Every sailor learns weather eventually, usually the hard way. The forecasts say "seas 4 to 6 feet" and the actual conditions are something else entirely. The morning starts calm, the afternoon builds to 25 knots, and nobody warned you. A tropical system that was supposed to curve north doesn't. Marine weather isn't harder to understand than land weather — it's different in specific, learnable ways. The ocean surface both generates weather and modifies it in ways that make coastal and offshore forecasting distinct from anything that works on land.
Sea Surface Temperature: The Engine Behind Everything
Water temperature controls more marine weather phenomena than any other single variable. Warm surface water evaporates readily, pumping moisture into the atmosphere and supplying the energy that fuels storm systems. Cold surface water does the opposite — it chills the air above it, suppresses convection, and in the right conditions creates fog.
The California coast is the textbook case for cold-water fog. The California Current flows south along the coast, carrying cold subarctic water. In summer, persistent northwest winds drive a process called upwelling: surface water is pushed offshore by the wind, and cold water from several hundred feet down rises to replace it. Sea surface temperatures along the Northern California coast in July can be 50–55°F while air temperatures twenty miles inland are in the 90s. Warm, moist Pacific air moves onshore, hits the cold water surface, and the bottom of the fog layer drops to sea level. This is the mechanism behind San Francisco's legendary summer fog — not a precipitation event but a persistent marine layer that burns off, sometimes, by afternoon.
Warm water does the opposite, fueling convection and storm systems. The Pacific Decadal Oscillation (PDO) — a slow shift in North Pacific sea surface temperature patterns over periods of decades — determines whether the Pacific is in a warm or cool phase, affecting rainfall patterns across North America, fisheries productivity, and the frequency of certain weather patterns. During warm PDO phases, California tends toward drought; cool phases bring more precipitation. The PDO operates on cycles of 20–30 years, which is long enough to affect the entire career of a commercial fisherman but short enough to matter for anyone paying attention.
The Sea Breeze Cycle: The Most Predictable Marine Forecast
The sea breeze is driven by the differential heating of land and water. Land heats quickly in the morning sun — its specific heat capacity is much lower than water, meaning it absorbs radiation and raises temperature faster. By late morning, the air over the land is warmer than the air over the adjacent water. Warm air rises over the land, cooler marine air rushes in from the sea to replace it. This is the sea breeze.
The timing is consistent enough to set your watch by in stable summer weather. The breeze tends to establish by 10–11 AM and builds through the afternoon, often peaking around 3–4 PM. As the sun drops and the land begins to lose heat, the temperature differential reverses. The sea breeze weakens, dies, and is eventually replaced by a lighter land breeze blowing offshore — the reverse circulation, but weaker because the nighttime temperature differential between land and sea is smaller than the daytime one.
For sailors, the sea breeze is reliable afternoon wind — often reliable enough to plan passages around. The Southern California coast, the San Francisco Bay, the Chesapeake, and Long Island Sound all have well-documented sea breeze cycles that experienced local sailors exploit systematically. A race that starts in the morning doldrums and finishes in the afternoon sea breeze is being sailed by people who understand the local pattern.
Trade Winds: Why They Blow Where They Do
The trade winds are the persistent winds of the tropical ocean, and they are arguably the most reliable meteorological feature on earth. In the North Atlantic and North Pacific, they blow from the northeast. In the South Atlantic and South Pacific, from the southeast. They've been steering sailing ships across the tropics for five centuries.
The mechanism: solar heating is most intense at the equator. Warm air rises there, creating a persistent low-pressure zone. That rising air flows poleward at altitude and subsides in the subtropical high-pressure belts — roughly 30°N and 30°S. The surface air flows back toward the equatorial low, completing the Hadley cell circulation. If the earth weren't rotating, this air would flow straight toward the equator from north and south. The Coriolis effect deflects it: northward-moving air curves east, southward-moving air curves west. Air flowing from the subtropical high (30°N) toward the equatorial low (0°) deflects to the right in the Northern Hemisphere — producing northeast winds. In the Southern Hemisphere, it deflects left — producing southeast winds.
Where the northeast and southeast trades meet is the Intertropical Convergence Zone (ITCZ) — the doldrums of sailing lore. The ITCZ is a band of light, variable winds, heavy convection, squalls, and calms. Its position shifts seasonally, following the sun: further north in Northern Hemisphere summer, further south in winter. Crossing the ITCZ is a test of patience for passage-makers heading between hemispheres. The crossing can take anywhere from a few hours to several days depending on where and when you hit it.
NOAA Forecast Language: What "Seas 4 to 6 Feet" Actually Means
The significant wave height reported in NOAA marine forecasts is not the maximum wave height, nor the average. It is the average height of the highest one-third of waves in a given sea state. This is a specific statistical measure derived from ocean wave spectra — it corresponds approximately to what a trained observer would describe as the "typical" wave height when looking at a mixed sea.
