Real-Time Observations
Mariners rely on real-time marine weather and oceanographic observations to provide current conditions for waters they are navigating or plan to navigate. These observations are also used by weather forecasters in preparation of marine weather forecasts and warnings. They help generate surface-weather analysis and are often assimilated into numerical weather and oceanographic forecast models, providing greater accuracy in predictions to forecasters and mariners. The conditions that are relevant to mariners include winds, waves and swell, visibility, water levels, water currents, ice coverage and thickness, and air gap – the distance between the water surface and lowest point of a bridge. These conditions can vary based on season, geographic area, and ship position along the intended route.
Observations for coastal waters, offshore waters, and high seas are available to mariners from a variety of observing networks operated by NOAA’s National Ocean Service (NOS), National Weather Service (NWS), Center for Operational Oceanographic Products and Services (CO-OPS), Integrated Ocean Observing System (IOOS), and other federal agencies. Information on where to find real-time observations for each of the primary marine weather and oceanographic variables is given below, along with detailed descriptions of the various observing networks and programs.
It is important for mariners to understand surface wind direction, speed, gusts and tendency (increasing/decreasing), and to anticipate the wind’s impact on generating waves locally (wind waves) and waves far away (swells). This information is provided by wind sensors located on coastal and overwater platforms of stationary and mobile observing networks. A wind sensor can be located on the mast of a buoy or ship, a tower at an airport or pier, or the top of a lighthouse. The height above ground level where a wind sensor is located can vary by network and sometimes by stations within a network. In some cases, the agency responsible for the network will estimate the wind speed at 10 meters (33 feet) above the surface, making it easier for users to compare wind speed observations from nearby stations of different networks. Networks that provide real-time wind reports are listed in the table below.
The frequency of wind reports varies by network, ranging from every six minutes to every six hours. In addition, the average time period used for reporting sustained wind direction and speed varies, as does the specified time period used to determine wind gusts.
NOAA products and services that provide real-time wind observations and analysis are listed in the following table. Wind observations can be obtained in several graphical and text formats (see examples below). In addition, observations can be obtained via NOAA Weather Radio, automated phone reports, and web map services in voice and plain text message formats.
Data source/ product | Responsible office | Web link to latest observations |
National Data Buoy Center | NWS | www.ndbc.noaa.gov |
Voluntary Ship Observing Program (VOS) | NWS | www.ndbc.noaa.gov/ship_obs.php |
Physical Oceanographic Real-Time System (PORTS) | NOS/CO-OPS | https://tidesandcurrents.noaa.gov/ports_info.html |
National Water Level Observation Network (NWLON) | NOS/CO-OPS | tidesandcurrents.noaa.gov/met_info.html |
Regional Ocean Observing System of U.S. IOOS | NOS/IOOS & Regional Ocean Observing Associations | ioos.noaa.gov/data/regional-data-portals/ |
Automated Surface Observing System (ASOS) and Weather Observing System (AWOS) | NWS, DOD, and FAA | https://www.faa.gov/air_traffic/weather/asos/ |
Wave height, period, and direction are all important factors that mariners must consider. Waves can have an impact on vessels, and the severity of impact depends on the size of the vessel, the size and direction of the waves, and whether they break or not. Waves can be produced by local winds (wind waves) or from further away. Those that originate from far away are known as swells and have higher periods (longer wave lengths) than the waves produced by local winds.
Detailed wave reports, such as those provided by the National Data Buoy Center, often present multiple variables related to wave heights. Wind-wave and swell height both represent an average of the highest one-third of the respective wave type. Significant wave height is the average of the highest one-third of all waves present. Steepness, reported as either very steep, steep, average, or swell, takes into account both the significant wave height and the dominant wave period. Steeper waves present a higher risk for capsizing vessels or damaging marine structures.
Wave conditions are measured by sensors located on coastal and overwater platforms of several observing networks. Wave observations can also be manually reported by ships participating in the Voluntary Observing Ship Program (VOS) and the National Weather Service’s Voluntary Marine Observation and Mariner Reports Programs.
NOAA products and services that provide real-time wave observations and analysis are listed in the following table. Wave observations can be obtained in several graphical and text formats (see examples below). In addition, observations can be obtained via NOAA Weather Radio, automated phone reports, and web map services in voice and plain text message formats.
