Marine Weather Glossary | SeaLegsAI - Weather Terms Explained

A

Air Temperature

The temperature of the air at a specific location, usually measured at about 2 meters above the surface. On the water, air temperature affects comfort and can differ significantly from water temperature, especially in spring and fall.

Measured in: °F or °C

Apparent Wind

The wind felt on a moving boat, which is a combination of true wind and the wind created by the boat's motion. Apparent wind is always forward of true wind when sailing. Critical for sail trim and performance.

Atmospheric Pressure

See Barometric Pressure. The weight of the atmosphere at a given location, a key indicator for predicting weather changes.

B

Barometric Pressure

The weight of the atmosphere pressing down on the Earth's surface. Falling pressure often indicates approaching storms, while rising pressure typically means improving weather. Rapid pressure changes can signal severe weather.

Measured in: millibars (mb) or inches of mercury (inHg)

Beaufort Scale

A scale from 0-12 that relates wind speed to observed conditions at sea. Developed in 1805, it helps mariners understand wind strength: 0 is calm, 4 is moderate breeze (11-16 knots), 8 is gale (34-40 knots), and 12 is hurricane force (64+ knots).

Beam Sea

Waves striking a boat from the side (perpendicular to the direction of travel). Beam seas can cause uncomfortable rolling motion and are generally less comfortable than head seas or following seas.

Buoy

A floating device anchored to the seafloor. Weather buoys collect real-time data on wave height, wind speed, water temperature, and other conditions. NOAA operates a network of buoys that provide crucial marine weather observations.

C

Chop

Short, steep waves caused by local winds blowing against the current or in confined waters. Chop makes for an uncomfortable ride but is different from ocean swells. Common in bays, sounds, and near inlets.

Current

The horizontal movement of water caused by tides, wind, or differences in water density. Currents affect boat speed, fuel consumption, and can create dangerous conditions when opposing wind or waves.

Measured in: knots

Calm

Sea conditions with little to no wind (under 1 knot) and smooth water. Beaufort scale 0. While pleasant for swimming and anchoring, calm conditions can be challenging for sailors who rely on wind for propulsion.

Cloud Cover

The percentage of sky obscured by clouds. Affects visibility, UV exposure, and can indicate approaching weather systems. Increasing cloud cover often precedes rain or storms.

Measured in: percentage (%) or oktas (eighths of sky)

Confused Sea

Chaotic wave conditions caused by waves from multiple directions crossing each other, often where swells meet wind waves or at the confluence of currents. Creates unpredictable, uncomfortable conditions that can be dangerous for smaller boats.

Crosswind

Wind blowing perpendicular to a boat's direction of travel or a channel's orientation. Crosswinds can make docking difficult and affect fuel efficiency. Common challenge when navigating narrow channels or approaching marinas.

D

Dew Point

The temperature at which air becomes saturated and water vapor condenses into dew or fog. When air temperature approaches dew point, fog becomes likely. Important for visibility planning on early morning trips.

Measured in: °F or °C

Doldrums

A belt of calm winds near the equator where trade winds from northern and southern hemispheres meet. Historically dreaded by sailors for lack of wind. Characterized by light variable winds, high humidity, and frequent thunderstorms.

Doppler Radar

Weather radar that can detect precipitation intensity and movement, including rotation in storms. Used to track rain, storms, and estimate rainfall rates. Essential for monitoring approaching thunderstorms and severe weather.

Downburst

A strong downdraft of air from a thunderstorm that spreads out when it hits the surface, creating damaging winds. Can produce winds over 100 mph in severe cases. Arrives with little warning and is a significant hazard for boaters.

E

Ebb Tide

The outgoing tide, when water flows from the shore back to the sea. Ebb currents can be strong near inlets and passes. Opposing wind during ebb tide creates steep, dangerous waves at inlets.

ECMWF

European Centre for Medium-Range Weather Forecasts. Produces one of the most accurate global weather models, often called the "European model." SeaLegsAI uses ECMWF data alongside GFS for improved forecast accuracy.

F

Fetch

The distance over water that wind blows in a constant direction. Longer fetch produces larger waves. This is why large lakes and open ocean develop bigger swells than protected bays with limited fetch.

Front (Weather Front)

The boundary between two air masses of different temperatures. Cold fronts often bring sudden wind shifts, squalls, and thunderstorms. Warm fronts typically bring gradual weather changes with clouds and precipitation.

