Weatherguessing in the Late 18th Century

In most parts of the world, the spring and fall are the stormiest times of the year. Today, local weather forecasters use computer models that take weather data from all over the world, load it into a proprietary predictive model and voilà, out pops a forecast!

But if you were the captain of a merchant ship, privateer, or Royal or Continental Navy ship during the American Revolution, weather forecasting as we know it today didn’t exist. All the captain had was a thermometer, his eyes and the weather knowledge gained during his career.

The weather recording tools developed slowly over the centuries. Although Torricelli, Descartes, Pascal and Galileo all came to understand that the air has weight that changes with temperature and moisture content, it wasn’t until 1844 that the first practical aneroid barometer was invented by the French scientist Lucien Vidi. In 1480, Leonardo da Vinci invented the first hygrometer, the device to measure humidity in the air but it wasn’t until 1783 when Swiss physicist Horace Saussure invented the first practical hygrometer hair based on his observation that human and animal hair expands and contracts based on the amount of moisture in the air.

Measuring wind speed and direction through an anemometer began in the 1400s with Alberti’s first practical model, but anemometers were not widely deployed until the four rotating cup version invented by Thomas Robinson in 1846.

Modern weather forecasting was facilitated by an unlikely device called the telegraph that enabled weather data gathering stations to rapidly communicate with a central location. Two Royal Naval officers, Francis Beaufort (who created the Beaufort Scale for measuring wind) and Robert FitzRoy are credited with creating the modern science of weather forecasting. They even coined the word “forecast.” At the time, the British press ridiculed the idea!

Back to the quarterdeck of a ship setting sail from the Thirteen Colonies during the American Revolution. In the winter, to stay out of the cold, stormy North Atlantic, the captain heading to Europe, took his ship south past Bermuda where he tacked back and forth against the prevailing Trade Winds until the ship reached the Canary Islands. There, he set a course north to his ship’s destination. Travel time was two to three months.

In the late spring, summer and early fall, ships could follow what Ben Franklin first called the Gulph Stream and make it to Europe, if one’s ship averaged six knots over the bottom in about six weeks.

Sayings such as “red sky at night, sailors delight” or “red sky in the morning, sailor take warning” were part of the captain’s weather forecasting tools. Strangely enough, they are still accurate today but in the day when the ship’s daily weather forecast was governed by how far a lookout could see, it was all the captain had.

Ships in 1778 were tiny – generally about 150 – 175 feet long, 35 – 40 feet wide and displacing about 700 tons. Captains and their crews ventured forth into the oceans unable to forecast the weather and only able to determine their latitude and on charts we would call crude. They did it because that’s all they had.