What Hills Do to Wind

Collection: Field Notes — Old Fashioned Seamanship

Subject: What hills do to wind — gap acceleration, channel funnelling, split winds, rebel eddies, summit strengthening, mountain wave, katabatic events, and how to read the terrain before the wind arrives

In 1986 an instrument at the summit of Cairn Gorm in the Scottish Highlands recorded a gust of 173 miles per hour. At a station five hundred feet lower on the same mountain, the same gust was measuring 105 miles per hour. In the valley below, it was 63 miles per hour. The forecast had said it would be windy.

Tristan Gooley records this event in The Secret World of Weather not as a freak occurrence but as a precise illustration of something that happens at every scale on every coast and in every sea loch in the British Isles: the terrain multiplies, concentrates, redirects, and sometimes reverses the wind that arrives from the synoptic pattern. The forecast describes the wind that would be blowing if the land were flat. The land is not flat.

The previous post — Why the Forecast Is Always Wrong on Terrain-Influenced Water — explained why this gap between forecast and reality is structural: numerical models cannot resolve terrain effects at the scales sailors experience them. This post covers what those effects are, what they look like, and how to predict them from the chart and from observation before you arrive.

The gap wind: Theophrastus was right

Theophrastus, the Greek scholar who wrote about winds around 300 BC, observed that a wind passing through a narrow gap is always more forceful than the same wind in open country, and that in places where the main wind is calm, a gap will still produce a local breeze. Gooley cites this in The Secret World of Weather because the observation is as accurate now as it was then and the physics has not changed.

The mechanism is simple. A given volume of air must pass through a constriction in the same time it would take to pass across the open landscape. The only way it can do this is by accelerating. The narrower the gap, the greater the acceleration. This applies across all scales: between two tall buildings on a city street, between the mainland and an island a mile wide, between the flanking mountains either side of a sea loch entrance, between Orkney and Shetland, through the Strait of Gibraltar. The underlying physics is identical. The scale of the acceleration depends on the ratio of the open width to the constrained width and on the height of the constraining terrain.

Gooley describes his own experience of this threading the Canary Islands single-handed, where the wind transformed from a benign Trade Wind into sudden gusts of Force 7 or 8 as he entered the acceleration zones between the volcanic peaks. Simon Rowell covers the same phenomenon in Weather at Sea in the context of coastal navigation, noting that acceleration zones around headlands are usually visible in daylight as a distinct change in water surface texture — the wind line — where the rougher, darker accelerated wind meets the relatively calmer water on either side. At night this boundary is invisible until you are in it.

The practical read: any narrow passage between high ground on a chart is a potential gap wind zone whenever wind is blowing across that gap. The more perpendicular the wind direction to the gap, the stronger the effect. A light Force 3 with the wind pointing directly at a two-mile gap between five-hundred-metre hills can emerge from that gap as something considerably more vigorous. Cross-referencing the forecasted wind direction with the orientation of terrain features on the passage chart before departure is a direct and reliable preparation. This is not obscure knowledge. It is chart-reading applied to the third dimension.

The channel wind: valleys as pipes

Where a gap becomes a valley, the effect extends. Gooley describes channel winds — sometimes called the valley wind — as the continuation of a gap wind along any low route that the wind can follow through terrain. Once cold or dense air sinks into a valley aligned with its direction of travel, it follows that valley, accelerating if it narrows and maintaining its intensity far from the gap that started it.

The mistral is the most famous example in European waters: a high pressure over the Bay of Biscay drives air southward across France, but the Rhone valley and the gap between the Alps and the Massif Central funnel and concentrate it into the famous violent northerly that thunders down toward the Mediterranean. The Bora of the Adriatic — Rowell describes it in Weather at Sea — is a cold air mass that has pooled over the Dinaric Alps and Karst plateau driving southeast through the mountains toward the Adriatic coast. Both are famous winds with names precisely because they are so consistent and so dramatically stronger than the synoptic forecast would suggest.

