Solano weather glossary

A Solano-specific indicator measuring the historical precision of a weather model on your exact geographic point. It is computed by comparing past forecasts (J-1 to J-15) to the ERA5T reanalysis, via the MAE (mean absolute error).

This score answers the question: has this model, on this exact anchorage or col, been accurate in the past? It is computed separately per variable (wind, gusts, direction, rain, temperature, cloud, pressure) and per lead time (J+1 to J+7). Shown in the Accuracy tab of the Consistency panel.

A complement to the stability score (has the model changed its mind?) and the ensemble confidence (do the scenarios agree?): accuracy says whether it was right.

A zone of high pressure: the air sinks slowly, which dissolves cloud. The usual result: calm weather, clear sky, light wind. On an isobar chart it appears as closed lines whose values increase towards the centre, often marked with an H (or A).

A well-established anticyclone ("blocking high") produces very predictable situations, hence a high stability score. A caveat in summer: under a high, thermal breezes and heat thunderstorms can still develop in the afternoon.

The true wind is what a stationary observer feels — the wind of the bulletins and the models, given in absolute direction ("where it comes from", relative to north). The apparent wind is what a moving boat feels: the sum of the true wind and the "speed wind" created by the boat's own motion.

Consequences: close-hauled, the boat's speed adds to the true wind → the apparent is stronger and comes more from ahead. Downwind, the boat runs away from the wind → the apparent drops. Masthead wind vanes measure the apparent; instruments derive the true from it using boat speed. Solano shows the true wind everywhere, and the apparent along computed routes.

The process by which a numerical model folds in real weather observations to build its initial state — the most faithful possible snapshot of the atmosphere at launch time. These observations come from very diverse sources: surface weather stations, radiosondes, satellites, marine buoys, aircraft measurements in flight…

Assimilation reconciles this heterogeneous — sometimes contradictory — data with a first guess of the atmospheric state (from the previous run) by minimising the overall gap. The result is the initial condition from which each run starts. The quality of the assimilation directly conditions the accuracy of short-lead forecasts.

A measure of the energy available for vertical air motion. The higher the CAPE, the faster a parcel of air can rise if it is lifted, with an increased risk of deep convection (thunderstorms, cumulonimbus).

Interpretation reference points: CAPE < 300 J/kg = stable atmosphere, weak convection; 300–1000 J/kg = moderate convection, thunderstorms possible if triggered; > 1000 J/kg = unstable atmosphere, thunderstorms probable and possibly violent; > 3000 J/kg = extreme conditions, tornadoes possible (rare at our latitudes).

CAPE alone isn't enough — a triggering mechanism (front, terrain, convergence) is needed to activate convection, and it is always read with the CIN and the Lifted Index. Solano combines them in a thunderstorm risk index.

The energy (in J/kg) that prevents a parcel of air from rising to the level where convection runs away. It is the "cap" of the atmosphere: as long as it isn't overcome (by daytime heating, terrain, a front…), the storm doesn't trigger — even with an enormous CAPE.

Reference points: CIN < 25 J/kg = open cap, easy triggering; 50–150 = needs a forcing; > 300 = strong cap, convection suppressed. A large CAPE under a strong CIN is a "loaded gun": unlikely, but explosive if the cap breaks.

The study of the average weather of a place over several decades: what you should "normally" expect in a given season, independently of today's forecast. It isn't a forecast — it's a frame of comparison ("is this month windier than usual?").

Solano computes its climatology from the ERA5 reanalysis (monthly means, typical day, year-to-year variability) and also shows the seasonal trends of the SEAS5 model. Useful for choosing a cruising or trekking window, not for planning a specific outing.

A forward-looking indicator: for this precise day, do the possible scenarios agree? Solano compares a model's main forecast to the members of its reference ensemble forecast (for example ECMWF IFS against the 51 members of ECMWF-ENS) on wind, rain and pressure.

