Understanding the Marine Forecast



These days mariners have accurate weather forecasts at their fingertips. The science of meteorology, weather forecasting, and the tools we use to interpret weather data have improved so much that it may seem sailors needn’t worry about the weather at all. 

However, science is sometimes imperfect, and a good captain must closely monitor weather conditions at all times. 

marine forecast to be checked before sailing in open sea. Marina with sailboats looks calm, but the weather can get changed in few hours.

In this article, we’ll look at what a marine forecast is and how early humans interpreted the seas. We’ll also learn how captains and expert boaters interpret forecasts and how international agencies issue marine warnings.

The weather has a powerful impact on both pleasure boating and worldwide ocean shipping so keep reading to learn more. 

The Origins of Marine Weather Forecasting

When we think about early humans and sailing, we mostly wonder how they navigated their vessels without the aid of maps. But one critical aspect of sailing is the weather. Since ancient sailors couldn’t yet predict the weather or understand the science behind it, how were they able to sail successfully?

The answer is they weren’t often successful at all. 

But early humans, especially those that relied heavily on sailing like the Polynesians, took their weather clues from the world around them. What we might call lore today was, in fact, reasonably accurate science given the tools these ancient mariners had at their disposal. 

Observing Animal Behavior 

Before dropping a boat in the water, ancient sailors observed animals on land. Hunter-gatherers had extensive knowledge about the animals they hunted and the animals that hunted other animals. Their knowledge about these animals helped them understand more about the world around them. 

Ruminants like moose, elk and deer are susceptible to changes in the weather. When humans spotted a herd of these animals lying down, it was a fair assumption that a weather change was on the horizon. Since ruminants lose body heat quickly, they would lie down to warm themselves with the latent heat from the ground. 

When elk and deer moved to lower elevations, it was also a sign that a cold front might be moving in. 

Equids like horses and donkeys behave similarly to ruminants. When severe weather was approaching, these animals sought shelter under trees or in valleys. They would also group together to preserve body heat and protect younger herd members from wind and rain. 

Observing birds was a vital measure of approaching weather. While early humans didn’t know this, birds hear sounds on a different frequency. Birds can pick up on weather sounds from miles away, and observing seagulls flying inland meant a storm was rapidly approaching from the sea. 

Sharks are also very sensitive to weather changes. Modern studies show that sharks move to deeper water when hurricanes are approaching. Early humans may have noticed this behavior and, over time, learned to prepare for a severe storm.

Dolphins and whales may also offer clues to approaching weather. Rough seas make it difficult for cetaceans to breathe. When dolphins and whales seek out calm, protected waters, it may be a sign that bad weather is looming.

One of the common denominators here is barometric pressure. Animals are much more sensitive to changes in air pressure, which prompts their behavior during severe weather. Humans didn’t have the tools to measure pressure but watching animals gave them clues to weather changes. 

Look to the Skies 

While watching animal behavior was an important aspect of weather prediction, the best clues came from the skies. Cloud patterns and formations were impressive indicators of weather. By watching clouds, humans could determine the type of approaching storm, when it might arrive and how severe it might be. 

The color of the clouds was a significant factor in determining the storms they might bring. Black clouds indicated lower wind speeds, while brown clouds contained high levels of dust and indicated higher wind speeds. 

The height of the clouds also held clues about the weather. Low clouds often indicated rain, as the moisture added weight to clouds and prevented them from rising. Higher clouds often accompanied high winds. 

One of the drawbacks of cloud observation is the human eye’s lack of sight distance. In many cases, the observable clouds were too close for people to assess the weather and then plan to avoid it. 

Watch the Ocean’s Activity

Even the untrained human eye can observe the height difference in various waves. But the true nature of wave action is much more nuanced. Knowing the difference between waves and swells is an important factor in predicting the weather. 

Swells are large movements in the ocean generated by storms that could be hundreds of miles away. When sailors observed large swells, they would then use what they knew about wind direction and the appearance of clouds to gauge the likelihood of a storm. 

The wind speed and direction behind the swells also cause changes to the ocean’s surface. When a swell’s top has a round appearance, the wind driving it travels from farther away. A sharp peak on a swell indicates the winds are more localized. 

Currents can be hard to observe from the shore, but sailors would often launch their watercraft and sail a short distance from the coast. This allowed them the opportunity to observe the currents and add that information to what they already knew about imminent weather. 

The Early Science of Weather Forecasting 

People from all cultures had their own versions of predicting the weather. But it wasn’t until the masters like Copernicus and Galileo delved into the science of astronomy that people began to have a better idea of atmospheric science. 

But neither of these scientists could accurately predict the weather, and, if they could, they could not convey that information to others in a reasonable time.

The First Marine Weather Forecasts 

Unsurprisingly, the men who developed weather forecasting science were sailors themselves. Francis Beaufort and Robert FitzRoy, both members of the British Royal Navy, took it upon themselves to study weather patterns and use their knowledge to create early forms of weather predictions. 

