FEATURE ARTICLE
Revolutionary Minds
Thomas Jefferson and James Madison participated in a small "revolution" against British weather-monitoring practices
Susan Solomon, John S. Daniel, Daniel L. Druckenbrod
Consternation
In early 1790, Jefferson returned to the United States, spending a few months at his home at Monticello and taking daily meteorological data as he always did. These provided the first chance for direct comparison of simultaneous data from Monticello to those at the Madison plantation just 21 miles away. Jefferson was not specific about the location of his instrument while taking these data. He found to his apparent dismay that there were great differences in the two measurement series. He wrote to his son-in-law about the large discrepancy, trying to explain it by the change in elevation between the two, all the while noting that even in his own view this explanation seemed far-fetched:
You know that during my short stay at Monticello I kept a diary of the weather. Mr. Madison has just received one, comprehending the same period, kept at his father's in Orange. The hours of observation were the same, and he has the fullest confidence in the accuracy of the observer. All the morning observations in Orange are lower than those of Monticello, from one to, I believe, 15 or 16 degrees: the afternoon observations are near as much higher than those of Monticello. Nor will the variations permit us to ascribe them to any supposed irregularities in either tube, because, in that case, at the same point the variations would always be the same, which it is not. You have often been sensible that in the afternoon, or rather evening, the air has become warmer in ascending the mountain. The same is true in the morning. This might account for a higher station of the mercury in the morning observations at Monticello. Again when the air is equally dry in the lower & higher situations, which may be supposed the case in the warmest part of the day, the mercury should be lower on the latter, because, all other circumstances the same, the nearer the common surface the warmer the air. So that on a mountain it ought really to be warmer in the morning & cooler in the heat of the day than on the common plain; but not in so great a degree as these observations indicate. As soon as I am well enough I intend to examine them more accurately" (TJ to Thomas Mann Randolph, May 30, 1790)
Figure 6 shows the morning and evening observations taken in February 1790 at Monticello and Montpelier that were the subject of Jefferson's consternation. Elevation does not explain the differences, as despite being referred to by Jefferson as a "mountain," Monticello is only a few hundred feet higher than Montpelier. Jefferson's measurements reflect the clear influence of the thermal lag of his home, whereas Madison had already moved his instrument to the box on the porch by then and was therefore taking data with a morning-to-afternoon range in broad agreement with modern outdoor data as shown in Figure 3.
Many historians have puzzled over Jefferson's early observations and have wondered for example whether the apparently rather mild temperatures he recorded for July 4, 1776, are consistent with other suggestions that the Declaration of Independence was signed during a sweltering week. Figure 6 illustrates that the indoor bias was often more than 10 degrees. If the placement of Jefferson's thermometer in Philadelphia were similar to that at Monticello, then the outdoor conditions during the historic week of July 4, 1776, were virtually certain to have been much warmer than those recorded at that time by the man who had led the crafting of the document that would become an icon of American government.
Jefferson's observations through much of the 1790s largely suggest an indoor (or perhaps window-mounted) bias. Monticello was also under frequent remodeling and construction over decades, with roofing and other changes throughout. The northeast portico (Figure 4) took many years to complete, and in the late 1790s it became the location for the thermometer. But Jefferson doubted the data obtained there, writing in his log book that "all the thermometrical observations of the year 1798 and those of 1799 to June 17 inclusive are to be rejected. During that period my thermometer had been placed in the N.E. portico, newly built, and it was not until Jun 17 … that I discovered it to be artificially heated, probably by the mound of earth in it." Figure 3 makes it evident that the morning-to-afternoon differences for June of 1799 were actually more representative of the outdoors than his earlier data rather than less so. Although he did not realize it, he was not measuring the effect of an "artificially heated" mound but more likely a genuine improvement afforded by better ventilation of his instrument. His friend Madison had much earlier written a letter that should have provided a clue:
The most remarkable occurrence of late date here was the excessive heat on Sunday the first instant. At two Oclock the thermometer in its ordinary position was at 99°[°F]. At four it had got up to 103°. On being taken into the passage the coolest part of the House it stood at the former hour at 97° and at the latter at 98°.… (JM to TJ, July 5, 1792)
These remarks about instrument positioning demonstrate that not only his family but also Madison himself were quite engaged with the issue of thermometer location. But Madison's remarks went unanswered by Jefferson. Further, it is not clear whether Jefferson ever expressed his quandary over the differences in their observations directly to Madison.
History leaves no doubt that Jefferson and Madison were the closest of friends. But it would be natural for a Virginian aristocrat to decline to criticize data obtained by a friend at considerable time and expense, particularly when these observations had been acquired in direct response to a request. Their correspondence during the 1790s also makes clear that both men were engrossed in matters of far greater urgency and national importance, including the deepening rift with Alexander Hamilton. Jefferson described his conflicts with Hamilton in Washington's cabinet as akin to "cocks in a pit."
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