Flood Tide

The incoming tide, when water flows from the sea toward the shore. Flood currents bring water into bays, estuaries, and up rivers. Timing the flood tide can help when entering shallow areas.

Fog

A cloud at ground/sea level that reduces visibility below 1 kilometer. Marine fog often forms when warm air moves over cold water (advection fog) or during calm, clear nights (radiation fog). Extremely hazardous for navigation.

Following Sea

Waves approaching from behind the boat, traveling in the same direction. Can cause surfing or broaching in steep conditions. Generally more comfortable than head seas but requires attention to prevent being overtaken by waves.

G

Gust

A sudden, brief increase in wind speed. Gusts can be 30-50% stronger than sustained winds and can catch sailors off guard. Important for sail trim and anchoring decisions.

Measured in: knots or mph

GFS (Global Forecast System)

A weather forecast model produced by NOAA. It provides forecasts up to 16 days out and is one of the primary models used for marine weather prediction. SeaLegsAI uses GFS data in its forecasts.

Gale Warning

A warning issued when sustained winds of 34-47 knots (39-54 mph) are expected. Conditions are dangerous for most vessels. Small craft should remain in port; larger vessels should prepare for heavy weather.

Gulf Stream

A powerful warm ocean current flowing from the Gulf of Mexico up the U.S. East Coast. Creates significant weather effects, temperature differences, and strong currents. The western edge often has rougher conditions when wind opposes the current.

H

High Pressure System

An area where atmospheric pressure is higher than surrounding areas. Generally brings fair weather, light winds, and clear skies. Winds circulate clockwise around high pressure systems in the Northern Hemisphere.

Humidity

The amount of water vapor in the air. High humidity makes hot days feel hotter and can indicate unstable weather conditions that may produce thunderstorms.

Measured in: percentage (%)

Head Sea

Waves approaching from directly ahead of the boat. Head seas slow progress, increase fuel consumption, and cause pitching motion. Generally safer than beam seas but can be uncomfortable in steep waves.

Hurricane

A tropical cyclone with sustained winds of 64 knots (74 mph) or greater. Hurricanes bring extreme winds, storm surge, heavy rain, and dangerous seas extending hundreds of miles from the center. Hurricane season in the Atlantic runs June through November.

I

Inlet

A narrow waterway connecting the ocean to a bay, sound, or river. Inlets can have strong currents and dangerous breaking waves, especially when wind opposes tidal current. Always check conditions before crossing.

Inshore

Waters close to shore, typically within a few miles of the coastline in protected or semi-protected areas. Inshore conditions are usually calmer than offshore but can still be affected by local weather patterns.

Isobar

A line on a weather map connecting points of equal atmospheric pressure. Closely spaced isobars indicate strong winds; widely spaced isobars indicate light winds. The pattern of isobars shows high and low pressure systems.

K

Knot

A unit of speed equal to one nautical mile per hour (1.15 mph or 1.85 km/h). The standard unit for measuring wind speed and boat speed in marine contexts. Named after the knots tied in a ship's log line.

1 knot = 1.15 mph = 1.85 km/h

L

Land Breeze

A wind that blows from land toward the sea at night, caused by land cooling faster than water. The opposite of sea breeze. Land breezes are typically lighter than sea breezes and occur in the late night and early morning hours.

Lightning

An electrical discharge between clouds or between clouds and ground. Extremely dangerous on the water where boats are often the tallest objects. When thunder is heard, seek shelter immediately. Lightning can strike 10+ miles from a storm.

Low Pressure System

An area where atmospheric pressure is lower than surrounding areas. Associated with clouds, precipitation, and stronger winds. Winds circulate counter-clockwise around low pressure systems in the Northern Hemisphere.

M

Marine Forecast

A weather forecast specifically for ocean and coastal waters, issued by NOAA's National Weather Service. Includes wind speed, wave height, visibility, and weather conditions for specific zones. Updated multiple times daily.

Marine Layer

A layer of cool, moist air that forms over the ocean and can move inland. Often produces low clouds and fog, especially in coastal areas during spring and summer. Can significantly reduce visibility.

Microburst

A localized column of sinking air that produces damaging straight-line winds at the surface, often 60+ knots. Associated with thunderstorms. Can appear with little warning and capsize boats. Watch for rain shafts and darkening clouds.

