How President Thomas Jefferson became one of America's early 'weather geeks'

On 4 July 1776, Philadelphia was teetering on the brink of a new world.

The men who would sever a budding super-power from an empire were gathering in the building that would one day be called Independence Hall.

Among them was Thomas Jefferson, lawyer, philosopher, revolutionary, and soon to be the third president of the United States.

But before all that, on the most momentous morning in American history, he did something rather more ordinary. He checked his thermometer. He wrote down 68 degrees Fahrenheit. And then he got on with changing the world.

While the world was being shaped through speeches and cannon fire, Jefferson was, quietly and methodically, trying to measure it.

This is a side of Jefferson many people don't know. Yes, he wrote the Declaration of Independence and, as president, oversaw the Louisiana Purchase, roughly doubling the size of the young republic.

But buried in thousands of pages of letters and notebooks is another Jefferson, one meteorologists now regard as a founder of American weather observation.

He kept a weather diary for 50 years, in different countries and dozens of towns, and was still doing it days before he died.

Between July 1776 and June 1826, Jefferson made 19,000 weather observations across nearly 100 locations, from Monticello in Virginia to Paris, Philadelphia and Washington DC, recording temperature, pressure, wind direction, rainfall and snow. But also the quieter rhythms of the natural world: when the first peas appeared in his garden, and which birds had returned for spring.

His routine was iron-clad. "My method is to make two observations a day, the one as early as possible in the morning, the other from 3 to 4 aclock," he wrote, "because I have found 4 aclock the hottest and daylight [dawn] the coldest point of the 24 hours".

At Monticello - his primary residence - he installed a weathervane on the roof connected to a compass dial on the ceiling of the East Portico, so he could read the wind direction without going outside.

He also devised his own method for measuring rainfall, setting out the calculations in his weather records.

Dr James McClure, General Editor of the Papers of Thomas Jefferson at Princeton University, says Jefferson was arguably less interested in weather than in climate, trying to understand the mechanisms by which it changed over time. A question that feels, two centuries on, urgently familiar.

There was a political and patriotic dimension too.

The French naturalist Georges-Louis Leclerc, Comte de Buffon had argued that North America's cold, damp climate caused all life there to degenerate, its animals to be smaller and feebler, its people to be diminished in body and mind.

Jefferson devoted the longest chapter of his book - Notes on the State of Virginia - to disputing this with tables of data.

It wasn't enough. He wanted Buffon to recant in public, so he arranged for a friend in New Hampshire to find and stuff the largest moose available, antlers and all, and ship it across the Atlantic to Paris.

Jefferson enlisted friends on both sides of the Atlantic to keep their own weather diaries too, hoping to be "exchanging observations from time to time. I should like to compare the two climates by contemporary observations".

In the 1770s, he had planned to go further: a thermometer in the hands of one deputy in every county of Virginia, requiring twice-daily readings. It was the seed of an early national weather network, but the Revolutionary War put paid to the scheme.

When Meriwether Lewis and William Clark set off on the first US transcontinental expedition in 1804, Jefferson's official instructions required them to observe the climate "as characterised by the thermometer, by the proportion of rainy, cloudy and clear days, by lightning, hail, snow, ice."

The full significance of Jefferson's records is only now becoming clear.

Alison Dolbier, an editor of the Thomas Jefferson Weather and Climate Records Project, notes that coordinated weather networks didn't emerge until the 1850s, leaving scientists with less than two centuries of systematic data at a time when understanding long-term climate change matters more than ever.

Individual diaries and almanacs help fill that gap, she says: "Jefferson's Weather Memoranda, and other paper records buried in archives, add to the data record giving us a longer view on how climate changes naturally over time, which in turn will help determine the best courses of action for the future."

So why did he do it?

Dr Andrew Davenport, Vice President for Research at the International Center for Jefferson Studies, argues it wasn't separate from Jefferson's politics: "Jefferson believed that a nation would be virtuous if its people lived in harmony with their environment," he says, "and dedicated so much of his life to advance knowledge about the natural world."

Jefferson kept his weather records until late June 1826, just days before his death.

He died on 4 July, 50 years to the day after independence, a coincidence that struck contemporaries as remarkable in itself.

The networks he had tried to build eventually took shape. Weather observation spread across America, the telegraph made real-time forecasting possible, and in 1890 the US Weather Bureau, now the National Weather Service, was formally established.

As America marks 250 years of independence this 4 July, the satellites tracking hurricanes across the Gulf of Mexico, the forecasts warning millions of approaching storms, the vast network of stations blanketing the country: few individuals shaped that story more profoundly than one man recording the temperature before breakfast every morning, every day. Until the very end.