A high percentage = the scenarios converge → reliable forecast. A low percentage = they diverge → uncertain day, to watch. It is Solano's third angle, complementing the stability score (the model's recent past) and the accuracy score (its measured errors): the movie, the past, and the spread of possible futures.

A model whose grid is fine enough (≈ 1 to 3 km) to compute the updrafts of a storm itself, rather than estimating their average effect with a formula. Coarse-grid models (13 to 25 km: GFS, ECMWF, ARPEGE) don't "see" a cell a few kilometres across: they parameterise it. A convection-permitting model, on the other hand, draws it — it places cells, with a time and a place.

In Solano: AROME HD, ICON-2I, ICON-D2 and ICON-CH1/CH2. Their trade-off is a short horizon (34 h to 5 days) and a regional footprint — and a caveat in reading: the exact placement of a cell in a deterministic run at 2 km moves by tens of kilometres from one run to the next. They answer "where and when it may fall", not "is the atmosphere charged" — that is the role of the thunderstorm risk.

The storm cloud: a cumulus tower that has "punched" vertically up to the upper troposphere (often 10–12 km, sometimes more). Its top spreads into an anvil when it meets the tropopause. It is what produces lightning, heavy showers, hail and downbursts — brief, violent wind blasts under and ahead of the cloud.

A cumulonimbus forms and dies within 1 to 3 hours (isolated cell); under strong wind shear it can organise into a lasting system. At sea, the squall line that comes with it looks like a dark wall approaching: anticipate, shorten sail.

The second reading of thunderstorms in Solano, alongside the thunderstorm risk. Where the risk describes an environment at the basin scale (is it charged? is the cap holding? would it organise?), triggering answers a different question: does a convection-permitting model place a cell on this point, and at what time?

It reads from the total precipitation of the fine model, reinforced by a simultaneous gust. Counter-intuitive but measured: when a model resolves convection, the shower is in the total rain and not in the "showers" field, where it is now just a residue. Three levels: shower, formed cell, vigorous cell.

⚠️ It never enters the /10 score of the thunderstorm risk, and for a physical reason: a model that triggers a cell consumes the CAPE. A low CAPE at 5 pm may mean "storm in progress", not "calm". The two readings complement each other, they don't average.

A forecast that gives a single value per variable, hour and place: "tomorrow 2 pm, 18 knots from the south-west". This is the type of output Solano shows for each model (ECMWF IFS, ARPEGE, GFS, ICON…). It is the most readable, but it says nothing about uncertainty.

Its opposite is the ensemble forecast, which runs the model dozens of times with slight variations to produce a spread of scenarios. Solano keeps the deterministic reading in the foreground, and adds uncertainty through stability, accuracy and ensemble confidence.

A 0-to-9 scale describing the state of the sea by wave height, independently of the wind that raised it. It is used in marine bulletins and to log the observed sea in a logbook.

• 0 — calm (glassy) · 1 — rippled (0–0.1 m) · 2 — smooth (0.1–0.5 m)
• 3 — slight (0.5–1.25 m) · 4 — moderate (1.25–2.5 m)
• 5 — rough (2.5–4 m) · 6 — very rough (4–6 m)
• 7 — high (6–9 m) · 8 — very high (9–14 m) · 9 — phenomenal (> 14 m)

The heights correspond to the significant wave height (Hs). Solano shows waves in metres and recalls the Douglas step in some summaries.

Germany's national meteorological service. It produces the ICON family of models used in Solano: global ICON, ICON-EU (Europe, 7 km) and ICON-D2 (Germany and surroundings, 2 km, convection-permitting).

DWD releases its data openly, which lets services like Open-Meteo — and therefore Solano — redistribute them.

The European Centre for Medium-Range Weather Forecasts, based in Reading (United Kingdom). An intergovernmental organisation founded in 1975 and supported by 35 member states. Recognised as one of the world references in numerical weather prediction.

ECMWF operates the IFS model (Integrated Forecasting System) — available in Solano under the name ECMWF IFS — and produces the ERA5 reanalysis, used as the reference for the accuracy score. ECMWF also publishes the SEAS5 seasonal forecasts used in Solano's Climatology section.