Data source/ product | Responsible office | Web link to latest observations |
National Data Buoy Center | NWS | www.ndbc.noaa.gov |
Voluntary Ship Observing Program (VOS) | NWS | www.ndbc.noaa.gov/ship_obs.php |
Physical Oceanographic Real-Time System (PORTS) | NOS/CO-OPS | https://tidesandcurrents.noaa.gov/ports_info.html |
Regional Ocean Observing System of U.S. IOOS | NOS/IOOS & Regional Ocean Observing Associations | ioos.noaa.gov/data/regional-data-portals/ |
Significant height and direction time-series plot and interactive map viewer for the Southeast Regional IOOS System
Visibility is defined as a measure of the horizontal opacity of the atmosphere at the point of observation. Reduced visibility – caused primarily by fog when over water – can make navigating difficult for mariners, especially when visibility drops to one mile (1.6 kilometers) or less. Even with the advent of marine radar, ship collisions caused by reduced visibility still occur.
Visibility is measured manually by eye or by an automated light sensor. For measurements made by eye, such as those taken aboard ships, visibility is expressed in terms of the horizontal distance at which a person should be able to see a specified object. During the day, measurements are made to a prominent dark object set against the horizon, while at night they are made to a known, preferably unfocused, moderately intense light source. The reported surface visibility must be equaled or exceeded throughout at least half of the horizon circle. Manual visibility observations have limitations due to the observer’s experience, sun angle, day and night differences, and poor observing sites. Manual visibility observations are reported through the National Weather Service’s Voluntary Ship Observing Program.
Automated visibility sensors sample the volume of air at the sensor location rather than look at distant objects to calculate visibility. The sensor is configured to avoid sun glare and snow blockage, however, sensors over or near saltwater can often be contaminated by salt spray. Automated visibility sensors are located on land, coastal, offshore, and overwater platforms through the Automated Surface and Weather Observing Systems (ASOS and AWOS) and NOAA’s Physical Oceanographic Real-Time Systems (PORTS). ASOS sensors are located at airports and heliports near touchdown zones, while PORTS sensors are often located in areas susceptible to fog. Sensors from both systems are typically located 10 feet (3 meters) above ground level, but their upper limits of visibility vary: 10 miles (16 kilometers) for ASOS and 5.4 nautical miles (10 kilometers) for PORTS.
NOAA products and services that provide real-time visibility observations are listed in the following table. Visibility observations can be obtained in several graphical and text formats (see examples below). In addition, observations can be obtained via NOAA Weather Radio, automated phone reports, and web map services in voice and plain text message formats.
Data source/ product | Responsible office | Web link to latest observations |
Voluntary Ship Observing Program (VOS) | NWS | www.ndbc.noaa.gov/ship_obs.php |
Physical Oceanographic Real-Time System (PORTS) | NOS/CO-OPS | https://tidesandcurrents.noaa.gov/ports_info.html |
Automated Surface and Weather Observing Systems (ASOS and AWOS) | NWS, DOD, FAA, and some state agencies | https://www.faa.gov/air_traffic/weather/asos/ |
Text and graphical visibility outputs from PORTS sensors
Water-level data is integral to marine navigation. Its uses range from making nautical charts to supporting long-term sea-level monitoring. Water levels can vary frequently, as they are affected by tides, meteorological conditions such as winds and frontal systems, and factors like higher- or lower-than-normal river runoff. In harbors and coastal waterways, navigation depends on real-time water-level information. Mariners must know the water depth beneath the ship’s keel in order to safely pass over shallow areas.
A nautical chart shows the depth of water relative to a standard local datum level, but this does not necessarily reflect the real-time water level. Real-time water-level observations are provided by local gauges and are reported in reference to a local tidal datum. A tidal datum is a standard elevation defined by a certain phase of the tide, and it is referenced to a fixed point known as a benchmark. NOAA’s National Ocean Service maintains a modern water-level measurement system that allows for a variety of real-time, near real-time, and long-term applications, with stations reporting water levels at six-minute intervals.
In addition to contributing to safe navigation, real-time water-level observations can allow mariners to optimize their routes and the amount of cargo their vessels can carry. Traditionally, a minimum underkeel clearance is calculated in ports and harbors as a factor required to provide safe passage for a vessel. Once the available underkeel clearance is known, mariners can plan the most viable route to take based on current water levels and a vessel’s size, draft, and nature of cargo. This can lead to more efficient transits, reduced lightering, and substantial cost savings.
Transit in areas of limited water depth, in relation to a ship’s draft and available width of navigable water, is undertaken with extreme caution. Despite the application of a minimum underkeel clearance, the likelihood of grounding on immediately adjacent shallows is increased.
