Highs, Lows and the Jet Stream - Understanding Weather for Passage Planning
Collection: Field Notes — Old Fashioned Seamanship
Series Hubs: Weather Forecasting | Coastal and Offshore Passage Planning
Subject: Highs, lows and the jet stream — how the jet stream controls the storm track, how highs and lows interact and create wind, what a blocking high means for passage planning, and how to read the medium-range weather pattern for a window
In August 1947 a British South American Airways flight disappeared over the Andes after reporting its position as clear of the summits. The crew were experienced. Their dead reckoning was careful. What they had not accounted for was the jet stream — a river of fast-moving air at altitude that was pushing them backward significantly faster than any forecast wind had predicted. They descended into mountains they believed they had already crossed.
Tristan Gooley recounts this in The Secret World of Weather as the moment the aviation world was forced to take the jet stream seriously. Sailors have the same relationship with it now that those pilots had then: they know it exists, they see references to it in forecasts, but few have a working model of what it does and how to read its position and behaviour directly from available information.
The jet stream is not an obscure specialist topic. It is the fundamental organising structure of North Atlantic weather. The position of the jet determines where depressions form and track. Its configuration determines whether a high-pressure system will sit over the British Isles for a week or be swept aside in a day. Understanding it gives a sailor a medium-range planning framework that most forecast products do not make explicit.
What the jet stream is
The jet stream is a narrow band of very fast wind at altitude — typically six to nine miles above the surface, at the boundary between the troposphere and the stratosphere. In the North Atlantic, its average speed at the core is eighty to a hundred knots, occasionally more. It forms along the zone of greatest horizontal temperature difference in the upper atmosphere: where relatively warm tropical air meets relatively cold polar air. In the Northern Hemisphere there are two relevant jet streams — the subtropical jet and the polar jet — but the polar jet is the one that controls North Atlantic weather for British and northern European sailors.
As Rowell explains in Weather at Sea, the jet flows roughly from west to east but its exact track undulates north and south in large-scale waves called Rossby waves. When the jet is relatively straight and fast, it carries depressions rapidly across the Atlantic from west to east. When it buckles into large north-south meanders, it can strand weather systems in position for days — producing either prolonged stormy conditions or prolonged settled spells depending on which part of the meander you sit under.
The practical tool Rowell identifies for locating the jet on an upper-level chart is the 5,640-metre contour of the 500 hPa height chart. This contour line runs along the equatorward side of the jet's core. Depressions track along its poleward side; high pressure systems sit on its equatorward side. Wherever that line is on the chart, that is approximately where the boundary between stormy and settled weather runs.
How the jet steers depressions
The previous post described how Atlantic depressions form along the polar front and develop through a life cycle of warm front, warm sector, cold front, and occlusion. The jet stream determines where along the Atlantic that process begins and which direction the resulting system travels.
When the jet is positioned to funnel depressions directly across the North Atlantic toward the British Isles — its typical winter configuration — the sequence is familiar: low after low tracking northeast past Iceland or directly toward Ireland and Scotland, each dragging its frontal sequence over the UK within forty-eight to seventy-two hours of its predecessor. Westerly winds dominate. Fronts arrive regularly. The pattern is what British sailors call typical Atlantic weather.
When the jet lifts north in summer or dips south in winter, or buckles into a large-amplitude meander, the storm track shifts accordingly. A jet positioned well north of its usual track carries depressions over Iceland and into the Norwegian Sea, leaving the British Isles in a relative backwater south of the storm track — the classic condition for a blocking high. A jet dipping unusually far south sends depressions through the Bay of Biscay, producing southerly and southwesterly gales across the Western Approaches rather than the more typical westerlies.
Rowell notes in Weather at Sea that while forecasts beyond five days lose significant detail on individual systems, the broad position of the jet stream is typically well forecast out to ten days. This makes it the most useful single parameter for medium-range passage planning: if you can see from the five-to-ten-day upper-level forecast that the jet is tracking depressions north of Scotland, you have a reasonable basis for planning a Channel or Biscay passage. If the jet is positioned to drive systems directly at you, wait.
