President Jefferson Meets Mount Tambora

Guest Post by Willis Eschenbach

In the comments of my last post entitled “Spot The Volcano, 1815 Edition” someone mentioned that Thomas Jefferson had commented on what is often called the “Year Without A Summer”. This was the summer of the year 1816, one year after the eruption of Mt. Tambora in Indonesia, which occurred in April of 1815.

Some research turned up Jefferson’s weather notebook entitled “Analysis of Weather Memorandum Book, January 1817”. All of the data in this post is from his analysis. He opens by saying:

1817. January. Having been stationary at home since Mar.1 1809. with opportunity and leisure to keep a meteorological diary, with a good degree of exactness, this has been done: and, extracting from it a term of seven years compleat, to wit from Jan. 1. 1810. to Dec. 31. 1816. I proceed to analyse it in the various ways, and to deduce the general results which are of principal effect in the estimate of climate. the observations (3905. in the whole) were taken before sunrise of every day; and again between 3. and 4. aclock P.M. on some days of occasional absence, they were necessarily omitted. in these cases the averages are taken from the days of the same denomination in the other years only, and in such way as not sensibly to affect the average of the month, still less that of the year, and to be quite evanescent in their effect on the whole term of 7. years.

In other words, a good and careful observer. In addition, he took the maximum and minimum values at the same times of day, before dawn and between three and four o’clock. Well done, that man.

Since our interest is in the summers, the first graph that I made up from Jefferson’s data is of the May to September growing season in Monticello (see below for growing seasons). Here are the temperature means, minimums, and maximums.

Figure 1. May to September average temperatures at Monticello, Virginia, as recorded by Thomas Jefferson. Horizontal straight colored lines show the averages from 1810 to 1815.

Now, Figure 1 shows some interesting things. The average May to September temperature (yellow line) in 1816 was about the same that of 2015. However, the maximum temperature is about a degree warmer than 1815, and the minimum temperatures were about a degree lower than in 1815. Max goes up, mean unchanged, but the minimum was unusually cold. In particular, the months of July and August had cold minima. So Tambora may have had some effect on temperatures at Monticello.

Jefferson also recorded some other interesting weather data. He wrote down the days of “white frost”, also known as a “killing frost”. He recorded the last day of white frost in the spring, and also the first day of white frost in the fall. This lets us see if 1816, the “Year Without A Summer”, had killing frosts later or earlier than other years. Here’s that information.

Figure 2. Days of the first and last frosts at Monticello, Virginia, as recorded by Thomas Jefferson.

The year 1816 was not at all unusual as regards to frost. The last frost of the spring happened around its usual time. And although it had the earliest fall frost during the period, as Jefferson commented, “but we have seen, in another period, a destructive white frost as early as September.” Not to mention that the growing season was the fourth longest of the seven years of record.

How about rainfall? Here are the month-by-month values:

Figure 3. Monthly rainfall at Monticello, Virginia, as recorded by Thomas Jefferson.

I’ve highlighted the June to August periods because that period was indeed unusually dry in 1816. However, both May and September 1816 were unusually wet, and the year 1816 overall had the fourth highest annual rainfall of the seven years.

Finally, Jefferson considered that when the temperature was below 55°F you’d need a fire. So he listed the number of times that a fire would be necessary, both in the morning and in the afternoon. In his words:

“It is generally observed that when the thermometer is below 55.° we have need of fire in our apartments to be comfortable. in the course of these 7. years the number of observations below 55.° in each year were as follows.”

Here are the number of days per year that Jefferson thought would require a fire either in the morning, the afternoon, or both:

Figure 4. The number of days requiring fires in the morning and the afternoon at Monticello, Virginia, as recorded by Thomas Jefferson. Photo shows one of the many fireplaces in Jefferson’s home at Monticello.

As you can see, 1816 required the least morning fires of the seven years, and it only needed an average number of fires in the afternoon.

So … the growing season of the year of 1816 was dry and had both cold and warm months. In the fall of that year, before he wrote the summary of the data used in this analysis, Jefferson commented in a letter to a friend that “We have had the most extraordinary year of drought and cold ever known in the history of America.” However, his own records show that, as is the habit of humans the world over, he was exaggerating … shocking, I know, that a US President would exaggerate … but I digress.

I say he exaggerated because contrary to his claim, the facts in his meticulous recordkeeping for the year 1816 show less than a half-degree C of May – September cooling, no unusually late or early frosts, an average length growing season, no need for extra fires in the morning, and fewer fires than usual needed in the afternoons. And although June through August was dry, overall the year 1816 was fourth among the seven years in the total amount of rain.

So was there a “Year Without A Summer” at Monticello?

I’d say that the eruption of Tambora may well have some effects at Monticello, with a couple of quite cold months and a couple of quite dry months, but the reports of those effects have been greatly exaggerated, even by Thomas Jefferson himself …

My warmest regards to all,

w.

PS—When you comment, to avoid misunderstandings, please quote the exact words that you are discussing.