NOAA products and services that provide real-time water-level observations are listed in the following table. Water-level observations can be obtained in several graphical and text formats (see examples below). In addition, observations can be obtained via NOAA Weather Radio, automated phone reports, and web map services in voice and plain text message formats.
Data source/ product | Responsible office | Web link to latest observations |
National Water Level Observation Network (NWLON) | NOS/CO-OPS | https://tidesandcurrents.noaa.gov/stations.html?type=Water+Levels |
Coastal Inundation Dashboard | NOS/CO-OPS | https://tidesandcurrents.noaa.gov/inundationdb/ |
Physical Oceanographic Real-Time System (PORTS) | NOS/CO-OPS | https://tidesandcurrents.noaa.gov/ports_info.html |
NWLON water level data listing (table) and time-series plot for Mobile Bay, Alabama:
PORTS time-series plot for Houston/Galveston Bay gauges:
The speed and direction of water currents are crucial factors to consider for navigation, as strong currents can impact a vessel’s speed or cause it to veer off course. Water currents are driven by a number of factors, including winds and weather, tides, water level changes (e.g., inbound and outbound currents in the Great Lakes), or major ocean circulation systems (e.g., the Gulf Stream). Water current velocity is typically reported in knots (1 knot ≅ 1.15 mph), and the direction is reported as the direction in which the current is moving (e.g., easterly currents move from west to east).
Tidal currents are those that occur in conjunction with the rise and fall of the tide and are the most prevalent currents near U.S. seaports. The vertical motion of the tides near the shore causes the water to move horizontally, creating currents. When a tidal current moves toward land and away from the sea, it floods; when it moves toward the sea away and from land, it ebbs.
Real-time current data plays an important role for ships coming into or going out of port. Mariners need to account for currents as they coordinate transit time and vessel turns. Currents can change rapidly at depth or across a channel; therefore, NOAA’s National Ocean Service measures currents in real time using instruments that can be oriented in three different ways. These include buoy mounted (down looking) and bottom mounted (up looking) to measure how currents change at depth, and pier mounted at a fixed depth (side looking) to measure how currents change horizontally across the channel.
Having accurate, real-time data can improve the efficiency and safety of port maneuvers, thus reducing port congestion and traffic. Mariners can also use current data while transiting at sea for route optimization – by taking advantage of a strong current, a ship can operate well below its maximum speed and reduce its fuel use and greenhouse gas emissions.
NOAA’s real-time current information is provided through the NOS Physical Oceanographic Real-Time System (PORTS). A full list of actively installed current meter stations is available here. An example of the time-series plot and data output from a current meter station is provided below. The red curves represent speed in knots and green symbols represent direction in degrees.
Ice affects mariners operating in high latitudes during the winter months, in areas such as the North Atlantic, North Pacific, Arctic Ocean, the Great Lakes, rivers (e.g., Hudson River), and estuaries and seaports (e.g., Port of New York/New Jersey and Delaware Bay). Ice conditions that mariners need to be aware of include:
- Ice concentration – Concentration refers to the amount of the water surface that is covered in ice, expressed as a fraction of the whole area (reported in tenths).
- Ice type/stage – There are many different types of ice that describe important properties of the ice, such as its thickness. Both sea ice and lake ice contain several different types; a full list can be found here.
- Ice floe size – Ice floes are sheets of ice that are at least 20 meters (65 feet) across. Both sea ice and lake ice can contain ice floes.
Operating in ice-covered waters often requires ice-strengthened ships whose hulls have been reinforced for that purpose. Commercial shipping in these waters relies on U.S. and Canadian Coast Guard icebreakers to provide escort and support when ice is present. These ice breakers are crewed by highly skilled and experienced specialists in the field of ice navigation, ice breaking, and ice escort.
Commercial ship and icebreaker crews rely on ice climatologies and analyses that are based on satellite observations. The information is compiled into ice charts for navigation use. For U.S. waters, these charts are generated by the U.S. National Ice Center, the National Weather Service’s Alaska Sea Ice Program, and the Canadian Ice Center. Links to ice charts, observations, and analyses for different geographic regions are included in the table, and examples of ice charts are provided below.
