Ocean & Coastal Weather Quiz

Wind flowing around a headland is forced to compress into a smaller cross-section around the obstruction. Conservation of mass requires it to speed up to maintain flow continuity — the same mechanism that accelerates water around a rock in a river. The effect can double wind speed close to the headland and typically persists for 2–3 miles beyond the point before flow normalizes.

Shadow zones in the lee of terrain provide genuine wind shelter, but that shelter ends abruptly at the edge of the terrain's influence. When you sail out of a shadow zone, you can transition from near-calm to 20+ knots in seconds as you exit the terrain's blockage. Anticipate the transition before rounding: trim sails conservatively, warn the crew, and be ready for sudden load.

Upwelling on the West Coast is driven by Ekman transport: northerly and NW winds create a net offshore movement of surface water (at 90° to the wind, due to Coriolis). This offshore movement of surface water is replaced by cold, deep water upwelling along the coast. The California Current contributes to cold surface temperatures but is not the primary upwelling driver — the dominant NW wind is.

Summer (May–September) is peak fog season on the California coast because NW winds are strongest (Ekman transport maximum), coastal upwelling is most active, and the contrast between cold SST and warm moist Pacific air is greatest. In winter, southerly storms suppress upwelling, SSTs warm, and fog frequency drops significantly. Peak upwelling = peak fog.

The temperature inversion is a layer where temperature increases with altitude — the opposite of the normal atmospheric lapse rate. The inversion is a stable barrier: the cool, dense marine layer air below cannot penetrate the warmer, less-dense air above. The inversion acts as a physical lid that traps the marine layer and its moisture, condensing it into stratus at the inversion base.

Dew point depression is the difference between air temperature and dew point temperature. When this gap is near zero (1–2°F or less), the air is at or near its saturation point — fog forms when dew point depression reaches zero. A 1°F dew point depression means fog is either already present or will form with any further cooling. This is the most direct real-time fog risk indicator.

Headland acceleration is well-documented: wind compresses around terrain obstructions and accelerates significantly at and just beyond the headland tip. A synoptic forecast of 15 knots represents the average over a broad area and does not capture the local enhancement. Wind speed can be 50–100% higher at the headland than in the protected bays on either side. Check local observations (NOAA buoys, VHF weather forecasts with zone-specific information) before committing to a headland rounding.

The Papagayo (Costa Rica/Nicaragua) and Tehuantepec (Mexico) are classic gap winds — continental trade winds funneled through narrow gaps in the Central American cordillera to the Pacific. The terrain channeling accelerates these winds to gale force (30–50+ knots). They are seasonal, predictable, and well-documented in cruising pilot guides. Sailors transiting this coast during gap wind season should check forecasts and time passages for gap-wind-free windows.

July peak fog on the California coast is driven by the seasonal maximum of the NW wind pattern, which creates peak Ekman transport and maximum coastal upwelling. Cold SSTs in July against warm summer air temperatures and high Pacific dew points maximizes the temperature-dew point convergence that generates advection fog. Winter and fall have warmer coastal SSTs and reduced upwelling — fog frequency drops significantly.

Morning stratus clears through a specific mechanism: solar radiation heats the surface, reducing the stability of the lower marine layer and weakening the inversion from below. As the inversion erodes, the stratus lifts and thins. On most summer days, this produces a clear afternoon after a gray morning — the diurnal cycle that California sailors know well. The process can be disrupted by a particularly strong, deep marine layer or by weak solar heating from persistent overcast.

The cold SST (51°F) and the air dew point (59°F) are only 8°F apart. As moist air flows over water just a few degrees cooler than its dew point, it cools rapidly toward condensation. If the SST drops further to near the dew point, fog will form. The dropping SST along this track is the diagnostic sign of entering a cold upwelling zone and its associated fog risk. Check the dew point depression: the closer the gap between SST and dew point, the higher the fog probability.

