How to Read a Synoptic Chart
Collection: Field Notes — Old Fashioned Seamanship
Series Hubs: Weather Forecasting | Coastal and Offshore Passage Planning
Subject: How to read a synoptic chart — isobars, pressure centres, fronts, troughs, thickness lines, wind barbs, deriving wind direction and strength from the chart, what the chart cannot show you, and how to use analysis and forecast charts together
A synoptic chart is a snapshot of the atmosphere at a given moment, displayed as a map. Like any map it uses conventions that require translation before they yield information. Unlike most maps, the conventions are few and the translation is fast once you know them. The difficulty is not learning to read the chart but acquiring the habit of reading it regularly and critically.
Simon Rowell is precise about what synoptic charts actually are in Weather at Sea: large-scale weather maps covering oceanic or continental areas, showing atmospheric pressure and its associated features as they existed at a given analysis time, or as they are predicted to exist at a future forecast time. There are two types — analysis and forecast — and treating one as the other is one of the most common errors in passage planning. The analysis chart shows what the atmosphere did. The forecast chart shows what the model predicts it will do. Reading both together, noting where the model prediction has diverged from the subsequent analysis, is how you calibrate your confidence in the current forecast.
This post covers the conventions for reading a surface pressure chart, how to derive wind direction and strength from what you see, what the various symbols mean, how to use thickness lines, and what the chart can and cannot tell you.
Isobars: the foundation of everything
Isobars are lines of equal surface pressure, labelled in millibars or hectopascals — the same unit, different names. Rowell's comparison in Weather at Sea is exact: they work precisely like contour lines on a land map. Closely spaced isobars indicate a steep pressure gradient, just as closely spaced contours indicate steep terrain. Widely spaced isobars indicate a shallow gradient. The wind blows along the isobars, and its strength is proportional to how close together those isobars are.
A chart with isobars drawn at four hectopascal intervals — which is standard on Met Office charts — gives a direct visual read of relative wind strength across the chart. A stretch of ocean where the isobars are fifty miles apart represents much lighter conditions than a stretch where the same four-hectopascal intervals are stacked twenty miles apart. No calculation is required. The visual comparison is immediate and reliable.
Standard isobar pressure values run in multiples of four: 976, 980, 984, 988, 992, 996, 1000, 1004, 1008, 1012, 1016, 1020, 1024, and so on. When you see a value labelled on a line, you can immediately read every surrounding isobar by counting up or down in fours. If you find a closed loop of isobars — a complete oval or circle — it is either a high or a low pressure centre.
Pressure centres: H and L
The centre of a high pressure system is labelled H on most charts; some agencies use an X for the centre of rotation regardless of type. The centre of a low is labelled L. In the Northern Hemisphere, as Rowell explains, air circulates anticlockwise around a low and clockwise around a high. In the Southern Hemisphere these directions reverse.
The H and L labels alone tell you a great deal about what is happening across a large area. A high with tightly packed isobars on one side and widely spaced on the other is generating strong winds on the tight side and light conditions on the open side. A low in the western approaches to the English Channel, with isobars running northeast-southwest, is producing southwesterly winds ahead of it and northwesterly winds behind it. Reading the shape of the isobar field around any centre gives you the wind pattern across the chart.
Pressure values are also worth attending to. Most lows affecting British waters sit between 960 and 1000 hPa at their centres. A low at 960 hPa in the Atlantic is likely to be producing severe gale conditions in its immediate vicinity; a low at 995 hPa may be producing only moderate conditions. But absolute pressure values matter less than isobar spacing for predicting what you will experience at any specific location — the gradient between systems determines the wind, not the central pressure alone.
Deriving wind direction from the chart
The wind at sea does not blow exactly parallel to the isobars. Rowell explains the reason in Weather at Sea: surface friction slows the air and reduces the Coriolis effect, causing the wind to angle inward toward the low pressure centre. At sea, where friction is relatively low, this inflow angle is approximately fifteen degrees. Over land, where friction is greater, it is approximately thirty degrees.
The practical method is straightforward. Find the isobar nearest your position on the chart. The wind direction is approximately parallel to that isobar but angled fifteen degrees toward the nearest low pressure centre. In the Northern Hemisphere, with a low to your left as you face along the isobar in the direction the air is moving, the wind blows anticlockwise around the low. Backing your bearing fifteen degrees gives you the surface wind direction at sea.
This method is accurate enough for passage planning. The main sources of error are terrain effects — covered in What Hills Do to Wind and Why the Forecast Is Always Wrong on Terrain-Influenced Water — which the chart cannot show at the scales it operates on, and frontal discontinuities where the wind shifts sharply over a short distance.
Fronts: symbols and meaning
A warm front on a surface chart is drawn as a line with filled semicircles on the side toward which it is advancing. A cold front is drawn with filled triangles on its advancing side. An occluded front, where the cold front has caught up with the warm front, carries both symbols — alternating triangles and semicircles on the same line. Old or weak frontal remnants are often shown with open rather than filled symbols.
