To commemorate the event, the Unscientific Society threw their stone into Helsinki’s South Harbour on Wednesday. Surface waters will remain warm for a while, despite the supposedly cooling effect of Jaakko’s rock.

”This year Jaakko once again will throw a warming stone, provided that the forecast of hot weather for the end of the week is correct,” says Johanna Korhonen, a hydrologist at the Finnish Environment Institute.

Whatever predictive value Jaako’s stone did have was destroyed by the switch from the Julian to the Gregorian calendar in 1753. That involved the elimination of eleven days in the year, and naming days were moved.

Recent warmth makes water feel cooler

Up until that point, Jaako’s naming day had been on 5 August, around the time Finland’s lakes, rivers and coastal waters do tend to begin to cool.

Korhonen and her colleague Esko Kuusisto have trawled the records from 35 different locations since the 1960s. They found that in some locations, such as Tuusulanjärvi just north of Helsinki and Lohjanjärvi in the south-west, temperatures are a couple of degrees below the long-term average.

Despite that, this summer’s cool waters are unexceptional. Korhonen points out that the last couple of years’ hot, sunny summers may have skewed perceptions enough to make a fairly normal year seem unusually cold.

Record in 2001

Waters were cooler during the summers of 2004, 2008 and 2009 than they are this year.

Sometimes surface water temperatures can drop as many as five degrees in June. That kind of change relies on windy weather churning up lakes and bringing cooler water to the surface.

”The biggest drop was in 1970 in Pielinen (a lake in North Karelia), where the temperature dropped 12 degrees in one day,” says Korhonen. ”Pelinen always sees quite large fluctuations because it is a deep lake.”

The highest temperature ever recorded in a Finnish lake was 28.4 degrees Celsius at Tuusulanjärvi in 2001.

Korhonen points out that the official measurements are always taken at 8am, meaning that in all likelihood the lake hit an even higher temperature by the middle of that day, and gave Jaako’s stone an awful lot of cooling work to do that year.