In the British Isles the same effect operates at smaller scale but with equal predictability. Any sea loch running northeast-southwest in the Scottish west coast will funnel a southwesterly along its length. The wind arriving at the entrance may be the forecast Force 4. The wind at the head of the loch, compressed between the hills and channelled the full length of the water, may be considerably more. Coming out of that loch entrance in the same wind, the sailor experiences the reverse: the compressed valley wind emerges over the open water and fans out, losing speed but gaining direction unpredictability as it expands.

Gooley describes seeing the signature of this effect from high ground: dark bands reaching out over the sea from low points along a complex coastline, the wind painting the shape of the land onto the water surface. This is precisely the visible signature of the wind leaving valley exits. The dark rippled band stretches out over otherwise calmer water and reveals the alignment of every valley or pass in the coastal terrain behind it. At night or in reduced visibility these bands are not visible but they are still present, and a boat passing through them will know it immediately.

The split wind: what happens at a headland

When wind meets a promontory or headland projecting into the flow, it cannot pass through. It goes around. Gooley describes what this means in practice: the wind splits as it meets the obstruction, part of it deflecting around one side, part around the other. In the immediate lee of the headland the two deflected streams reunite, but having travelled around different sides of the obstacle they arrive from different directions. The result is a zone of confused, gusty, and directionally inconsistent wind in the lee of any significant headland, even when the main wind away from the headland is steady and predictable.

For a sailor working around a headland this is familiar experience: the wind in the bay on the approach is steady; the wind as you round the point is gusty and backing and veering; the wind in the bay on the other side is steady again but may have a completely different character reflecting which side of the headland it arrived from. None of this is in the forecast. All of it is predictable from the chart by reading the headland shape and the wind direction.

Rowell identifies a related effect in Weather at Sea: where wind blows roughly parallel to a coastline, the side of the channel nearest the low pressure will experience divergence and lighter winds near the shore, while the opposite side will experience convergence and strengthened winds. A beat along a coast in a fresh breeze will behave differently on each tack not just because of wave state but because the wind itself is structured differently on either side of the channel by the presence of the land.

The rebel wind: eddies and rotor zones

Gooley names eddies forming in the lee of obstacles the rebel wind — the wind that blows against the main flow, toward what you would think was the windward side. It forms wherever the main flow passes a sharp or jutting obstacle and creates a circulation in the downwind shadow. Part of that circulation flows back toward the obstacle, counter to the main wind.

This is exactly the mechanism that produces the rough, disturbed water in the lee of cliff faces described in What Waves Know. The water is rough not just because reflected waves are crossing incoming waves, but because the wind in that zone is itself in chaotic rotation. A boat seeking shelter from a strong westerly in the lee of a cliff-backed island may find the water calmer there but the wind directions entirely unreliable — gusting from the west as the main flow sweeps overhead, then from the east as the rotor brings air back toward the cliff face.

At the largest scale, this rotor effect creates the rotor clouds Gooley describes in the cloud chapter: ragged, warped cumulus forming in the turbulent zone in the immediate lee of a ridge or summit, a visible marker that dangerous chaotic winds exist at that level. When a rotor cloud is visible on the lee side of high ground and you are in a vessel downwind of that ground, the cloud is indicating conditions at mast height that you cannot feel at deck level until you are in them.

Summit strengthening and what it means for coastal passages

Gooley's Cairn Gorm example establishes a principle that applies to any coastline backed by hills: the higher the exposed ground, the stronger the wind, because upper air is progressively less slowed by surface friction. The wind you feel in the anchorage at the head of a sea loch is the wind that has been braked by the water surface, the hillsides, and the woodland on the banks. The wind at the top of the ridge above you is the geostrophic wind — the wind that would blow if there were no surface friction — and it may be two or three times stronger.