Geographic area | Responsible office | Web link to latest ice analysis |
Great Lakes | U.S. National Ice Center | https://usicecenter.gov/Products/GreatLakesHome |
Canadian Ice Center | https://iceweb1.cis.ec.gc.ca/Prod/page2.xhtml?CanID=11080&lang=en&title=Great+Lakes | |
Mid-Atlantic States | U.S.National Ice Center | https://usicecenter.gov/Products/MidAtlanticHome |
East Coast | Canadian Ice Center | https://iceweb1.cis.ec.gc.ca/Prod/page2.xhtml?CanID=11091&lang=en |
Alaska | NWS Alaska Sea Ice Program (ASIP) | https://www.weather.gov/afc/ice |
U.S. National Ice Center: USCG District 9 Great Lakes Ice Concentration and Level Ice Thickness Chart
Large vessels often have to pass under bridges on their way to and from seaports. The distance between the water surface to the lowest structure of a bridge is known as the air gap. It is critical for a mariner to know a bridge’s air gap to ensure safe passage. The air gap can vary due to tides, wind and wave setup, bridge height variations caused by varying traffic loads, and temperature-related structural expansion and contraction. Air gaps are measured by sensors placed on bridges that use microwave technology to measure the distance to the water surface. This ensures the sensor is not affected by weather conditions, waves, vibration, or radio frequency noise.
Real-time air gap measurements are provided by NOAA’s National Ocean Service via the Physical Oceanographic Real-Time System (PORTS). Air gap observations are available in text and graphical formats, in the form of a table and time series plots. Mariners can also obtain the information via NOS PORTS automated phone reports. An example of air gap observations is provided below.
PORTS air gap graphic and time series plot for Lake Charles I-210 Bridge
There are several observing networks and programs that provide the real-time data and observations useful for mariners. Often, this data is compiled by and disseminated through national centers or local forecast offices. These include:
- National Data Buoy Center (compiles data from buoy networks and the Coastal-Marine Automated Network, or C-MAN)
- Voluntary Observing Programs (including the U.S. Voluntary Observing Ships Program, SKYWARN, MAREP, and MAROBS)
- Physical Oceanographic Real-Time System (PORTS©)
- National Water Level Observation Network (NWLON)
- nowCOAST™
- Automated Surface and Weather Observing Systems (ASOS and AWOS)
- U.S. High Frequency (HF) Radar Network
- Land-Based Cloud-to-Ground Lightning Detection Network
Descriptions of these networks and programs can be found below.
National Data Buoy Center (NDBC)
The National Data Buoy Center (NDBC) provides quality observations in the marine environment to support the understanding and predictions of changes in weather, climate, oceans, and coast. The NDBC compiles coastal and offshore weather observations from NOAA fixed and drifting data buoys and C-MAN stations. Moored buoys are deployed in all U.S. coastal and offshore waters, from the western Atlantic to the Pacific Ocean around Hawaii, and from the Bering Sea to the South Pacific. NDBC's moored buoys measure and transmit barometric pressure; wind direction, speed, and gust; air and sea temperature; and wave energy spectra from which significant wave height, dominant wave period, and average wave period are derived. Even the direction of wave propagation is measured on many moored buoys.
The National Weather Service’s C-MAN was established by the NDBC in the early 1980's. C-MAN stations have been installed on lighthouses, at capes and beaches, near shore islands, and offshore platforms. C-MAN station data typically includes barometric pressure, wind direction, speed and gust, and air temperature. Some C-MAN stations are designed to also measure sea water temperature, water level, waves, relative humidity, precipitation, and visibility. These datasets are processed and transmitted hourly to users in a manner almost identical to moored buoy data. In addition to the conventional method of data transmission, certain C-MAN stations are equipped with telephone modems that allow more frequent data acquisition, data quality control, and remote payload reconfiguration or restarting.
U.S. Voluntary Observing Ship (VOS) Program
The U.S. Voluntary Observing Ships (VOS) Program is organized for the purpose of obtaining weather and oceanographic observations from moving ships. VOS is an international program under the World Meteorological Organization and includes 49 countries with over 900 participating vessels. The U.S. program is the largest in the world. Observations are coded in a special format known as the ship's synoptic code, or BBXX format, and are distributed on national and international circuits for use by meteorologists, oceanographers, ship routing services, fishermen, and many others.
Other voluntary marine observation programs include the National Weather Service’s SKYWARN, MAREP, and MAROBS programs. SKYWARN is a nationwide network of volunteer weather spotters who report to and are trained by NWS. These spotters report many forms of significant or severe weather (e.g., severe thunderstorms, hail, flooding, etc.). The MAREP program allows mariners to report current coastal weather conditions to local weather forecast offices. Unlike the VOS and SKYWARN programs, pre-registration and training is not usually a prerequisite for participation. Local NWS Weather Forecast Offices can provide more information on SKYWARN and MAREP activities. Finally, the MAROBS Program is an experimental NWS program that is seeking the participation of any mariner not already involved in the VOS program. The goal is to collect as many marine observations as possible. The MAROBS report format will be identical to coded reports of the VOS Program, with the exception that MAROBS will replace BBXX. There will be additional differences between MAROBS and VOS, such as MAROBS observation elements representing only a subset of those collected in the full VOS report.