Francis Beaufort focused on wind and tides. He kept his own records and encouraged sailors at the British Coast Guard’s more than 200 stations to keep their own tide records as well. Beaufort’s wind studies would become the Beaufort wind force scale, which is a method still used today. 

Beaufort’s tide tables were instrumental in giving sailors insight into the ocean’s movement, allowing sailors to plot their courses with more success. 

Robert FitzRoy asked ship captains to collect rudimentary weather data from all their voyages which he later collated into semi-reliable weather forecasts. But the main problem still remained: how could this new information be relayed fast enough to allow the ship’s captains the ability to avoid bad weather? 

The Electric Telegraph

When Samuel Morse invented the telegraph in 1843, he likely had little knowledge of its impact on weather forecasting. The use of the telegraph was reliant on the use of cables, so its progress was slow. 

But by the 1850s, engineers laid a telegraph cable between New York City and London, and operators could relay weather information in minutes rather than weeks or months. Robert FitzRoy used the telegraph to his advantage and set up weather monitoring stations in key locations. 

Now that information from those stations was more readily accessible, FitzRoy was ready to consolidate his information and create a weather system analysis everyone could understand. 

The International Cloud Atlas

In 1896, the first edition of the International Cloud Atlas was published. This document contained years of data describing the patterns and behaviors of cloud formations. Several scientists contributed to this project under the assumption that cloud patterns were largely similar all over the world. 

The science community first published the atlas in English, French, and German. However, the publishers added additional languages as subsequent editions came out with updated information. The atlas became a vital part of seafaring and an integral tool used for assessing sea conditions. 

Modern Marine Forecast Techniques

As science and marine travel moved into the 20th century, atmospheric scientists made great strides to improve the way the marine forecast was estimated and delivered. Not only did communication techniques improve, but new ways to observe weather patterns also allowed people to better understand major storm systems and their behavior. 

Weather Science Takes to the Skies 

As early as the 18th century, scientists used kites and balloons to study upper air patterns. While this method was ineffective at precise weather calculations, it did set the foundation for further aerial research. 

By the 20th century, scientists began attaching meteorographs to kites and balloons. These instruments measure wind speed, barometric pressure, and humidity. Some of these balloons could reach the stratosphere, but the scientists couldn’t collect the data until the balloons burst and fell to the ground. 

Unfortunately, the data was lost if the scientists couldn’t locate the fallen meteorograph. 

By the 1930s, airplanes were deployed with meteorographs on board, but many of the same problems occurred. The data could not be retrieved until the plane landed, and the planes weren’t capable of flying in severe weather, which is precisely the information weather scientists were looking for. 

In the years leading up to World War ll, weather prediction became even more important. Naval ships and cargo vessels needed accurate weather information more than ever. Radio transmitters called radiosondes were deployed and could relay real-time data, allowing forecasters to deliver more accurate marine weather information. 

The National Oceanic and Atmospheric Association

In 1807 President Thomas Jefferson established the US Coast and Geodetic Survey as a way to provide sailors with nautical charts of the US ports and coastline. The US Weather Bureau came into existence in 1870. These combined agencies were the first governmental scientific agencies in history. 

Working together, these groups provided mariners with weather forecasts to the extent that current science would allow. In 1970, the National Oceanic and Atmospheric Association was formed. The National Weather Service, having gone through several name changes, was also established in 1970. 

Both entities helped pioneer the use of weather satellites and radar to provide accurate weather forecasts to people on land and at sea. 

Nautical Weather Terms

Communication between NOAA and other marine monitoring agencies is done with specific nautical terminology. Without knowledge of these terms, sailors may not understand the potential hazards looming on the horizon. 

Nautical Mile

Distance is measured differently in oceans than it is on land. One nautical mile equals 1.15 land miles (also known as statute miles), and one nautical mile equals 1 minute of latitude. 

Latitude and Longitude

Latitude and longitude lines are used to precisely map any location on earth. Latitude lines run east to west and are parallel to the equator. Longitude lines run north to south and are parallel to the prime meridian. 

When these lines intersect, they form coordinates that link to a specific place on earth. Latitude and longitude are written in degrees, minutes, and seconds. One degree of longitude equals about 70 miles. 


In sailing, a knot isn’t just a way to fasten lines. It’s a specific nautical term used as a measurement of wind and ship speed. One knot of wind speed equals one nautical mile per hour, or 1.15 statute miles per hour. 


Gust is a measure of intermittent wind speeds. When wind speeds hit 10 knots or more with lulls in between, this is considered a gust.

Sustained Wind Speed

Sustained wind speeds are winds that maintain a steady speed over a period of time. Sustained wind speeds and gusts can happen simultaneously. An example would be sustained winds of 50 knots with gusts of a higher speed. 


Doldrums is an antiquated word, but it describes a specific area of the Atlantic Ocean known for its calm waters and minimal winds. Before sailors knew to navigate around the area, they could spend days or longer without wind to power their sails. 

Gale Warning

A gale warning indicates sustained winds of 34 to 47 knots. Gale warnings are not issued in hurricane or cyclone conditions. 

