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Benjamin Franklin’s map of the Gulf Steam (A), which he made in 1770, and a recent NOAA satellite image of the Gulf Stream (B) in which differences in sea surface temperature are shown with different colors. Blues represent the coldest water while orange and yellow represents warmer water.
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(A) The NOAA Photo Library and (B) NOAA

Picturing the Gulf Stream Current

When Benjamin Franklin heard complaints in 1769 that the mail boat from North America got to Europe much faster than mail boat traveling the other direction, he took these complaints very seriously. He was a postmaster in the American colonies, so mail delivery was important to him. Yet Franklin was also a scientist and so he turned to science to figure out what was going on with the mail delivery delay. He wanted to get to the bottom of this postal mystery.

At that time, before the invention of the airplane, all letters and packages were transported across oceans by boats. The boats travelling from Europe to North America took weeks longer than mail boats heading in the opposite direction.

Benjamin Franklin crossed the Atlantic several times by boat, taking temperature measurements and making other observations of the ocean. He discovered that an ocean current was to blame for the mail delay. Boats going to Europe were sped up by the current. Boats going to North America were slowed by the current. He called the current the Gulf Stream.

The map that Benjamin Franklin made of the Gulf Stream in 1770 was the first map ever made of this ocean current. Mapping this current’s path was very helpful for sailors.

Franklin noticed many features that made the Gulf Stream water different from the surrounding ocean. One of the most characteristic was a difference in temperature. “I find that it is always warmer than the sea on each side of it,” he wrote in a letter.

The warmer temperature of the Gulf Stream shows up in satellite images of Sea Surface Temperature data (SSTs). Purple and blue represent the coldest water and orange and red represents the warmest water. The Gulf Stream is the warmest water in this satellite image. It reaches from the Caribbean to Delaware before heading east - amazingly the same location where Benjamin Franklin mapped the current by hand over two hundred years before. He used temperature measurements to identify the ocean current.

Last modified September 18, 2008 by Lisa Gardiner.

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