Field Forecasting - Be Your Own Meteorologist

Collection: Field Notes — Old Fashioned Seamanship

Subject: Field forecasting — assembling a short-range forecast from barometric trend, wind direction and its changes, cloud sequence, and sea state without a numerical model

A 12 to 24-hour field forecast is a realistic and achievable goal for any sailor who has been paying consistent attention to the available inputs. Beyond that horizon, confidence falls off sharply without access to professional model output. Within it, the barometer, the wind direction, and the cloud sequence together give more useful local information than the forecast alone can provide.

Jim Woodmencey opens his field forecasting chapter in Reading Weather with this exactly calibrated statement of what the skill can achieve — and the ceiling matters as much as the capability. A field forecast is not a replacement for the synoptic forecast. It is a continuously updated layer of local observational information that cross-checks and supplements what the professional forecast is saying, and which often captures what the model misses in the gap between regional scale and local reality.

Everything in this series to this point has been assembling the individual instruments for this work: the barometer in The Kit, the cloud reading in Clouds Overhead, the sea state in What Moving Water Tells You and The Beaufort Scale, the local wind effects in What Hills Do to Wind and The Sea Breeze and the Land Breeze. This post puts them together into a working method.

What you are actually trying to do

A field forecast answers one of two questions, as Woodmencey puts it plainly in Reading Weather: if conditions are currently good, when will they worsen and what are the signs? If conditions are currently bad, when will they improve and what are the signs? Everything else is input to answering one of these.

The practical output for a coastal or offshore sailor is usually a go/no-go decision for a passage, or a timing decision for departure. The passage planning posts that follow in this collection apply this to specific waters. Here the focus is on assembling the forecast from the available inputs, regardless of destination.

The barometer: primary instrument

The barometer reading in isolation is nearly worthless. The barometer trend — the rate and direction of change over three to six hours — is primary. Woodmencey's rate table in Reading Weather provides useful thresholds: a fall of roughly one to two hectopascals in three hours signals some change developing, possibly slowly. A fall of two and a half hectopascals or more in three hours signals significant change, possibly developing rapidly. Over a twelve to twenty-four hour period, a fall of six to twelve hectopascals indicates change developing slowly; twelve or more hectopascals indicates rapid development.

These thresholds are worth knowing in order of priority, not precision. The most useful practical rule from Rowell's Weather at Sea — that a fall of approximately one hectopascal per hour sustained over several hours signals the approach of a frontal system — is a reliable working rule for British waters. A fall of two hectopascals per hour or more means something significant is developing and the passage planning window is compressing.

Equally important is knowing what a rising barometer does not mean. A rapid post-frontal rise — the pressure climbing quickly after a cold front passage — is not simply settled weather arriving. Rowell notes in Weather at Sea that a rapid rise followed by a renewed fall is one of the more dangerous North Atlantic sequences, indicating a fast-deepening secondary low following the primary. Logging pressure every three hours through a passage and being able to read the shape of the trend — not just the direction — gives this kind of warning.

Woodmencey adds a useful observation in Reading Weather about daily pressure variation: there is a natural semi-diurnal oscillation of roughly one hectopascal, with highs around 1000 and 2200 and lows around 0400 and 1600 local time. This natural fluctuation is small at British latitudes and can be mentally filtered, but it is worth knowing exists to avoid misreading a very small fall in late afternoon as something more significant.

Wind direction: the most underused forecasting tool

Tristan Gooley argues in The Secret World of Weather that sensitivity to wind direction is the most underappreciated forecasting tool available — and that for most people it operates only when the wind changes dramatically enough to be impossible to miss, which is too late for advance warning.

The core principle: a change in the main wind direction means a different air mass is approaching. Since the weather is the air mass, a wind direction change is a weather change — the two are not separate events but the same event viewed from different angles. As Gooley puts it, the wind is like mail with a recognisable postmark: the direction it comes from tells you what it is carrying with it.