N

Nautical Mile

A unit of distance used in marine navigation, equal to one minute of latitude (1.15 statute miles or 1.85 kilometers). Based on the Earth's circumference, making it practical for navigation with charts.

1 nautical mile = 1.15 miles = 1.85 km

NOAA

National Oceanic and Atmospheric Administration. The U.S. government agency responsible for marine weather forecasts, buoy data, and ocean conditions. Primary source of marine weather data in the United States.

Nearshore

The zone between inshore and offshore waters, typically 3-20 miles from the coast. Nearshore areas are exposed to ocean swells but still influenced by coastal effects. Popular for fishing and diving.

O

Offshore

Waters beyond the nearshore zone, typically more than 20 miles from the coast. Offshore conditions are dominated by open ocean swells and require larger, well-equipped vessels. Weather can change rapidly with fewer options for shelter.

Offshore Wind

Wind blowing from land toward the sea. Offshore winds flatten waves near shore and can push boats away from land. Can be deceptively calm at the beach while conditions deteriorate further out.

Onshore Wind

Wind blowing from the sea toward land. Onshore winds build waves toward the shore and can make landing difficult. Often develops in the afternoon as a sea breeze when land heats up.

P

Precipitation

Any form of water falling from clouds, including rain, drizzle, sleet, snow, or hail. On the water, precipitation reduces visibility and can indicate unstable weather patterns.

Measured in: inches or millimeters

Primary Swell

The dominant wave pattern, usually from the most distant or powerful storm. Swells travel thousands of miles across oceans and arrive at coastlines as organized, rolling waves distinct from local wind chop.

Probability of Precipitation (POP)

The likelihood that precipitation will occur at any point in a forecast area. A 40% POP means there's a 40% chance of rain at your location. Useful for planning but doesn't indicate intensity or duration of rain.

Measured in: percentage (%)

R

Rip Current

A strong, narrow channel of water flowing away from shore, typically through the surf zone. Rip currents can pull swimmers and small vessels seaward rapidly. Look for gaps in breaking waves, discolored water, or debris moving offshore.

Speed: up to 8 ft/sec (5+ mph)

Ridge (High Pressure Ridge)

An elongated area of high pressure extending from a larger high pressure system. Ridges typically bring fair weather and can steer storms around an area. A strong ridge often means stable, predictable conditions for boating.

Restricted Visibility

Any condition that limits visibility on the water, including fog, rain, mist, haze, smoke, or spray. Navigation rules require reduced speed and extra caution when visibility is restricted. Sound signals become critical.

S

Sea State

A description of the overall ocean surface conditions including wave height, period, and direction. Ranges from calm (0-1 ft) to phenomenal (46+ ft). Critical for planning offshore trips.

Significant Wave Height

The average height of the highest one-third of waves. This is the standard measurement used in marine forecasts because it closely matches what an observer would estimate. Note: 10% of waves will be higher than this number.

Measured in: feet or meters

Small Craft Advisory

A warning issued when winds are 21-33 knots and/or seas are 7+ feet. Conditions can be hazardous to small boats. Not legally binding but strongly recommended to heed, especially for vessels under 26 feet.

Squall

A sudden, violent wind often accompanied by rain or thunderstorms. Squalls can increase wind speed by 20-30 knots within minutes. Watch for dark, rapidly approaching clouds as a warning sign.

Swell

Long-period waves generated by distant weather systems. Unlike chop, swells are smooth and rolling with consistent spacing. Swell direction and period affect how waves break at inlets and beaches.

Sea Breeze

A wind that blows from the sea toward land during daytime, caused by differential heating. Sea breezes typically develop late morning and strengthen through afternoon, often bringing cooler temperatures and building waves near shore.

Storm Surge

An abnormal rise in sea level caused by strong winds and low pressure during a storm. Storm surge can add 10-20+ feet to normal tides during hurricanes, causing catastrophic coastal flooding. The most dangerous aspect of tropical systems.

Storm Warning

A warning issued when sustained winds of 48-63 knots (55-73 mph) are expected. Dangerous conditions for all vessels. All small craft should be in port; larger vessels should seek shelter or avoid the area.

Slack Water

The brief period when tidal current stops before reversing direction. Occurs between flood and ebb tides. Optimal time for crossing difficult inlets or diving because current is minimal.