High Seas Forecast

High seas can occur in any ocean and the Great Lakes and refer to wave height often generated by storms. A high seas warning is issued when waves reach approximately 18 meters high. 

A high seas warning is one that sailors should always take seriously. High seas have the potential to sink ships and severely damage offshore oil rigs. Early warning and immediate action are necessary to avoid disaster.

Small Craft Advisories

Any boat less than 66 feet in length is considered a small craft. Sea conditions that won’t affect larger ships will affect small craft vessels. Small craft advisories for wind are issued when wind speeds reach 22 knots or higher.

Small craft advisories for hazardous seas are issued when wave heights reach 10 feet, but wind speeds are less than 22 knots. 

Tropical Storm Warning

Tropical storm warnings occur when sustained winds are 34 to 63 knots. Tropical storms often have the potential to reach hurricane status. 

Typhoon Warning

Typhoons and hurricanes are similar storms. However, when one of these storms occurs west of the international date line, it’s called a typhoon. When it occurs east of the international date line, it’s called a hurricane. 

A typhoon warning is issued when wind speeds are 64 knots or higher, significantly affecting the shipping forecast. 

Special Marine Warning

A special marine warning is usually an isolated event that lasts no more than two hours. Typically a special marine warning is issued when sudden winds reach 34 knots or waterspouts have been observed.


A fathom is a measure of depth in the ocean. A fathom may sound like it refers to significant depth, but in reality, a fathom is only six feet. Most ocean depths are named in feet or meters, but you may run into a salty old ship’s captain who prefers to use this term.

Weather Hazards Facing Today’s Ships

Modern sailors have a wealth of weather information at their fingertips. The World Meteorological Organization has been instrumental in gathering weather data and making it available to anyone engaged in water activities. 

Primarily using satellite data, nautical buoys, and GPS technology, information regarding all of these weather hazards and warnings is available to sailors and must be monitored at all times.


Wind speeds and direction are among the top pieces of information a sailor should know. The force of the wind and the direction it’s coming from can mean that a captain has to alter the course or stay at the port until the wind conditions are more favorable. 

Wind speeds are even more impactful for smaller vessels. Small craft advisories and warnings are issued when wind speeds are particularly threatening to sailboats and pleasure craft. 


Waves and large swells can be disastrous for watercraft of all sizes. Large waves prevent bigger ships from making headway and burning precious fuel in the process. And while the hull construction of large ships allows them to handle large waves, the safety of the crew and the cargo may be at risk. 

Smaller watercraft can easily be inundated by small waves, making it critical that pilots of small watercraft heed all marine warnings at all times. 

Port officials also need to pay attention to wave warnings. Large wave action prevents the safe offloading of cargo and can cause damage to both the ship and the dock. During significant swells, many vessels are required to anchor offshore until the wave height diminishes. 


Even though modern ships have state-of-the-art marine radar systems, visibility is paramount to ocean safety. Fog, sea smoke, mist, low cloud cover and heavy rains all impact visibility. 

When visibility is less than two nautical miles, the captain must choose to proceed with caution. When visibility drops to less than one nautical mile, the captain must slow the forward motion, or the vessel needs to come to a complete stop. Ignoring low visibility warnings can lead to catastrophic collisions. 

Docking in low visibility is also hazardous. The port master may choose to suspend port activity until the visibility improves. 

Squalls and Thunderstorms 

Significant rain events like squalls and thunderstorms happen in waters all over the world. Even though these storms are commonplace, they can potentially create serious hazards for ships and crew. 

Cruise ships and cargo vessels are especially vulnerable to the effects of a thunderstorm. The length and height of these ships put them at risk of heeling over.

While rainfall makes deck conditions hazardous, it’s the associated wind and wave action that make thunderstorms at sea dangerous. Lightning is also a grave concern. 

Since a ship’s masts are high enough to attract lightning, crew members must stay off the deck and out of the water during a thunderstorm.

The Ocean’s Currents 

One of the key aspects of marine navigation is the ocean’s currents. Currents determine how a captain plots a course, manages fuel consumption, and alerts the crew to potential hazards, like icebergs, that may be driven by the currents. 

Currents can also have an impact on storm development. 

Hurricanes and Tropical Cyclones 

Prior to modern weather forecasting, hurricanes and tropical cyclones were deadly hazards for seafarers. With today’s advanced warning systems, most vessels can avoid a direct hit by a hurricane. However, since hurricanes can span hundreds of miles of the ocean, avoiding the storm’s outer bands is not always possible. 

When a hurricane warning is issued, it’s charted in latitude and longitude. Less experienced captains and crew may not be familiar with these positions, putting themselves, their crew, and their boat in danger. 

Stay Safe by Understanding Marine Weather Forecasts

Whether you’re a pleasure boater or the captain of a Capesize or Panamax cargo ship, the marine forecast is crucial to keeping your crew and your cargo safe from harm. Taking classes is a great way to learn the terminology and nuances of marine weather and how to navigate safely in stormy seas. 

Are you interested in learning more about life and work on the high seas? Have a look at some of our other articles to boost your knowledge and hone your safety skills.

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