For British and northern European waters, the general rules are well established. Southwesterly and westerly winds bring Tropical Maritime air: mild, moist, overcast, moderate visibility in rain and drizzle. Northwesterly winds bring Polar Maritime air: cold, showery, excellent visibility between showers, active cumulus. Northerly winds in winter bring cold, dry Continental or Arctic air. Easterly winds bring Continental air: cold in winter, warm and humid in summer, often with sea fog on east-facing coasts as the warm dry air passes over cold North Sea water.

The direction of a wind shift matters as much as the resulting direction. Woodmencey establishes a simple and reliable rule in Reading Weather: backing winds — shifting anticlockwise, for example from northwest to southwest — indicate approaching low pressure and worsening weather. Veering winds — shifting clockwise, for example from southwest to northwest — indicate high pressure approaching and improving weather. The memory aid he offers is direct: veering means clearing.

Gooley deepens this in The Secret World of Weather by connecting it to the warm front and cold front sequences covered in the previous posts. As a warm front approaches, the wind backs — turns anticlockwise toward a more southerly direction — and strengthens. As the warm front passes, it veers briefly. In the warm sector the wind steadies. As the cold front approaches, the wind strengthens and veers sharply before the front, then backs again at the front itself. After the cold front passes, the wind veers to northwest and the pressure rises.

The cross-winds rule Gooley describes is a practical application of this directional analysis. Standing with your back to the main wind — the wind moving the lowest clouds — you observe the direction of the highest cirrus. If the cirrus is moving from your left to your right, a frontal system is approaching. If the high and low cloud layers are moving in the same direction, no significant change is imminent. If the upper cloud moves right to left, improvement is more likely. This single observation, made in ten seconds from any open deck, integrates the synoptic-scale pressure pattern and the local wind simultaneously.

The cloud sequence: reading the progression

The cloud sequence as a forecasting tool operates on two timescales. The long-range sign — cirrus appearing on a clear day — may give twelve to thirty-six hours of warning. The short-range confirmation — a rough, dark cloud base with falling pressure and backing wind — gives hours or less.

The warm front sequence, as Gooley describes in The Secret World of Weather: cirrus first, thin wispy cloud at high altitude, often showing a consistent west-to-east trend that reveals the front's approach. Then cirrostratus — a thin overcast through which the sun or moon shows a halo — indicating the moisture is increasing and the cloud layer thickening. Then altostratus — the sun now producing no shadows, visibility beginning to deteriorate. Then nimbostratus and rain. Total time from first cirrus to rain: twelve to thirty-six hours. Each stage confirms and narrows the timing estimate of the previous.

Woodmencey identifies specific trigger observations in Reading Weather that compress this progression into a practical decision rule. A halo around the sun or moon, followed by clouds that continue to lower and thicken over the next six to twelve hours with the barometer falling slowly: expect precipitation within twelve to twenty-four hours. Warming temperatures combined with a falling barometer and southerly winds: a frontal system approaching. Rapidly building cumulus on a summer afternoon: afternoon convective development likely.

The seven cloud patterns covered in Clouds Overhead feed directly into the forecasting sequence: clouds getting lower, multiple cloud types simultaneously visible, small clouds growing — each is a confirmation that the atmospheric state is moving in the direction the barometer trend already suggested.

Signs of improving weather

Woodmencey's list of improving signs is equally important, and often less attended to than the deterioration list. Heavier precipitation late in a storm is frequently an early sign that the worst is passing. Cooling temperatures and a rapidly rising barometer, with the wind shifting to a more northerly direction: a cold front has passed. Cloud breaks revealing no higher cloud above them: the storm is nearly over. Cloud bases rising with precipitation decreasing: conditions are improving. A barometer that has been rising steadily for twelve hours or more is as close to a guarantee of high pressure arriving as the weather system provides.

For passage timing purposes, identifying these signs as a storm is passing is as valuable as identifying the approach signs. A cold front passage — the sharp wind veer, the violent brief squall, then the clearing — is a reliable threshold after which conditions improve rapidly. Timing a departure to follow the cold front rather than precede it by a few hours, or to wait for the cold sector air rather than crossing in the warm sector, is the most consistently useful application of this pattern recognition.