High pressure systems: why they matter differently from lows
Rowell's description of high pressure systems in Weather at Sea contains a detail that most sailors overlook: highs are fundamentally more stable and slower-moving than lows. A depression forms, develops, tracks across the ocean, occludes, and fills within a typical life cycle of three to five days. A high-pressure system, once established, can sit in the same general area for a week or more.
The physics explains this. A low pressure system depends on continuous energy input — the temperature contrast between the air masses feeding its conveyor belts, the upper-level dynamics supporting its circulation. Remove those inputs and the low fills. A high pressure system depends on convergence at altitude and divergence at the surface, producing descending dry air. Descending air suppresses cloud and precipitation, which means the surface beneath a high warms in sunshine, which helps maintain the warmth that keeps the high in place. Highs are self-reinforcing in a way that lows are not.
The practical consequence is that the arrival of a high — confirmed by rising pressure, veering wind, clearing skies — is a more reliable indicator of sustained improvement than the departure of a low. When a high moves over you, it tends to stay. The question for passage planning is how long it will stay and from which direction the next depression will eventually dislodge it.
The character of high pressure weather
Rowell identifies a seasonal difference in the character of high pressure conditions that is directly relevant to planning. In summer, a high over the British Isles produces the kind of weather that seems ideal for sailing but is in some respects the most meteorologically complex: calm conditions in the morning, sea breeze development as described in The Sea Breeze and the Land Breeze, hazy visibility from the inversion that the descending high-pressure air creates, possible afternoon thunderstorm development as the sea breeze front triggers convective instability. The Field Forecasting skills are at their most useful here — the synoptic situation is settled but the local weather is dynamic.
In winter, the same high pressure system produces entirely different conditions. The solar heating that maintains the summer inversion is weak or absent. The descending dry air of the high produces crystal-clear visibility, sharp frosts, and often very cold temperatures at night. The wind is light or calm and the sea surface glassy. These conditions are ideal for coastal passages in terms of visibility and sea state but demand vigilance for fog — specifically the radiation fog that forms in estuaries and river mouths on calm clear nights when the land surface radiates heat away faster than the sea, as described in Fog on Inland and Coastal Waters.
Blocking highs and what they mean for passage planning
A blocking high is a high-pressure system that has become locked in position by the configuration of the jet stream and is preventing the normal westerly progression of weather systems. Instead of the low-high-low-high sequence sweeping eastward across the Atlantic, the blocking high sits stubbornly in place — sometimes for ten days or more — and deflects depressions north and south around itself.
Rowell shows in Weather at Sea how this happens over the North Atlantic and Europe. When the jet stream bucks northward into a high-amplitude meander, a high can push up over the British Isles and into Scandinavia. Depressions that would normally track across Scotland or Ireland are deflected north toward Iceland or south toward the Azores. The British Isles sit under the high, experiencing settled conditions. This can be the best extended sailing weather of the year.
The problem with a blocking high, from the passage-planner's perspective, is the transition at its end. When the jet eventually returns to a more zonal pattern and a vigorous depression comes up against the eastern side of the block, two things can happen. The depression either forces the high to retreat rapidly — producing a rapid transition from calm to stormy — or the two systems sit side by side with a very tight pressure gradient between them, producing extreme winds in the zone between high and low. Rowell illustrates this mechanism in Weather at Sea: a low pushing up against a high creates strong southerly winds in the narrow zone between the two systems. As the low moves east and the isobars compress into that corridor, the gradient winds can become severe even though neither system alone would be particularly threatening.
The end of a blocking high is therefore a time to be cautious rather than opportunistic. The very settled conditions that make a long passage attractive are also the conditions that precede the sharp transition when the block breaks down.