Instead of running a model once, it is run dozens of times (the "members"), perturbing the initial state and the physics very slightly. Each member gives a slightly different scenario; together, they draw the spread of plausible futures.

When the members look alike, the forecast is robust; when they diverge, the uncertainty is real and measurable. Examples: ECMWF-ENS (51 members), GEFS (NOAA), ICON-EPS (DWD). Solano uses this for ensemble confidence and the spaghetti chart.

ERA5 is the atmospheric reanalysis from ECMWF covering the period from 1940 to today. It is not a direct field measurement: it is a coherent reconstruction of the state of the atmosphere hour by hour, obtained by data assimilation of sparse observations — surface weather stations, radiosondes, satellites, marine buoys — into ECMWF's IFS model.

Why "sparse data"? Weather stations and buoys only cover a fraction of the globe's surface (the oceans in particular). ERA5 fills these empty zones coherently, using the physics of the atmosphere to interpolate between observation points. The result is a best estimate of what happened — not an absolute truth.

ERA5T is the "real-time" version of this reanalysis, available with roughly a 1-to-2-day delay. Solano uses it as the reference for computing the MAE (accuracy score) and for feeding the 📡 Verif. tab of the Consistency panel.

The altitude at which the air temperature is 0 °C. It is a key thermal boundary: below it, precipitation falls as rain; above it, as snow. In winter or in disturbed weather, the freezing level can drop down to the coastal zone.

Useful for mountain activities (hiking, skiing), for estimating the risk of ice at altitude, or for understanding why it snowed at 800 m but not at 400 m. In Solano, the freezing level is available via the Snow/Freezing level layer.

Note: ECMWF IFS and ARPEGE Europe don't transmit this variable via Open-Meteo — the layer may be absent for these models.

A narrow transition zone between two air masses of different temperature, usually attached to a low. It is at the passage of a front that weather changes fast: wind shift, pressure fall or rise, arrival or end of precipitation.

• Warm front — warm air glides gently over cold air: cloud thickening hours ahead, steady rain, then milder air and a veering wind.
• Cold front — cold air undercuts the warm air, more abrupt: a line of showers or storms, gusts, sharp cooling, then clearing showers behind.
• Occluded front — the cold front has caught up with the warm front; the low is in its final stage.

A brief (a few seconds) strengthening of the wind above its mean value. The "sustained" wind is a 10-minute average; the gust is the peak. It is the gust that heels the boat, that breaks things, and that matters when choosing sail area or calling off an outing.

The gust factor (gust ÷ mean wind) is typically 1.3 to 1.5 at sea, more over land, under a squall or in unstable air where it can exceed 1.8. Solano shows the mean wind AND the gusts, and the gap between the two is itself information about the nature of the wind.

An estimate of the strongest wave in a given sea state, over an observation window of the order of 20 minutes. The models only provide the significant wave height; Solano derives Hmax from a coefficient (≈ 1.86 × Hs, Rayleigh distribution) or calibrated on nearby buoys where there are any.

Useful to anticipate the waves that can catch you out at the helm or at anchor, well beyond the "average" sea announced. Hmax remains a statistic: over a long duration, even higher waves stay possible.

A line joining points of equal atmospheric pressure on a map. Isobars let you visualise the structure of the pressure field at a glance and identify weather systems:

• Closed, tight isobars with values decreasing towards the centre = a low (low pressure, disturbed weather)
• Closed isobars with values increasing towards the centre = a high (high pressure, stable weather)
• Isobar spacing = the strength of the pressure gradient → strong wind if tight, weak if spread out

In Solano, isobars are available as a map overlay (MSL pressure parameter), drawn every 4 hPa.

A unit of speed used in marine meteorology, aviation and navigation. 1 knot = 1 nautical mile per hour = 1.852 km/h. Solano expresses wind speeds and currents in knots.