The practical implication: in "seas 4 to 6 feet," individual waves will routinely be higher. A seas-of-8-foot forecast implies that some waves — statistically, the largest 1% — may approach 15 feet. The relationship between significant wave height and the largest individual wave in a long period is well-established: expect the largest wave in a given hour to be roughly 1.8 times the significant height. In "seas 4 to 6," plan for an occasional 10-foot wave. In "seas 8 to 10," the occasional 18-footer is real.
Wave period matters as much as height, and the NOAA forecast often buries it. Steep, short-period waves (8–10 seconds) at 8 feet are far more violent than long-period swells (15–18 seconds) at the same height. A boat pitching through 8-foot steep chop in a 20-knot wind is more uncomfortable and more dangerous than riding 10-foot swells from a passing storm three days away. Read the period. The extended forecast discussion on the marine page usually discusses sea state in more useful terms than the headline numbers.
Hurricane Formation and Tracks
Tropical cyclone formation requires a few specific conditions, and their absence at any point terminates development. The threshold sea surface temperature — 26°C (79°F) to a depth of about 50 meters — provides the fuel: warm water evaporation that sustains deep convection. The Coriolis effect organizes that convection into a rotating system; this is why tropical cyclones do not form within 5° of the equator, where the Coriolis effect is negligible. Low vertical wind shear — the difference in wind speed and direction between lower and upper atmosphere — allows the system to maintain its vertical structure without being torn apart.
Atlantic hurricanes typically form from African easterly waves — disturbances in the atmosphere that propagate westward off the African coast. Those that survive the passage across the Atlantic, maintain warm SSTs beneath them, and find favorable upper-level conditions may develop into tropical storms and eventually hurricanes. The tracks are governed by the large-scale steering flow: initially westward in the trades, then curving north and northeast as the system moves into the mid-latitudes and encounters the westerlies. The classic Atlantic hurricane track curves away from the US coast and into the open Atlantic — when it follows that track. When the steering pattern is anomalous — a ridge blocking the northward curve, a trough drawing the system inland — the track departs from the climatological mean in ways that defeat the forecast.
The cone of uncertainty in NHC track forecasts represents the area within which the storm center is expected to fall roughly 66% of the time. The common misreading: the cone does not represent the storm's size, nor the extent of hazardous conditions. A storm with a 400-mile wind field whose center is forecast to pass well east of your location may still produce conditions well inside the cone boundaries.
ENSO: El Niño, La Niña, and the Atlantic Hurricane Season
The El Niño-Southern Oscillation (ENSO) is the most important year-to-year climate signal on earth. The Southern Oscillation is a seesaw of atmospheric pressure between the eastern and western tropical Pacific; El Niño and La Niña are its ocean surface temperature expression.
During an El Niño year, anomalously warm water pools in the central and eastern tropical Pacific. This warms the atmosphere above it, alters the position of the jet stream, and — through teleconnections that span ocean basins — increases vertical wind shear over the tropical Atlantic. Higher shear suppresses Atlantic hurricane development. El Niño years tend to be below-average Atlantic hurricane seasons.
La Niña reverses the pattern. Cooler than normal water in the eastern Pacific reduces shear over the Atlantic. La Niña years are frequently active Atlantic hurricane seasons. The 2020 season — 30 named storms — occurred during La Niña conditions. For anyone sailing in Atlantic waters June through November, NOAA's ENSO outlook, updated monthly, is worth reading.
Reading a Synoptic Chart: H, L, Isobars, and Fronts
A synoptic chart — what used to come off the single-sideband radio as a weather fax — shows the surface pressure field and its associated fronts. The H markers are centers of high pressure (anticyclones); the L markers are centers of low pressure (cyclones). Isobars connect points of equal pressure, and their spacing encodes wind speed: tightly spaced isobars indicate a steep pressure gradient and strong winds; widely spaced isobars indicate light air.
Frontal symbols: a line with triangles pointing in the direction of movement is a cold front. A line with semicircles is a warm front. A line with alternating triangles and semicircles is an occluded front. The sequence of a mid-latitude low matters: ahead of the warm front, expect southeast winds veering to southwest as the warm sector passes, then a wind shift to northwest with the cold front. The tightest pressure gradient — highest winds — typically lies near the center of the low and along the cold front.
Offshore, the weather fax tradition has largely been supplanted by GRIB file downloads via satellite messenger or SSB modem. The GRIB data is richer — wind vectors, pressure gradients, wave heights, all on a single display — but reading it well requires the same underlying understanding of what the pressure patterns and fronts mean. The map is different; the meteorology is the same.