Wave period determines the time between crests — at 14 seconds, waves arrive every 14 seconds; at 7 seconds, every 7. Short-period waves are steeper relative to their height and arrive more frequently, causing violent, rapid motion that is physically demanding and structurally stressful. Long-period swells are less steep and give the vessel time to respond to each wave in a rhythmic motion. Long-period waves also carry more energy per wave — but this energy is expressed over longer time intervals.

The Northeast Trades are the equatorward flow on the eastern side of the subtropical high pressure cells (Azores/Bermuda High in the Atlantic; North Pacific High in the Pacific). Air flows from the high pressure toward the equatorial low pressure trough (ITCZ), deflected to the right by Coriolis to become northeasterly. The subtropical highs are large, persistent, semi-permanent features — their stability is why the trades are consistent.

Warm SST heats the lower atmosphere over the Gulf Stream, increasing its instability. This enhanced convective mixing allows faster upper-level winds to mix down to the surface, increasing surface wind speeds. The effect is measurable in satellite scatterometer data — surface winds are consistently enhanced over warm SST like the Gulf Stream compared to adjacent cooler water in the same synoptic wind field.

The Gulf Stream flows northeast. A SW wind blows with the current's direction — favorable both for vessel motion and for sea state. A NW wind is roughly perpendicular — it doesn't create the worst wind-against-current sea state. A NE wind blows directly against the northward-flowing Stream, creating the dangerous steep, confused seas that make the Gulf Stream so hazardous in that combination. SW or NW windows are the times to cross.

The subtropical high (Azores/Bermuda High) is the dominant pressure feature of the mid-Atlantic. Its position determines whether frontal systems track north (favorable for mid-Atlantic passages) or south (threatening), where the trade wind belt begins, and what the passage conditions will be for the first 1,000–1,500 nm of any Atlantic crossing. Routing decisions — when to depart, which course to take — are largely based on exploiting the High and avoiding the low-pressure systems that circle its northern flank.

Model disagreement signals genuine forecast uncertainty — the atmosphere is in a state where small differences in initial conditions produce meaningfully different outcomes in the models. In this situation, prudent offshore seamanship means building margin: earlier departures to widen the window, courses that maintain more sea room from potential hazards, readiness to heave-to or divert. Model agreement gives confidence; model disagreement demands conservatism.

A NE gale in the Gulf Stream is not a moderate inconvenience — it is a potentially catastrophic sea state. Model disagreement on this scenario represents genuine uncertainty. If the ECMWF is correct, sailing into the Stream with a NE gale forecast is high risk. The prudent decision is to wait for the models to converge on a forecast, or to depart in a subsequent window where both models agree. Never gamble a Gulf Stream crossing on one model.

The ARC (Atlantic Rally for Cruisers) departs in late November to catch the well-established NE Trade winds while also clearing the statistical peak of hurricane season (August–October). Late November offers consistent 15–20 knot NE Trades for the downwind run to St. Lucia, with low tropical storm risk. Earlier departures face residual hurricane season risk; later departures can encounter weakening trades or early winter frontal influence in the northern part of the route.

Warm SST increases low-level atmospheric instability — the warmer the surface, the more the lower atmosphere is heated and destabilized. This enhanced mixing transfers momentum from the faster upper-level winds down to the surface, increasing surface wind speeds. This SST-wind coupling is observable in satellite scatterometer data: surface wind speeds are measurably higher over the Gulf Stream than over adjacent cooler continental shelf water in the same pressure gradient.

The mid-latitude westerlies are the background flow in the 30°–60° latitude band, but they are not steady like the trade winds. Frontal cyclones (extratropical lows) track through regularly, particularly in winter, generating alternating fronts, wind shifts, gales, and swell. Mid-latitude offshore passage planning is largely about navigating the intervals between these frontal systems — the 'sailing in the gaps' strategy that is the foundation of offshore Atlantic and Pacific routing.

