NATIONAL MARITIME HISTORICAL SOCIETY

Listening to the weather report to plan your weekend or to decide what clothes you might wear is something most Americans do every day. Today, people can use apps on their phones that provide live weather radar and relatively accurate forecasting. Mariners use these tools too, but they also rely on the National Weather Service’s scheduled marine weather broadcasts on the marine radio and online.

What did sailors use to predict the weather before modern times? Well, for one thing, marine weather forecasts have been around for a long time. The US Navy started issuing 3-day marine weather forecasts for the North Atlantic in 1901. That responsibility transferred to the newly established Weather Bureau in 1904, and in the 120 years since then, meteorological science has evolved with the use of new technologies, including satellites, radar, aircraft monitoring, and high-tech buoys that broadcast data from wherever they are located at sea.

What did mariners use before 1901? People have been going to sea for thousands of years, and, by necessity, sailors are very much in tune with the natural world around them. Then, as now, they used observations of the sky, seas, and even animal behavior. By looking skyward, mariners observe cloud types, formation, and movement. With a little practice, they can predict when squalls are headed their way or a cold front is in the offing.

Learning your cloud types is the first place to start, and it can be fun to try out. You don’t need any special instruments, and you can do it wherever you happen to be. The National Oceanic and Atmospheric Administration (NOAA) has some great tools online to guide you. Visit www.weather.gov/education to learn more. When you’ve mastered identifying these basic cloud types and want to learn more, visit www.noaa.gov/jetstream/clouds.

Clouds are classified according to their altitude and appearance. There are four core cloud types: cirrus, cumulus, stratus, and nimbus, with lots of combinations that occur in between, such as cirrocumulus, altostratus, and cumulonimbus, for example.

Cirrus clouds are high, wispy clouds made of ice crystals. A day with cirrus clouds against a blue sky is usually a fair–weather day, but cirrus clouds are often the first sign of an approaching warm front, which will bring a change in the weather.

Cumulus clouds look like big white cotton balls. They indicate the vertical motion of air taking place in the atmosphere. The base of a cumulus cloud is often flat compared to its upper half, where it is round and puffy shaped. Cumulus clouds are usually present in nice weather, but they can change quickly and dramatically and produce an intense storm system, with lightning, strong winds, and heavy rain.

Stratus clouds (from the Latin word for layer) are usually big broad blankets of gray clouds without a distinct shape. They tend to occur along warm fronts. Stratus clouds can be free of precipitation or they can produce light rain that can last all day.

Finally, when the word “nimbus” is attached to one of these three cloud types (cumulonimbus or stratonimbus, for example), you can expect rain. Under a cumulonimbus cloud, you should anticipate heavy rain, often with thunder, lightning, and strong winds. If you are sailing and see a cumulonimbus cloud to windward, you would be wise to alter course and put some distance between your vessel and that cloud system.