Signs of continued good weather

Woodmencey identifies several indicators of continued settled conditions in Reading Weather. A barometer that is high and steady, showing no fall trend over six or more hours. Scattered fair-weather cirrus coming from a northwesterly direction — indicating the front of a high-pressure ridge rather than the leading edge of a warm front. Dew or frost in the morning without fog: clear skies, cold radiation night, no moisture being advected in. No afternoon cumulus development on a summer day: the atmosphere is very stable and dry. These signs together describe the classic settled anticyclonic situation.

The important caveat Gooley makes in The Secret World of Weather is that the persistence forecast — "tomorrow's weather will be like today's" — is correct roughly half the time. That is simultaneously reassuring (it gives a baseline) and sobering (it is no better than a coin flip without additional input). The field forecasting skill adds value precisely because it gives a directional probability from actual observations, rather than a persistence assumption.

Assembling the forecast: a practical method

Woodmencey proposes a forecast pyramid in Reading Weather — a hierarchical structure for organising the inputs from the most certain at the base to the specific prediction at the top. For a sailor, this reduces to the following sequence.

Step one: establish the synoptic background. What does the professional forecast say is happening at the large scale? What pressure systems are in the vicinity and where are they moving? This is the context within which local observations make sense. A falling barometer in an established high-pressure system means something different from a falling barometer ahead of a forecast depression.

Step two: check the barometer trend. What has it done in the last three hours? The last six? Is the trend consistent with the synoptic forecast, ahead of it, or behind it? If conditions are changing faster than the forecast predicted, something is developing more rapidly and the margin for the passage may be shorter than the forecast suggested.

Step three: note the wind direction and any change. Has it backed or veered since the last observation? Is this consistent with the frontal sequence the synoptic chart predicts? A backing wind when the chart shows a warm front approaching is confirmation. A backing wind when the chart shows settled high pressure is a warning that the chart may be running behind the actual development.

Step four: read the cloud sequence. What cloud types are visible at what levels? Is the trend toward lowering and thickening, or toward breaking and rising? Is there cirrus at high level that was not present this morning? Are the bases rough or smooth? Is there mixing of multiple cloud families?

Step five: apply the local modifiers. What will the terrain do to the predicted wind? Is the forecast wind direction one that produces acceleration at the headland on this passage? Is the temperature and season consistent with sea breeze development? Is the air mass type one that produces the haar on this coast? These modifiers — covered in Why the Forecast Is Always Wrong on Terrain-Influenced Water and What Hills Do to Wind — are the gap between the professional forecast and actual conditions.

Step six: make a specific prediction. Not a probability — a statement. Conditions will remain settled for the next eight hours; the cold front will arrive around 1800; the sea breeze will establish around 1300 and veer by 1500. This specific statement is the field forecast. It may be wrong, but making it specifically forces you to commit the observations to a conclusion, which means the next set of observations will either confirm or correct it — and a corrected field forecast is more useful than a vague one that was never specific enough to test.

Woodmencey's final observation in Reading Weather is worth carrying as the governing principle: if the forecast you heard says fine weather and you are getting rain, trust your observations and make your own forecast. The forecast describes a region. Your observations describe where you are. When they conflict, you are the primary instrument.

Jim Woodmencey's Reading Weather (FalconGuides) covers the full field forecasting methodology, including the forecast pyramid, the barometer change rate table, wind backing and veering rules, the signs of worsening, improving, and continued good weather, and the limitations of persistence forecasting. Tristan Gooley's The Secret World of Weather (Sceptre) covers wind direction as a forecasting tool, the cross-winds rule, frontal progression from the observer's ground-level perspective, and the connection between synoptic-scale pressure systems and the local wind signs a sailor can read directly. Simon Rowell's Weather at Sea (Fernhurst Books) provides the professional framework within which field observations sit, including the barometer fall rates associated with approaching depressions and the post-frontal pressure signature.

The Scottish west coast and Orkney/Shetland weather specifics are woven into the Passage Planning posts. The synoptic chart context for these local observations is in How to Read a Synoptic Chart and The Anatomy of an Atlantic Depression. The full series index is at Weather Forecasting.