Marine weather describes the atmospheric and sea surface conditions that impact operations and travel over open water and along coastlines. This specialized branch of meteorology focuses on the environment where air and ocean interact. Understanding forecast elements, from wind speed to wave height, is fundamental for safety, navigation, and the success of any maritime activity. It provides the context for predicting hazards that are distinctly different from those experienced on land.
Why Marine Weather Is Unique
The primary difference between marine and terrestrial weather is the lack of friction over a large body of water. On land, mountains, buildings, and vegetation slow down wind speeds, but over the open ocean, the wind meets minimal resistance. This lack of friction means that wind speeds are higher at the surface over water than they are over land for the same pressure gradient, which directly influences wave generation.
The immense heat capacity of water also plays a major role, as the ocean temperature changes much more slowly than land temperature. This difference influences atmospheric stability and is a main driver for phenomena like sea fog. Sea fog, known as advection fog, forms when warm, moist air moves over a cooler water surface, cooling the air to its saturation point. Because the ocean temperature is stable, this fog can persist for long periods, unlike land fogs that dissipate quickly as the ground heats up.
Another unique factor is “fetch,” the uninterrupted distance and duration over which the wind blows across the water. The longer the fetch, the more energy the wind can transfer to the water, leading to larger waves. This relationship between wind speed, duration, and fetch dictates the intensity of the sea state, a concept irrelevant to land-based forecasting.
Key Elements of the Sea State
The sea state is the general condition of the water’s surface, defined by several measurable components. Wind speed is measured in knots (KT), where one knot is approximately 1.15 miles per hour. Mariners often use the historical Beaufort Scale to visually estimate wind strength based on the appearance of the sea, such as wave size and the amount of white foam or spray.
Waves are separated into two categories: “sea” (or wind waves) and “swell.” Sea waves are generated by local wind conditions, characterized by being short-crested and coming from various angles. Swell consists of waves generated by distant weather systems that have traveled out of their generating area. Swells are more organized, exhibit smoother crests, and have a longer period than local wind waves.
Wave height is primarily reported as the “significant wave height,” the average height of the highest one-third of the waves observed. This statistical measure is used because the sea is highly irregular, providing a reliable estimate of the wave conditions a mariner will encounter. Wave period, the time in seconds between successive wave crests, is also reported and indicates the wave’s power and energy.
Visibility over water is another component, as it can be rapidly reduced by precipitation or sea fog. Dense fog advisories are issued when visibility is expected to drop below a certain threshold, typically one nautical mile. Reduced visibility heightens the risk of collision and is a safety concern for all vessels.
Navigating Marine Forecasts and Warnings
Official marine weather information is distributed by government agencies, such as the National Oceanic and Atmospheric Administration (NOAA) in the United States, through services like NOAA Weather Radio and marine forecast websites. These forecasts utilize standardized terminology to communicate potential hazards. The wind speed reported is the sustained wind, the average speed measured over a period of at least one minute.
A common alert for non-severe conditions is the Small Craft Advisory, issued for winds ranging from 20 to 33 knots, or sea conditions hazardous to small vessels. The wind and wave height thresholds for this advisory vary by geographic region. When conditions worsen, a Gale Warning is issued for sustained winds between 34 and 47 knots.
The most severe non-tropical warning is the Storm Warning, which signifies sustained winds of 48 to 63 knots, indicating a threat to life and property at sea. Special Marine Warnings are used for short-duration, localized hazardous events, such as severe thunderstorms or waterspouts, that are not covered by broader warnings. These official products provide mariners with the information to make informed decisions about venturing out onto the water.