The Beaufort scale turns a wind force into a number from 0 to 12, with the corresponding sea state offshore:

The rate at which temperature falls with altitude, in °C/km. Solano uses the mid-tropospheric 700→500 hPa lapse rate: the steeper it is, the more unstable the atmosphere and the more vigorous the convection for the same CAPE.

Reference points: < 5.5 °C/km = soft profile; > 7 °C/km = steep profile (explosive convection). This is one of the reasons why "a CAPE of 2,000 isn't the same" depending on the season and the situation.

The interval between the moment a forecast is issued and the moment forecast. J+0 denotes the run's issue time (an "instantaneous" forecast), J+3 a 3-day forecast, J+7 a 7-day one, etc.

Uncertainty grows with lead time: models are generally reliable up to J+3 to J+5, and their skill decreases rapidly beyond that. That is why Solano's accuracy score is measured separately for each lead time — a model can be excellent at J+2 on your spot and mediocre at J+5.

An instability index (dimensionless): a parcel of low-level air is lifted to 500 hPa and its temperature is compared to that of the environment. Negative = unstable (the parcel is warmer, it keeps rising); positive = stable.

Reference points: LI > 0 = stable, no storm; 0 to −2 = marginal; −2 to −4 = unstable, thunderstorms probable; < −6 = very unstable, violent thunderstorms. More robust than CAPE alone for judging instability, because it is less sensitive to the season.

A zone of low pressure: air converges and rises, forming cloud and precipitation. It is the engine of disturbed weather in temperate latitudes — sustained wind, fronts, built-up seas. On an isobar chart, these are closed lines whose values decrease towards the centre, marked with an L (or D).

The tighter the isobars around the centre, the stronger the pressure gradient and the harder the wind blows. In the northern hemisphere, the wind turns anticlockwise around a low. A mobile low often gives a lower stability score: the models hesitate on the track.

A statistical measure of the gap between a model's forecasts and a reference. In Solano, the reference is the ERA5T reanalysis. The formula:

This calculation is carried out separately for each model, each variable (wind, gusts, direction, precipitation, temperature, cloud, pressure) and each lead time (J+1 to J+7). A wind MAE of 2 knots at J-3 means the model was on average 2 knots off over your last 15 days when forecasting three days ahead.

The official forecast written by a national weather service for navigation. The BMR — regional coastal bulletin (up to ≈ 20 miles offshore) gives the general situation, wind, sea, swell, weather and visibility, by named zones, usually twice a day.

The BMS — special marine bulletin is a gale or storm warning: it is only issued when danger thresholds are exceeded (strong wind, very rough sea, nil visibility). It is numbered and stays in force until its explicit cancellation. Solano aggregates the bulletins of Météo-France, AEMET (Spain), MeteoAM (Italy) and the Met Office (United Kingdom), and lets you subscribe by zone.

A named geographic division that serves as the grid for marine bulletins. Offshore, the METAREAs are large numbered ocean zones, coordinated worldwide for broadcasting safety warnings. Closer to the coast, each national service defines its own coastal zones with names familiar to sailors (Sole, Biscay, Provence, Ligurian…).

A bulletin or a warning applies to a whole zone: Solano detects the zone(s) that contain your point and shows only the relevant bulletins, with the option to subscribe by zone.

A European warning system that gathers, in a common format, the warnings issued by each national weather service — down to the region. Where a BMS covers the open sea, MeteoAlarm also carries warnings on land and along the coast: violent wind, thunderstorms, coastal flooding, snow-ice, heat…

Each warning carries a colour level (yellow → orange → red), a sender and a validity period. Solano shows the MeteoAlarm warnings concerning your point precisely, alongside the marine bulletins, and can send an email alert.

France's national weather service. It produces the ARPEGE models (global, with a grid tightened over Europe) and AROME (France, 1.3 km, convection-permitting), the MFWAM wave models, the marine bulletins (BMR / BMS) and the weather vigilance.

Solano uses these models via Open-Meteo for the map and the forecast, and via the Météo-France APIs directly for Routing and the marine bulletins.