The Transpac route from California to Hawaii requires navigating around or through the Pacific High to reach the consistent NE Trade winds. The High sits in the way — sailing straight west leads through its light-wind center. The routing challenge is deciding whether to sail north of the High (longer distance but stronger winds) or south (more direct but potentially lighter air). The High's exact position changes seasonally and annually, which is why professional weather routing is used for the Transpac race.

When a current flows against an oncoming wave, it slows each wave, compressing the wavelength. Energy must be conserved, so wave height increases as wavelength decreases — making the wave steeper. Steep waves break more readily and create the confused, dangerous seas characteristic of wind-against-tide conditions in inlets and channels.

At flooding tide, the current flows the same direction as the incoming swell — extending wavelength and reducing wave steepness. At slack, the current adds no energy in either direction. An ebb tide against oncoming swell is the most dangerous combination — it compresses the swell into steep, breaking waves at the bar. Time inlet entries to flood or slack; time exits to after the swell eases or at high water slack.

The Gulf Stream flows northward. A northeast wind blows southward — directly against the current. This creates a classic wind-against-current sea state: waves are compressed, shortened, and steepened to the point of breaking, superimposed on the gale sea state. The result is one of the most dangerous sea conditions in the North Atlantic. The Bermuda Race and offshore East Coast passages have produced numerous casualties in this combination.

The California Current brings cold subpolar water southward along the coast, and coastal upwelling (wind-driven) continuously replaces surface water with cold deep water. Warm, moist Pacific air flowing eastward over this cold coastal water cools to its dew point, creating persistent advection fog. This is classic advection fog — large-scale, persistent, and unrelated to overnight cooling.

Storm surge is directly additive to the astronomical tide. A 6-foot surge arriving at a 4-foot high tide produces 10 feet of water above mean low water — 6 feet above the normal high-water mark. This is why storm surge arriving at high tide is so much more destructive than the same surge at low tide: 6 feet of surge at mean low water produces only 6 feet total; the same surge at high tide reaches the same 10-foot level.

As storm surge propagates up a narrowing estuary, it is concentrated into a progressively smaller cross-section. Conservation of energy requires the surge to rise higher as the channel narrows — the same volume of water compressed into a smaller space. This funneling effect amplifies surge height significantly upstream of the coast, which is why historic surge disasters like the 1953 North Sea flood and Katrina's interior Louisiana damage were worst far from the open coast.

An ebb current flowing seaward against incoming swell is exactly the wind-against-tide mechanism applied at a bar — the current compresses the swell, shortening wavelength and producing steep, breaking waves. Waiting for slack water or a flooding tide (which extends swell wavelength and reduces steepness) is the correct decision. Many US coastal casualties occur at inlet bars on ebb against swell.

The Gulf Stream flows northeast; a northeast wind blows against it. This wind-against-current interaction in the Gulf Stream is one of the most dangerous sea conditions in the North Atlantic, producing steep, short-period, breaking seas on top of an existing gale sea state. The combination of 25-knot winds and Gulf Stream opposition has produced waves of 20–30 feet with confused, breaking crests.

Storm surge is fully additive to the astronomical tide. At high tide, the water is already 5 feet above mean low water. A 12-foot surge on top of that produces 17 feet above mean low water — 12 feet above the normal high-water mark. This is why peak surge at high tide is so catastrophic: 17 feet of water will overtop structures designed for a 5-foot tidal range.

California's coastal upwelling maintains cold sea surface temperatures even in summer. The contrast between this cold water and the warm California interior creates an enhanced sea breeze — a thermal circulation driven by temperature difference. Cold upwelling amplifies the gradient, producing the strong, reliable afternoon NW wind system that is a defining feature of California sailing. The Central Valley trough also plays a role, but the cold upwelling SST is the primary amplifier.

