franklininstitute

Meteorological Imaginations and Conjectures, [May 1784]

The Mystery Franklin Was Investigating

Franklin was trying to explain two connected phenomena from 1783-1784:

1. A strange “dry fog” that covered much of Europe (and possibly North America) during the summer of 1783
2. An exceptionally harsh winter that followed in 1783-1784

Franklin’s Scientific Observations

About Weather in the Upper Atmosphere: Franklin begins by theorizing about the upper atmosphere, noting that there must be a region high above where it’s always winter and frost exists continually - his evidence being that hail falls even in summer. He explains how water vapor condenses and freezes in these cold upper regions, then grows larger as it falls through more vapor.

The Mysterious Dry Fog: Franklin describes how during the summer of 1783, there was an unusual, persistent fog over Europe and parts of North America. This wasn’t ordinary fog - it was dry and couldn’t be easily dispersed by sunlight. Most tellingly, the sun’s rays were so weakened by this fog that when concentrated through a magnifying glass, they could barely light brown paper.

Franklin’s Revolutionary Theory

Franklin proposed that this dry fog significantly reduced the sun’s heating effect on Earth’s surface during summer 1783. Because the Earth couldn’t warm up properly:

• The ground froze earlier than usual
• Snow accumulated and stayed on the ground instead of melting
• The air became more chilled
• The resulting winter of 1783-84 was exceptionally severe

What Caused the Fog?

Franklin offered two possible explanations:

1. Meteors/Cosmic debris - smoke from burning celestial objects that Earth encountered in its orbit
2. Volcanic activity - specifically mentioning eruptions from Hekla in Iceland and a new volcanic island that emerged from the sea

Why This Matters

Franklin’s work was remarkably prescient. Modern science has confirmed that:

• The dry fog was indeed caused by the massive Lakagígar volcanic eruption in Iceland that began in June 1783
• Volcanic aerosols in the atmosphere can significantly affect global climate patterns
• The connection between the 1783 fog and the severe winter that followed was correct

Franklin also suggested that studying historical patterns of such fogs might help predict future harsh winters, allowing people to better prepare for potential disasters like flooding from frozen rivers breaking up in spring.

This paper demonstrates Franklin’s keen scientific observation and his ability to make connections between seemingly unrelated phenomena - a hallmark of his contributions to early meteorology and climate science.​​​​​​​​​​​​​​​​