A computer program that solves the equations of fluid dynamics (Navier-Stokes equations, thermodynamics, cloud microphysics…) on a three-dimensional grid of the atmosphere. The world is divided into cells of a few kilometres to a few tens of kilometres — each cell exchanges mass, heat and momentum with its neighbours at every time step.

The model is initialised from an "initial state" (see data assimilation) and computes the probable evolution of the atmosphere hour by hour up to several days. It starts from scratch at each run. The horizontal resolution varies: AROME France HD = 1.5 km, ICON-EU = 7 km, ECMWF IFS = ~9 km, GFS = ~13 km.

The models available in Solano: ARPEGE Europe, AROME France HD, ECMWF IFS, GFS NOAA, ICON-EU, ICON seamless, UKMO (UKV + global), GEM GDPS/RDPS/HRDPS (Canada), ICON-2I (Mediterranean).

A US federal agency. Its NCEP centre produces the global GFS model (~13 km, up to 16 days), the GFS-Wave wave model and the GEFS ensemble forecast, all used in Solano.

NOAA data is in the public domain, which explains its near-universal presence in consumer weather apps.

Very short-range forecasting — from now to 2–6 hours — based no longer on numerical models but on extrapolated observation: satellite imagery, radar, lightning detection, storm cells tracked frame by frame.

It is the only reliable regime for "is this storm going to hit me within the hour?". In Solano, the "See the sky now" button of the thunderstorm risk switches to this reading: infrared, lightning and RDT cells.

A service that gathers the weather models of the major national centres (ECMWF, NOAA, Météo-France, DWD, UKMO, ECCC…) and redistributes them under a single interface, free for non-commercial use and open source.

It is Solano's main source for the forecast, the map and the verification history (ERA5 archive). Solano adds its own processing — stability, accuracy, ensemble confidence, thunderstorm risk — and also queries some national APIs directly (Météo-France, EUMETSAT, bulletins).

The position of the sailboat relative to the direction of the true wind. It is named by the angle between the boat's axis and where the wind comes from: head to wind (0°, you can't advance, you must tack) · close-hauled (≈ 45°) · beam reach (≈ 90°, wind on the side) · broad reach (≈ 135°) · running (180°, wind from behind).

The point of sail decides the achievable speed: a sailboat can't sail straight into the wind and is often faster on a reach than dead downwind. This is what the polar diagram describes. In Solano, Route mode and Routing estimate the point of sail hour by hour along a track.

A table (or curve) that gives a sailboat's speed for each combination of wind angle × wind strength. It sums up everything a boat "can do": how high it points, where its best downhill angle lies, from what strength it needs to reduce sail.

Solano's Routing relies on the boat's polar (imported or edited in "My boats") to compute, from the wind fields, the fastest route between two points by the isochrone method. Without a polar entered, it uses that of a generic 11 m cruiser.

The rate at which pressure changes as you move across the map. It is what makes the wind: air flows from high to low pressure, and the sharper the difference over a short distance, the harder it blows.

On an isobar chart the gradient reads by eye: tight isobars = strong wind, spread isobars = light wind. The real wind doesn't blow straight from high to low — the Earth's rotation (Coriolis force) turns it to run along the isobars.

A weather observation made by a weather balloon fitted with a radiosonde — a box of a few hundred grams that captures temperature, pressure, humidity and wind speed as it climbs through the atmosphere, up to about 30 km altitude.

About 900 stations worldwide launch two radiosondes a day, at the synoptic hours of 00Z and 12Z. These vertical measurements are one of the most valuable sources for data assimilation: they provide a full profile of the state of the atmosphere where satellites only see integrated layers.

The balloon generally bursts between 20 and 35 km (expansion from depressurisation), and the radiosonde comes back down under a parachute. Some are recovered and reused.

A satellite product (NWC SAF, EUMETSAT) that automatically detects and tracks thunderstorm cells from their development phase, using geostationary imagery. It spots nascent storms even before the first lightning, and — being satellite-based — also covers the open sea, where ground radars see nothing.