As storm surge propagates into a narrowing estuary, the same volume of water is compressed into a progressively smaller cross-section. Conservation of energy requires the surge height to increase as the channel narrows — this is surge funneling. The Thames Barrier exists specifically to stop North Sea surge from funneling up the narrowing Thames Estuary to flood central London. The same mechanism amplified Katrina's surge damage far inland through Louisiana's coastal marshes and waterways.

Significant wave height (Hs) is the average of the highest one-third of waves — approximately what an experienced observer would call the characteristic wave height. Individual waves in the same sea state will range from less than Hs to 1.5–2 times Hs. Forecasted Hs is not the maximum; individual waves can significantly exceed it.

A fully developed sea represents an equilibrium state where energy going into the waves from the wind equals energy being dissipated by wave breaking. At this point, the sea cannot grow further regardless of additional duration. Achieving a fully developed sea requires sufficient wind speed, fetch, and duration acting together.

In coastal waters, fetch is typically the limiting factor. A bay or harbor mouth may have only 5–10 miles of open water, which caps wave height regardless of wind speed or duration. This is why an offshore passage can encounter much larger seas than any coastal anchorage even at the same wind strength.

Dispersion is the sorting mechanism: waves of longer wavelength (higher period) travel faster than shorter-wavelength waves. As the mixed storm sea propagates outward, the long-period energy races ahead while short-period energy lags behind. What arrives at a distant coast is the organized, sorted remnant of the storm — a smooth ground swell with a dominant period.

Wave energy is proportional to the square of height and the period. The 6-foot swell at 18 seconds carries far more energy per wave than the 4-foot wind sea at 8 seconds. The long-period swell also shoals dramatically as it enters shallow water. In terms of vessel impact and coastal hazard, the ground swell dominates.

Force 7 (Near Gale, 28–33 knots) produces a sea that heaps up with wave heights of 13–19 feet and streaks of foam blown by the wind. These are serious offshore gale conditions. Most recreational sailors should be well-reefed or hove-to. Force 5 (17–21 kt) is the moderate-whitecap condition; Force 12 is the extreme survival-conditions description.

Sea state lags behind wind speed significantly. Waves built by 30 knots of wind contain substantial energy that dissipates slowly. Residual confused seas from the old wind direction persist 12–24 hours after the wind drops. A forecast of decreasing winds does not mean immediate sea state improvement — especially if the wind is shifting direction, creating crossing seas.

Long-period swell (18 seconds) carries enormous energy and shoals dramatically as it enters shallow inlet water, even when local wind conditions are benign. A 10-foot swell at 18 seconds can create dangerous breaking conditions over a shallow bar regardless of local winds. Always check the swell forecast — height, period, and direction — before any inlet or bar transit.

Swell speed in knots approximates 1.5 times the period in seconds. A 20-second swell travels at roughly 30 knots. This explains why distant-storm swell can arrive days before the associated weather — a storm 1,500 nm away generates 20-second swell that covers that distance in roughly 50 hours.

Two swell systems from different directions create crossing seas — a complex, confused sea state where crests from both systems periodically align (constructive interference), producing individual waves larger than either component. When the 8-foot and 7-foot crests align, combined wave heights of 15 feet are possible. Crossing seas are more dangerous than either component alone and cause significant vessel motion and stress.

Sea state lags significantly behind wind speed. Waves built by 35-knot winds contain substantial energy that dissipates over many hours. Residual seas from a northeaster typically persist 12–24 hours after the wind drops. The post-frontal window when winds ease but seas remain high is the classic 'too soon to go' departure mistake. Wait for the wave period to lengthen and heights to decrease.

Beaufort Force 10 (Storm, 48–55 knots) produces very high waves with overhanging crests, sea surface white with foam, and significant wave heights of 29–41 feet. These are extreme survival conditions for small vessels — active storm sailing with drogue deployment, lying ahull, or other survival tactics. Force 7 (Near Gale) is the offshore gale condition with foam streaks; Force 5 is the whitecap-and-spray sailing breeze.