The wind between systems: reading the pressure gradient
One of the most practically useful observations in Rowell's Weather at Sea is the point that strong winds do not come only from severe systems — they come from steep pressure gradients, which can develop between any combination of high and low pressure wherever the isobars are closely spaced.
On any synoptic chart, the areas of strongest wind are always the areas of tightest isobar spacing — regardless of whether those isobars belong to a deep low, a strong high, or the corridor between the two. A modest low at 990 hPa sitting adjacent to a strong high at 1030 hPa produces a forty-hectopascal gradient across the gap between them. The isobars in that gap will be tightly packed and the gradient wind in that zone will be strong. If the gap happens to be the English Channel or the Irish Sea, those winds will be funnelled and potentially accelerated further by the channelling effects described in What Hills Do to Wind.
Equally, the areas between systems where the isobars are widely spaced — what Rowell calls the flat spots — can produce near-calm conditions even when severe weather is occurring nearby. The ridge between two highs. The sector ahead of a slow-moving depression where the isobars have barely closed. A flat spot on the chart is a passage window if you can exploit it before the systems either side of it converge.
The jet stream in the sky: cirrus as a medium-range indicator
Gooley's account in The Secret World of Weather of the jet stream's visible signature is directly useful here. The jet creates its own brand of cirrus — very long, roughly parallel streaks of white cloud covering large swathes of the sky, which Gooley calls jet stream ropes. Where ordinary cirrus commas cover no more than a fist-width of sky, jet stream ropes typically stretch across more than half the visible sky in roughly parallel lines. Their alignment tends toward west-east but can vary considerably.
The practical significance is that jet stream ropes indicate the jet is close to overhead, and lows running just north of the jet will therefore arrive within twenty-four hours. This is not a precise instrument — you cannot determine the jet's exact position from cirrus observation alone — but as a general twenty-four-hour warning sign it is reliable and requires no chart at all. Combined with a falling barometer and a backing wind, jet stream cirrus confirms that conditions are about to change significantly.
The speed of the cirrus provides an additional clue. Gooley notes that all cirrus appears to move slowly due to its altitude, but jet stream cirrus moves noticeably faster relative to any fixed foreground reference — a tree, a mast, the edge of a building. Fast-moving parallel cirrus is a stronger signal than slow-moving or stationary cirrus.
Building the medium-range picture: a practical method
For any passage requiring a weather window beyond three days, the useful habit Rowell describes in Weather at Sea is to track the jet stream's position on successive upper-level charts alongside the surface analysis. The 5,640-metre contour of the 500 hPa height chart is the proxy: lows track along its poleward side. If the contour is sitting north of the passage area, depressions are tracking north of you. If it passes directly over the passage area, the full frontal sequence will follow. If it is well to the south, you are in a blocking high situation with the caveats that brings.
The medium-range question for any offshore passage is not "what will the weather be?" but "where will the jet be, and what does that mean for the storm track relative to my route?" A chart-reading sailor who checks the 500 hPa height forecast alongside the surface pressure forecast is working with significantly more information than one who reads only the surface isobar picture. The surface forecast tells you what will happen. The upper-level chart tells you why, and whether the model's confidence in its prediction is justified by a stable or volatile jet stream configuration.
The next posts apply this understanding to specific passage decisions: Clouds at Sea and Fog at Sea address the offshore observational picture; When to Go and When to Wait brings the whole framework together as a decision structure.
Simon Rowell's Weather at Sea (Fernhurst Books) is the primary source for this post, covering jet stream structure and its role in steering depressions, high pressure system mechanics, the seasonal character of highs, blocking patterns, the interaction of highs and lows in producing strong winds, and the 5,640-metre 500 hPa contour as a practical jet stream proxy. Tristan Gooley's The Secret World of Weather (Sceptre) provides the jet stream cirrus signature — the jet stream ropes — as a visual medium-range indicator available without chart access.
Both series hub pages: Weather Forecasting | Coastal and Offshore Passage Planning.
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