Each cell is outlined by a contour and labelled with its minimum top temperature (°C): the colder it is, the higher the top and the more powerful the cell. Reference points: > −40 °C = weak (cumulus/congestus) · −40 to −55 °C = moderate · −55 to −65 °C = strong · < −65 °C = very intense (overshooting tops).

In Solano, RDT is an overlay layer of the satellite imagery, alongside lightning and infrared. The "See the sky now" button of the thunderstorm risk activates it automatically to move from forecast to observation.

A coherent, homogeneous reconstruction of past atmospheric states, obtained by passing archives of historical observations through a modern numerical model via data assimilation. Unlike a direct measurement, a reanalysis fills the zones without stations and produces a global state at every hour across the whole globe.

Advantages over raw observations: spatial and temporal consistency, global coverage (including oceans and sparsely observed regions), derived variables available (boundary layer, fluxes…). Limits: accuracy depends on the density of observations available at each period — recent decades are better analysed than the 1940–1960 years.

Solano's reference reanalysis is ERA5 (ECMWF). Other reanalyses exist: MERRA-2 (NASA), JRA-55 (JMA), CFSR (NOAA).

One launch of a forecast model at a precise moment, called the "run validity time". Operational models run at regular intervals: most produce 4 runs a day at the synoptic hours of 00Z, 06Z, 12Z and 18Z (UTC). Some high-resolution models run every hour or every 3 hours.

Each run starts from scratch with the most recent available assimilation and produces forecasts up to its time horizon (from 2 to 16 days depending on the model). Two successive runs of the same model can give appreciably different forecasts for the same target day — it is this variability that Solano's stability score measures.

The delay between a run's validity and its public availability (processing delay) varies: AROME France HD is available ~1h30 after its validity time, ECMWF IFS in ~3h, GFS in ~4h.

A forecast at 1 to 6 months that doesn't announce the weather on a given day but a trend: will the coming month be warmer, drier, windier than normal, and with what confidence? It relies on slow signals (ocean temperature, El Niño…) and is always read as a departure from climatology.

SEAS5 is the ECMWF seasonal system. Solano shows it in the Climatology section, by lead time and by variable. Take it as an orientation ("rather a good window"), never as an outing forecast.

The average height of the highest third of the observed waves (hence the name H1/3). It is the value bulletins and wave models announce when they say "sea 1.5 m": it roughly matches what an experienced sailor would estimate by eye.

It is not the biggest wave: some waves exceed Hs, and the strongest of a given sea state is statistically ≈ 1.6 to 2 × Hs — that is Hmax. Solano shows Hs by default and lets you enable Hmax.

A chart that overlays, for one day and one place, the forecasts of all the recent runs of a model (and of several models). Each curve is a run; the tangle of curves forms the "spaghetti".

Reading: bunched curves = successive runs tell the same story, well-held situation; scattered curves = the model changes its mind, strong uncertainty. It is the visual version of the stability score. A Verif. tab adds the curve of the ERA5 reanalysis to confront past runs with what actually happened.

A line of thunderstorms or showers, often along a cold front, advancing like a wall. Its passage brings a sharp wind shift, a downburst (the "squall" proper — a sudden strengthening of 15 to 30 knots for a few minutes), a dense shower, then a lull and cooling.

At sea, a squall line can be seen coming: a dark roll cloud (arcus) at its base, a whitening horizon, a pressure drop. The rule is to shorten sail before it arrives.

A Solano-specific indicator, scored from 0 to 10, measuring the consistency of a single model's successive runs. A high score (8–10) means the model "hasn't changed its mind" between its recent runs for the target date — the sign of a well-established, predictable atmospheric situation. A low score (0–4) means significant divergence between runs, revealing strong uncertainty.

Recent runs count more than older ones: the weighting decreases exponentially (yesterday's run = high weight, a run 4 days old = low weight). The score measures the internal consistency of the forecasts, not their accuracy — a model can be very stable and very wrong if the atmospheric situation is poorly initialised.

Complement: the accuracy score (ERA5T MAE) answers the accuracy question.

