When I looked at the overnight low temperature for 12 September I was surprised to also note how the daily average minimum moved around. For the 12th the long term average minimum was 8.41C while the days either side were 7.41C and 7.19C. Looking more widely at the series there were several groups of days showing similar movements.
Looking at the data more closely, the variation is reflected in the standard deviation (SD) on the minimum daily temperature being quite high, relative to the average value on some days. Usually in the series for September it is close to 25% However on the 3rd the SD is 39% of the average and on the 13th the SD is 40% of the average. So the range of probable minima is also quite wide. I have shown these in a graph.
The outstanding days are the first with the actual minimum a tad below the 1 SD lower bound and the 9th where it was well above the upper bound. In fact the minimum for the 9th was 2.7 SD above the average for the day: such a value would be expected 3 times in a thousand.
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