Why does this matter for a sailor not climbing that ridge? Because katabatic events bridge the gap between summit and water surface. Rowell describes in Weather at Sea how a plateau of cold air that has accumulated over high ground during a period of settled weather can be displaced by even a light gradient wind and begin to flow downslope. As it descends, the cold dense air accelerates under gravity. The wind arriving at the water surface can be dramatically stronger than the gradient wind that triggered the descent, sometimes reaching sixty knots or more from an apparently calm gradient. The high plateau of northern and western Scotland, the Icelandic coast, the Norwegian fjords, the Adriatic Bora zones are all areas where this happens with catalogue regularity. It is essentially unforecastable at the time and location of any specific event, but it is entirely predictable as a class of hazard whenever you are anchored or sailing near high ground after a period of cold settled weather.

The mountain wave: lens clouds as a wind warning

When a strong wind blows over a ridge or mountain and descends the lee slope, it creates an oscillating flow on the downwind side — a standing wave in the atmosphere analogous to the standing waves that form in water downstream of a submerged rock. Gooley describes this in detail in The Secret World of Weather using exactly that water analogy. The mountain wave is invisible, but it has a visible signature: lens clouds — smooth, layered, saucer-shaped formations — forming at the crests of the wave where the air is rising and cooling.

The practical implications are two. First, if you can see lens clouds over or downwind of high ground from your position on the water, wind speeds on the water near those clouds may be highly variable — relatively calm under the lens cloud crests where the air is riding highest, then significantly stronger between the crests where the wave brings the fast upper air down to the surface. Second, lens clouds are a reliable indicator that the atmosphere is stable and the wind strong and consistent — they form only in these conditions. Their presence means the wind is unlikely to change direction suddenly, even if its speed is variable. Their disappearance, by contrast, signals that conditions are changing.

Foehn winds and what they mean on the approach to high coasts

Rowell describes the Foehn wind in Weather at Sea — the effect that produces warm, dry, clear conditions on the lee side of a mountain range when moist air from the other side has been forced upward, rained out its moisture, and descended warm and dry on the sheltered side. The same mechanism operates in miniature wherever any wind crosses a ridge.

For the sailing context this matters most on passages approaching coasts backed by high ground. The Scottish west coast in a northerly has the entire depth of the highlands to the east. The Norwegian coast in an easterly has the Scandinavian plateau. In both cases the wind arriving at the water is not the same wind that began its journey on the other side of that ridge: it has lost its moisture going up and gained warmth coming down. The cloud patterns and air character it produces — often clear but gusty, drier than expected, with good visibility but strong squalls — are the foehn signature.

Reading the terrain before departure

The method for applying all of this practically is straightforward: before any passage in terrain-enclosed waters, orient the forecasted wind direction on the passage chart and identify what the terrain will do to it.

Draw a mental line in the direction the wind is blowing and note where it encounters hills, headlands, valleys, and gaps. Hills accelerate the wind at their summits and produce eddies on their lee sides. Valleys aligned with the wind funnel and strengthen it. Gaps between high ground compress and accelerate it. Headlands split it and produce confused wind in their immediate lee. A sea loch entrance running into the wind is a funnel. A bay sheltered by high ground upwind is a potential katabatic zone in settled weather. A ridge perpendicular to the flow is a potential mountain wave generator.

None of this requires instruments or models. It requires a chart with contours and an ability to read the terrain in three dimensions — which is simply an extension of the chart-reading skill described in Dead Reckoning Without Electronics into the vertical plane. The forecasted wind is the input. The terrain is the modifier. The output is what will actually happen on the water.

Tristan Gooley's The Secret World of Weather (Sceptre) covers all the local wind types discussed in this post — gap winds, channel winds, split winds, rebel winds, summit winds, mountain wave winds, sea breezes, and land breezes — with the physical principles behind each and a consistent emphasis on reading the terrain to predict them. Simon Rowell's Weather at Sea (Fernhurst Books) covers the same territory from a professional forecasting perspective, including Buys-Ballot's Law, convergence and divergence, headland acceleration zones, katabatic winds, and the Foehn effect.

The companion post on sea breeze mechanics is The Sea Breeze and the Land Breeze. The water surface effects that terrain-influenced wind produces are covered in What Moving Water Tells You and What Waves Know. The full series index is at Weather Forecasting.