The conventions encode physical information. The warm front — semicircles — is advancing with the warm air mass leading. As it approaches, the characteristic cloud sequence described in Clouds Overhead and Field Forecasting appears: cirrus, cirrostratus, altostratus, nimbostratus. The wind ahead of the warm front is southerly or southeasterly in the Northern Hemisphere, backing as the front approaches, then veering sharply as it passes.
The cold front — triangles — advances from behind. As Rowell describes in Weather at Sea, cold air cannot ride over the warm sector air as warm air rides over cold ahead of a warm front. Instead it pushes under the warm air as a steep wedge, forcing it sharply upward and generating towering cumulonimbus, heavy rain or hail, and sudden wind shifts. The cold front arrives violently and briefly. After it passes the wind veers sharply to the northwest in the Northern Hemisphere, the sky clears dramatically, and the air is cold and clean.
The occluded front — triangles and semicircles alternating — represents the later stages of a depression's life cycle. Rowell describes it as the closing zip of the system: the cold air behind the cold front has caught up with the cold air ahead of the warm front, lifting what remains of the warm sector above the surface. The weather under an occlusion is wet and persistent rather than dramatic — low cloud, drizzle, poor visibility — and it typically signals that the system is weakening.
Troughs
Troughs on a surface chart appear as dashed lines or lines without frontal symbols, often shaped like elongated wedges extending outward from a low centre. Rowell describes them as upper-level features made visible on the surface chart: regions of lower pressure that are not quite fronts but where rising air produces cloud and often a line of rain in and ahead of the trough.
For passage planning, a trough on the chart is worth treating with similar respect to a weak front. The wind shift associated with a trough passage is less dramatic than at a cold front but is real, and the rain line ahead of a trough can reduce visibility significantly. An otherwise clean passage plan that happens to run through a trough line shortly after departure is worth reviewing.
Thickness lines: rain, sleet, or snow
Thickness lines appear on some charts as additional contours, typically labelled in decametres. They represent the vertical distance between the 1,000 hPa level and the 500 hPa level — roughly the depth of the lower half of the atmosphere by mass. The thicker this layer, the warmer and more expanded the air. Rowell provides the practical rule in Weather at Sea: a thickness of less than 528 decametres means any precipitation will fall as snow; between 528 and 546 decametres produces sleet or a wintry mix; above 546 decametres the precipitation will be rain.
The 528 decametre line is the snow boundary. In winter, watching this line advance southward across a forecast chart tells you directly whether a frontal system approaching from the northwest will deposit snow on your route or rain. The 546 decametre line separates the rain zone from the sleet zone. For offshore passages in winter these lines are operationally relevant: deck ice from freezing spray is a serious hazard at higher latitudes, and knowing whether the approaching front will bring rain or snow at your position is directly useful.
Wind barbs
On upper-level charts — and on some surface analysis charts from certain agencies — wind is shown by barbs rather than arrows. Rowell explains the convention in Weather at Sea: each barb is positioned at a specific location with its point indicating the location itself, and the line extending in the direction the wind is coming from. Small barbs on the line each represent five knots; large barbs represent ten knots; a filled triangle represents fifty knots.
Reading a barb of one large and one small pennant at a position mid-Atlantic gives you fifteen knots from the direction the barb stem points. A barb with two large pennants and one small gives twenty-five knots. A triangle with one large pennant gives sixty knots. The convention is consistent across all agencies that use it, though some use metres per second rather than knots as the unit — always check the chart legend.
Analysis versus forecast charts: a critical distinction
Rowell is clear on the distinction in Weather at Sea. An analysis chart shows the atmosphere as it was at the time of the most recent observation network input. A forecast chart shows what the model predicts will happen at a future time — six, twelve, twenty-four, forty-eight, or seventy-two hours ahead. The accuracy of the forecast chart degrades with forecast horizon.
Using these two types together is more informative than using either alone. If the current analysis chart matches what the previous twelve-hour forecast predicted for this time, the model is running well and confidence in the current forty-eight-hour forecast is relatively high. If the analysis shows significant differences from what was forecast — a low that has deepened faster than predicted, a front that has stalled rather than progressed — then the current forecast should be treated with corresponding caution.
The most practically important consequence of this is that a passage plan based on a forty-eight-hour forecast should be actively reviewed against the analysis at twenty-four hours and again at departure. The forecast that looked ideal two days ago may describe a pattern that has already diverged significantly from the model's prediction. The analysis chart at departure is the most current truth available.
