DAILY AIR TEMPERATURE AND PRESSURE SERIES ... - BALTEX

188 ANDERS MOBERG ET AL.
Figure 7. Differences of smoothed 10th, 50th and 90th percentile sequences for the periods
1961–1998 and 1756–1840. Positive values indicate that 1961–1998 has the warmer temperatures.
The thick curves highligt variability on time scales longer than about two weeks (Gaussian filter
with σ = 4.2). The thin curves highligt variability on time scales longer than about three months
(Gaussian filter with σ = 30).
of summer temperatures is, on the contrary, almost equal for all three percentiles.
There is only a slightly smaller change of the 10th percentile than of the 50th and
90th percentiles. The summer cooling has thus been connected with a general shift
of the entire temperature distribution, with a small tendency towards fewer hot days
dominating an increase in number of chilly summer days.
4.5.4. Frequency of Hot Summer Days and Cold Winter Days
A time-series plot of the changes in frequency of hot summer days and cold winter
days is shown in Figure 8. We define a hot summer day as a day with its mean
temperature exceeding the 95th percentile for all July days 1961–90 (+21.1 ◦ C).
Accordingly, we define a cold winter day as a day with mean temperature below
the 5th percentile for all January days 1961–90 (–13.4 ◦ C).
Many years with a high frequency of cold winter days occurred around the
1770s, 1780s and 1800s. From the 1810s to the 1890s cold winter days became
less frequent. The frequency of cold winter days declined abruptly around 1900
and they were very rare 1900–1939. After the extremely cold 3-year period 1940–
42, the frequency of cold days has varied from decade to decade. Notably, 1985
was one of four years with the highest frequency (23) of cold days in the whole