Two hot days in March 1777

I’m reading Gilbert White’s Natural History of Selborne slowly, and checking his weather against the instrument record whenever he gives me something to check. In a letter to Daines Barrington dated November 22, 1777, he writes:

You cannot but remember that the twenty-sixth and twenty-seventh of last March were very hot days; so sultry that everybody complained and were restless under those sensations to which they had not been reconciled by gradual approaches.

…for on those two days the thermometer rose to sixty-six in the shade; many species of insects revived and came forth; some bees swarmed in this neighbourhood; the old tortoise, near Lewes in Sussex, awakened and came forth out of its dormitory; and, what is most to my present purpose, many house-swallows appeared and were very alert in many places, and particularly at Cobham, in Surrey.

But as that short warm period was succeeded as well as preceded by harsh severe weather, with frequent frosts and ice, and cutting winds, the insects withdrew, the tortoise retired again into the ground, and the swallows were seen no more until the tenth of April…

The Met Office’s daily Central England Temperature series (HadCET, Parker et al. 1992) begins in 1772, so it covers those days. Before looking, I wrote down three guesses:

March 26 and 27 are each at least 5 °C above the 1772–1800 average for those dates, and both are among the three warmest days of March 1777.
November 4, 1777, the “dark and mild, and soft” morning when twenty martins reappeared for a few hours, is at least 3 °C above average.
The five days before and the five days after the heat are both colder than average.

Results.

Yes, strongly. March 26 was 14.6 °C (+8.5) and March 27 was 14.8 °C (+9.2), the two warmest days of that March. Their mean, 14.7 °C, is the warmest March 26–27 in the entire series, 1772–2026, 2.5 °C ahead of second place (2006, 12.2 °C). A daily mean near 15 °C fits a shade maximum of 66 °F (19 °C).
Yes. November 4, 1777 was 11.5 °C, +5.0.
Half. The five days after average −1.3 °C, as he says. The five days before average +1.4 °C, because the warm spell had already started on the 24th and 25th. The “harsh severe weather” before it is in the record, but it was earlier in the month: March 7–14 sat between 1 and 4 °C.

Caveats. Early daily CET is built from only a few stations, so a single 1777 value carries more uncertainty than a modern one, and a record margin this large is a reason for some suspicion. What makes me trust it more is that White is a separate witness in a different county, with his own thermometer and a list of things the heat did (insects, swarming bees, a tortoise, swallows). As far as I know, none of the series’ stations was at Selborne. Some of his evidence is second-hand: the tortoise was in Sussex, the swallows in Surrey.

What he made of it. White used these days as an argument that swallows hibernate nearby: “we cannot suppose that… house-swallows should leave the districts of Africa to enjoy, in March, the transient summer of a couple of days.” He was wrong about the swallows, and right about the days. An instrument record 249 years later finds his “couple of days” exactly where he put them.

Data: HadCET daily, downloaded October 19, 2026. Text: Project Gutenberg’s Natural History of Selborne*, letter XXXVI to Barrington. The guesses were written into the command before the numbers were printed. Earlier that evening, before writing this, I had pre-registered five other guesses about White’s weather (I’ve made more since, and I’m keeping a full scorecard). Two passed (a very wet autumn and winter in 1773–74, second-wettest of sixty; a dry spring in 1775, fifth-driest). Three failed: November 1771 was a little warmer than my “ordinary” guess allowed (+0.8 °C, consistent with White’s “warm noons”); June 1774, his “black cold solstice”, was average by the month and by the eleven days around the solstice (a three-day cold snap on June 21–23 turned up only when I went looking after both tests had failed, so I don’t count it); and September 1775, “as wet a month as ever was known”, was wet but ninth of sixty.*