T

Tidal Current

Horizontal water movement caused by tides. Strongest near inlets, passes, and narrow channels. Can reach several knots and significantly affect navigation, fuel consumption, and wave conditions.

Tide

The regular rise and fall of sea level caused by gravitational forces of the moon and sun. Most areas experience two high and two low tides daily. Essential for navigation in shallow waters and timing inlet crossings.

Thunderstorm

A storm with lightning, thunder, and often heavy rain, strong winds, and sometimes hail. Marine thunderstorms can produce sudden wind gusts over 50 knots, waterspouts, and dangerous lightning. Watch for towering cumulus clouds and darkening skies.

Tropical Storm

A tropical cyclone with sustained winds of 34-63 knots (39-73 mph). Brings dangerous winds, heavy rain, and rough seas. Can strengthen into a hurricane. Tropical storm watches and warnings indicate when conditions are possible or expected.

Trough

An elongated area of low pressure, the opposite of a ridge. Troughs often bring unsettled weather, clouds, and precipitation. They can steer fronts and storms, and are important features to watch in marine weather charts.

True Wind

The actual wind speed and direction relative to a fixed point, as opposed to apparent wind felt on a moving boat. True wind is what weather forecasts report and is measured by stationary instruments like buoys.

U

Upwelling

A process where cold, nutrient-rich water rises from the deep ocean to the surface. Common along coastlines when wind blows parallel to shore. Creates cooler water temperatures but excellent fishing conditions due to increased nutrients.

UV Index

A measure of ultraviolet radiation intensity from the sun. On the water, UV exposure is amplified by reflection off the surface. Index values: 1-2 (low), 3-5 (moderate), 6-7 (high), 8-10 (very high), 11+ (extreme).

Scale: 0 to 11+

V

Veering Wind

A clockwise shift in wind direction (e.g., south to west to north). In the Northern Hemisphere, veering winds typically indicate improving weather as a cold front passes. The opposite of backing wind.

VHF Weather Radio

NOAA Weather Radio broadcasts continuous marine forecasts, warnings, and observations on VHF frequencies. Available on channels WX1-WX7 on marine radios. Essential for receiving real-time weather updates while on the water.

Frequencies: 162.400-162.550 MHz

Visibility

The distance at which objects can be clearly seen. Reduced by fog, rain, haze, or spray. Critical for safe navigation. When visibility drops below 1 nautical mile, extra caution and navigation lights are required.

Measured in: nautical miles or statute miles

W

Wave Height

The vertical distance from wave trough to wave crest. Forecast heights are typically "significant wave height" (average of highest third). Individual waves can be 1.5-2x higher than forecast significant height.

Measured in: feet or meters

Wave Period

The time in seconds between successive wave crests passing a fixed point. Longer periods (12+ seconds) indicate organized ocean swells; shorter periods (under 6 seconds) indicate local wind waves or chop. Longer periods generally mean more comfortable conditions.

Measured in: seconds

Wind Direction

The compass direction FROM which the wind is blowing. A "north wind" blows from north to south. Critical for planning routes, anchoring, and understanding wave development.

Measured in: degrees or compass points (N, NE, E, etc.)

Wind Speed

How fast the air is moving, typically measured as sustained speed (average over 2 minutes) and gusts (brief peaks). Marine forecasts use knots. Light winds are under 10 knots; 20+ knots is considered moderate to strong.

Measured in: knots, mph, or km/h

Waterspout

A rotating column of water and spray formed by a whirlwind over water. Fair weather waterspouts are weaker and more common; tornadic waterspouts are dangerous. Both can damage boats. Typically short-lived but unpredictable in movement.

Whitecaps

Foamy white crests on waves caused by wind breaking the wave tops. Generally appear when winds reach 10-15 knots. The presence and density of whitecaps help estimate wind speed when no instruments are available.

Wind Waves

Waves generated by local wind, as opposed to swells from distant storms. Wind waves are typically shorter, steeper, and more chaotic than swells. They develop quickly when wind blows and subside soon after wind stops.

Wind Chill

The perceived temperature on exposed skin due to wind speed. At sea, wind chill is a significant factor for comfort and safety. A 50°F day with 25-knot winds feels like 38°F. Critical for planning appropriate clothing and exposure time.

Measured in: °F or °C