NOS Physical Oceanographic Real-Time System (PORTS©)
The National Ocean Service’s PORTS system provides commercial vessel operators with accurate and reliable real-time information for marine and weather conditions in and around U.S. seaports. PORTS includes observations and predictions for conditions such as water levels, currents, bridge air gap, and many meteorological parameters (e.g., winds, waves, atmospheric pressure, visibility, air and water temperatures). This data is recorded and disseminated every six minutes.
National Water Level Observation Network (NWLON)
The National Water Level Observation Network is an observation network with more than 200 permanent water level stations on the coasts and Great Lakes. This system allows NOAA to provide the nation with official tidal predictions. Accurate water level data is critical for safe and efficient marine navigation and for the protection of infrastructure along the coast. The NWLON also provides the national standards for tide and water level reference datums used for nautical charting, coastal engineering, international treaty regulation, and boundary determination. The NWLON is widely recognized as the key federal component of the Integrated Ocean Observing System (IOOS).
nowCOAST is NOAA’s web-mapping portal for near real-time observations, analysis, tide predictions, model guidance, watches/warnings, and forecasts for the coastal United States. It incorporates information from across NOAA, other federal agencies, and regional ocean and weather observing systems. Users can display maps of this information using the nowCOAST interactive map viewer or by connecting to its map services. Mariners can utilize nowCOAST for real-time observations of marine weather, surface winds, sea-surface temperatures, and precipitation. For future conditions, nowCOAST provides information on marine weather advisories, watches, and warnings; surface winds; waves; water levels; temperature; salinity; and currents.
Automated Surface and Weather Observing Systems (ASOS and AWOS)
The Automated Surface Observing System serves as the nation's primary surface weather observing network. It is a joint product of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DOD). For mariners, ASOS stations located along the coast can provide real-time observations for the following weather variables: wind direction, speed, and character; visibility and obstructions to vision; and basic current weather conditions. ASOS detects significant changes and disseminates hourly and special observations from ASOS stations. An ASOS station transmits a special report when conditions exceed preselected weather element thresholds (e.g., visibility decreases to less than three miles).
The Automated Weather Observing System (AWOS) is a nationwide network operated and controlled by the FAA. The system predates ASOS and is designed to support aviation operations, as well as weather forecast activities. AWOS stations are located at airports and heliports along the coast and offshore. AWOS stations can be level I, II, III, or IV depending on what sensors are installed. The most common level is AWOS III, which reports the following variables: wind speed and wind gusts, wind direction, visibility, sky condition, cloud ceiling, air temperature and dew point temperature.
U.S. High Frequency (HF) Radar Network
NOAA provides maps of near-real-time surface water currents in coastal and some offshore waters based on data from the U.S. National High-Frequency (HF) Radar Network and the Canadian network. The radars utilize high frequency radio waves to measure the direction and speed of surface-water currents in the coastal ocean. Radar antennas, typically in pairs, are positioned on the coast and can measure surface water-currents (located at the top 3-6 feet, or 1-2 meters, of the water column) up to 125 miles (200 kilometers) away, with spatial resolutions ranging from 310 miles (500 kilometers) to 3.7 miles (6 kilometers) depending on the individual radar’s frequency. The observations of the surface current speed and direction are usually one-hour averages (e.g., 11:30 to 12:30).
Surface-current data based on HF Radar can be accessed through interactive maps via the Integrated Ocean Observing System (IOOS). Additionally, NOAA’s Center for Operational Ocean Products and Services (CO-OPS) HF Radar product displays both near-real-time surface current observations and surface tidal current predictions.
Land-Based Cloud-to-Ground Lightning Detection Network
The NWS Ocean Prediction Center provides near-real-time lightning strike density data. The purpose of this data product is to provide mariners and others with enhanced awareness of developing and transitory thunderstorm activity, to give users the ability to determine whether a cloud system is producing lightning, and if that activity is increasing or decreasing. Lightning strike density, as opposed to display of individual strikes, highlights the location of lightning cores and trends of increasing and decreasing activity. The 15-minute gridded source data is updated every 15 minutes at 10 minutes past the valid time. NOAA’s nowCoast web mapping portal also displays the latest lightning-strike data.