GRIB data and model resolution: what the chart cannot show
Rowell discusses GRIB data in Weather at Sea in a way that is directly relevant to the chart-reading sailor. GRIB files — the numerical model output in gridded binary format — are the data that underlies both the professional synoptic chart and most weather app forecasts. The resolution of the model determines the smallest feature it can represent. A model with ten-kilometre grid spacing can only resolve weather features roughly forty kilometres across; its one-and-a-half-kilometre regional model can resolve features around six kilometres.
This means that features smaller than these thresholds are invisible to the chart. The wind acceleration around Portland Bill, the sea breeze circulation along a specific beach, the katabatic wind pouring off a Scottish hillside — none of these appear on any synoptic chart because they fall below the resolution limit of even the highest-resolution operational models. Rowell illustrates this with the example of Sugar Loaf in Guanabara Bay, Rio de Janeiro — a six-hundred-metre peak that was the most significant topographic feature at the 2016 Olympic sailing venue and was simply not represented in any model at any resolution available.
The synoptic chart is the correct tool for understanding the large-scale pattern. It is not the correct tool for predicting what will happen on the water at any specific inshore location. That is the gap that Field Forecasting and the terrain posts fill.
The Shipping Forecast areas
The UK Met Office Shipping Forecast divides the waters around the British Isles and across Biscay into named areas: Viking, North Utsire, South Utsire, Forties, Cromarty, Forth, Tyne, Dogger, Fisher, German Bight, Humber, Thames, Dover, Wight, Portland, Plymouth, Biscay, FitzRoy, Sole, Lundy, Fastnet, Irish Sea, Shannon, Rockall, Malin, Hebrides, Bailey, Fair Isle, Faeroes, Southeast Iceland. Each receives a forecast covering wind direction and force, weather, and visibility.
Rowell recommends in Weather at Sea listening to or reading the sea areas surrounding your own, particularly those to the west, since weather systems generally move eastward across British waters. An area forecast for Sole or Biscay that shows Force 7 to 8 with a cold front tells you what is approaching Plymouth or Fastnet in the next twelve to twenty-four hours. The Shipping Forecast is not a synoptic chart, but it is a text summary of the same information and is available without a chart viewer or data connection, which makes it the most practically accessible source of synoptic information for a vessel at sea.
Developing a shorthand for taking down the Shipping Forecast quickly is a skill worth acquiring before a longer passage. Rowell notes that any workable shorthand is adequate — the important thing is capturing wind direction, force, any increases or decreases, weather, and visibility for each relevant area. A notation system that takes the forecast live from the 0048 or 0520 BBC Radio 4 broadcast requires practice, but practice requires only a domestic radio and a pencil.
Secondary depressions
Rowell flags secondary depressions in Weather at Sea as one of the most dangerous development types in the North Atlantic, and names the 1979 Fastnet Race storm as the obvious example. A secondary depression forms on the trailing cold front of a mature primary system, typically in the area where the cold front extends southwestward from the main low. Under certain upper-level conditions, the wave that appears in this cold front can develop with great speed into a vigorous system in its own right — sometimes deepening by twenty hectopascals or more in twenty-four hours.
On the synoptic chart, a secondary depression appears as a small distortion or wave in a cold front, often initially labelled with a higher central pressure than the primary low. The first analysis showing a secondary is not necessarily alarming. It is the subsequent analysis twelve hours later that tells the story: if the central pressure has fallen sharply and closed isobars have formed around it, the secondary is developing. Passage plans that rely on a window behind a primary cold front should treat any secondary development on the trailing front as a potential hazard to that window.
Heat lows
Rowell describes the Iberian heat low in Weather at Sea as a specific and practically important feature for any passage planning along the Portuguese coast or across Biscay in summer. A heat low forms when intense solar heating over a large land mass creates a thermal low pressure centre that is not associated with a frontal system and has no fronts. Over the Iberian peninsula in summer this produces a persistent area of low pressure that in turn creates a strong pressure gradient against the adjacent North Atlantic high.
The effect, as Rowell describes it, is a localised but intense northerly wind along the Portuguese coast — frequently twenty knots or more — regardless of what the synoptic pattern suggests elsewhere. This is the Portuguese Trade, a reliable summer feature that can make the southbound passage from Finisterre to Lisbon significantly more challenging than the synoptic chart alone would imply. Any summer Biscay passage planning must account for it.
Simon Rowell's Weather at Sea (Fernhurst Books) is the primary source throughout this post, covering surface pressure chart conventions, isobar reading, wind direction derivation, frontal symbols, troughs, thickness lines, wind barbs, GRIB data and model resolution, Shipping Forecast area structure, secondary depressions, and heat lows.
The cloud sequences associated with the frontal systems this chart depicts are in Clouds Overhead and Field Forecasting. The anatomy of the Atlantic depression that produces these fronts in detail is in the next post: The Anatomy of an Atlantic Depression. Both Weather Forecasting and Passage Planning series hub pages: Weather Forecasting | Coastal and Offshore Passage Planning.
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