The most organised and longest-lived storm: a single cell whose updraft rotates on itself (mesocyclone), sustained by strong wind shear. It can live several hours, move in a deviated path relative to the general flow, and produce large hail, destructive gusts and, more rarely in Europe, tornadoes.

Rare but dangerous. In the Solano reading, an environment with high CAPE and strong shear tips the thunderstorm risk towards "Severe · organised".

The wind sea is raised by the wind blowing here and now: short, disorganised waves, breaking crests, in the wind's axis. The swell is what remains once those waves leave the wind zone and travel: long, regular, well-ordered undulations that can arrive from very far — sometimes from a storm you never saw, under a blue sky and no wind.

The period tells them apart: short (< 6 s) for the wind sea, long (> 8–10 s) for swell. The real sea state is often the sum of both, sometimes from different directions — hence a "cross sea", uncomfortable. Solano shows wave height, period and direction (where they come from).

A Solano-specific categorical index — Low · Moderate · Marked · Severe — computed hour by hour to go beyond reading CAPE alone. It combines the fuel (CAPE sharpened by the Lifted Index and the lapse rate) and a cap factor (the CIN): a lot of energy under a strong cap gives a low risk but a "loaded gun" flag ⚠️. The shear then refines the category — "Severe" is reserved for organised storms.

The index is anchored on the GFS model, the only one to provide every ingredient — so it describes the environment at the basin scale (point brought back to ≈ 25 km) and doesn't localise the cells, which is the role of triggering. It is accompanied by a measure of agreement between models (CAPE spread): when they diverge, the risk is flagged as uncertain. It describes a favourable environment, never a thunderstorm at a precise time.

An acceleration of the wind when it is channelled by terrain: between two islands, through a strait, at the mouth of a valley, off the end of a headland. Air forced through a gap speeds up — the wind there can be 1.5 to 2 times stronger than over the neighbouring open area, over just a few hundred metres.

Global models smooth it out; fine models and local knowledge recover it. In the anchorage analysis, the venturi effect is taken into account to judge the real shelter of a tight bay: an anchorage that looks "protected" on the chart can be swept by an acceleration.

The time between the passage of two successive crests, in seconds. It is what distinguishes a short, hard sea from a long, rolling one, at equal height.

Reference points: < 6 s = wind sea, tight and slamming waves, hits the hull; 8–12 s = well-formed swell; > 12 s = long swell from far away, which lifts the boat without brutalising it but generates surge in anchorages. A modest height with a long period can make a bay uncomfortable.

A meteorological symbol representing the direction and speed of the wind simultaneously. The shaft points in the direction the wind comes from (a symbol pointing up indicates a north wind). The ticks on the shaft (the "barbs") encode the speed:

• Short tick = 5 knots
• Long tick = 10 knots
• Pennant (filled triangle) = 50 knots
• Empty circle = calm (< 2 knots)

A barb with a pennant and a long tick = 60 knots. This convention is universal in meteorology and aviation.

A local wind created by the temperature difference between land and sea (or between a valley and a ridge). By day, land warms faster than water: air rises over it, cooler marine air moves in to fill the gap — the sea breeze, blowing from sea to shore, setting in late morning and freshening in the afternoon. At night the mechanism reverses: land breeze, weaker, from the shore out to sea.

Breezes add to the general wind and can reinforce it, cancel it or deflect it. Fine-grid models (regional AROME, ICON) represent them better than global models. It is also one of the local effects the ERA5 reanalysis can underestimate on a precise coastal point.

The variation of the wind (speed and direction) between the surface and altitude, measured here as the vector difference between the wind at 10 m and the wind at 500 hPa (≈ 5.5 km), in m/s. Shear doesn't create the storm but decides its organisation.

Reference points: < 10 m/s = isolated, short-lived cells (showers); 15–20 = organised systems, supercell possible; > 20 = strongly organised (squall lines, hail, downbursts). It's what distinguishes a thundery shower from a dangerous, lasting system.