold one?

Compare air beside the slope with air at the same height away from it.

A slope creates a local temperature contrast

During daytime heating, the sunlit slope warms the air touching it. That air becomes warmer and less dense than air at the same elevation away from the slope and rises along the incline: an anabatic wind. It is most evident with good sunshine, suitable slope exposure and a weak background flow. Cloud can develop over the higher terrain if the lifted air reaches saturation.

At night, radiative cooling chills the slope and the adjacent air. The colder dense air drains downhill as a katabatic wind and can pool in valleys or basins. Ordinary nocturnal drainage may be shallow, but a large cold reservoir over an ice sheet or plateau can drive powerful persistent downslope flow. Katabatic does not mean that all descent over mountains is caused by local night-time cooling; a pressure-driven downslope wind can have a different mechanism.

Sun angle, snow cover, cloud, roughness and synoptic wind alter the strength and timing. The local flow may be overwhelmed or reinforced by the larger-scale pressure gradient. A wind observation on the valley floor is therefore not a reliable substitute for assessing the exposed slope, pass or ridge along a flight path.

Why can a pass have much stronger wind than a nearby plain?

Separate the daily valley circulation from pressure-driven channelled flow.

Valley and mountain winds

The valley wind is the daytime up-valley tendency in a thermally driven mountain-valley circulation. Heating of the valley system and its slopes helps establish a pressure distribution that draws air towards the higher valley head. At night a mountain wind tends to flow down-valley as cooled air drains from the mountain and valley system towards lower ground. Anabatic and katabatic describe slope flows; valley and mountain winds describe the broader along-valley component.

A valley can also channel a synoptic flow along its axis even when the free-air wind has a different direction. A pressure difference across a gap can drive strong gap winds. Narrowing of the effective flow passage can accelerate the air, often called a Venturi effect, with a local reduction of static pressure. The atmosphere is not a sealed pipe: stable-layer depth, blocking, spill-over and three-dimensional escape routes determine whether and where the acceleration occurs.

Convergence where valleys join or where separate streams are forced into a narrower region can increase local wind and create upward motion. Flow separation and obstacles generate turbulence downwind of bends, ridges and gap exits. Do not infer the strongest point from the narrowest map contour alone; the upstream wind, stability and pressure pattern are essential parts of a mountain briefing.

Why can the coast turn onshore during a sunny afternoon?

Follow the temperature contrast and the local pressure difference it creates.

Daytime onshore flow

Land usually warms faster than the sea during sunshine. A local pressure-gradient circulation develops with low-level flow from the relatively cool sea towards the warmer land: a sea breeze. Air rises over the heated land and a compensating return component can develop aloft. The pattern is clearest with weak large-scale wind and a strong land-sea temperature contrast, rather than being guaranteed on every summer coast.

The sea-breeze front is the leading boundary of the advancing marine air. It can bring a wind shift, a temperature fall, a humidity change and convergence. If the air ahead is sufficiently moist and unstable, the lifting can produce cumulus, showers or thunderstorms. If it is dry or capped, the boundary may have little cloud. Opposing coastal breezes can converge over a peninsula or island, enhancing ascent.

At night, land can cool below the sea temperature and reverse the low-level contrast. A land breeze then blows from land towards sea and is often weaker than the daytime sea breeze. Cloud, coastline geometry and the background wind can reinforce, weaken, delay or mask both circulations. The sea-breeze boundary is a potential low-level wind-change feature, not simply a pleasant cooling effect at the beach.

How can a low-level jet develop after sunset?

Track the loss of daytime mixing and the changing momentum balance above the stable layer.

Decoupling changes the wind profile

During the day, turbulent mixing transfers momentum between the surface and the air above it. After sunset, cooling can establish a stable surface layer and reduce that exchange. Air above the cooled layer loses some of the frictional influence that had slowed it, while the surface wind itself may weaken. The elevated flow can accelerate and turn as its force balance adjusts, creating an evening or nocturnal low-level wind-speed maximum.

An inertial oscillation is one mechanism: after the drag changes, the wind vector oscillates around its new balance under Coriolis. Terrain and regional pressure gradients can reinforce the jet, so not all low-level jets have identical causes, heights or timing. The term describes a local speed maximum at low altitude, distinct from the polar or subtropical upper-tropospheric jets.

Marked shear can occur between the calm runway environment and the faster jet above it. An aircraft climbing or descending through that layer can encounter rapid changes in headwind or crosswind and sometimes turbulence. A low surface wind report therefore does not rule out an adverse wind profile. Use low-level forecasts, soundings, reports and any local wind-shear information appropriate to the airport.

Why can the lee side be warmer and drier?

Use the parcel's thermodynamic history, while recognising that precipitation is not the only foehn mechanism.

The classical moist-ascent explanation

Air forced up a mountain initially cools at the dry adiabatic rate until saturated. During saturated ascent, condensation releases latent energy and reduces the rate of cooling. If water is lost as precipitation and the air then descends unsaturated on the lee side, it warms at the dry adiabatic rate. At the same final elevation it can be warmer and have a lower relative humidity than on the windward side. The detailed numerical calculation is developed in the thermodynamics lessons.

Foehn is the general warm, dry lee-wind effect; Chinook is a regional name associated with the eastern side of the Rocky Mountains. The basic thermodynamics explains one important pathway, but foehn can occur without substantial precipitation. Drawdown of warmer air from above a blocked cold layer, turbulent mixing of warmer air into the descending flow and radiative effects can contribute. A wet windward slope is therefore not an absolute requirement for every foehn event.

Better visibility can hide a dangerous wind field

Windward ascent may produce extensive cloud and precipitation, with a cap cloud or sharp foehn wall near the crest. The lee can have clearer, warmer and drier air, but strong gusts, downslope acceleration, mountain waves, rotors and severe turbulence may occur. Lenticular cloud can identify wave crests if there is enough moisture. Dry lee conditions are not evidence that the air is smooth or that a mountain crossing is safe.

Which air mass is being channelled or carried into the region?

Connect a local wind name to its direction, temperature and visibility implications.

Mistral and Bora

The Mistral is a cold, often dry north to north-westerly flow towards the Gulf of Lion, strongly channelled through the Rhône region. A pressure gradient between higher pressure inland and lower pressure towards the Mediterranean can accelerate it. Clear air may coexist with strong gusts and terrain turbulence.

The Bora is a cold, gusty north-easterly downslope flow towards the Adriatic. Cold continental air spilling across the Dinaric barrier and through gaps can produce intense local winds. Despite compression warming during descent, its cold source can leave it colder than the maritime air it replaces; downslope does not always mean a warm foehn.

Harmattan

The Harmattan is a dry, dust-bearing north-easterly continental flow from the Sahara towards West Africa, especially prominent in the dry season. Suspended dust can reduce visibility far from the source and degrade slant visibility even without low cloud. A clear-looking sky overhead does not establish good horizontal or approach visibility.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.