*3.2. Hot Days and Heat Waves in the Years 1966–2015 (Pozna ´n–Ławica)*

Hot days (i.e., days above the 95th annual percentile—>27.8 ◦C) in Pozna ´n occurred on average 18 times a year. In individual seasons, the number varied from two days (1980) to as many as 37 days (2006), and the standard deviation was 9.1 days (Figure 3). In the period analysed, there was a statistically significant (*p* < 0.05) increase in the number of hot days (2.9 days/10 years). Days with Tmax > 27.8 ◦C were observed from April to October, although the largest number were reported in July and August, representing 39.8% and 31.3% of all hot days, respectively. In total, there were seven days in April and only one day in October.

**Figure 3.** Number of hot days in 1966–2015 and the frequency of hot days' occurrence in certain months.

In the analysed multiannual period, there were 42 heat waves in total, which lasted 310 days (Table 2). Five- and six-day waves were the most common and accounted for, respectively, 34.9% and 20.9% of all heat waves. The longest heat wave was recorded in 1994, which lasted 18 days (from 21 July to 7 August). Long heat waves were observed in 2015 (16 days, from 2 to 17 August), 2006 (14 days, from 18 to 31 July), 1969 and 1971 (12 days—27 July to 3 August and 24 July to 4 August, respectively). The average heat wave length in the multiannual period analysed was 7.4 days, while in the individual years it ranged from six (1996–2005) to 8.8 days (2006–2015). Heat waves occurred between May and August, although most of them (as much as 56%) occurred in July. The earliest recorded heat wave in 1979 was from 31 May to 7 June, while the latest one in 1997 occurred from 22 to 27 August.


The mean Tmax during the heat waves analysed was 30.5 ◦C, while Tmin was 16.2 ◦C (Table 2). The highest mean Tmax was found during the heat wave of 1992 (6–11 August), which was 33.9 ◦C, while the lowest average Tmax value, which was 28.9 ◦C, was seen in 1992 (24–28 July), 1997 (22–27 August) and 2001 (26–30 July). During the longest heat wave, i.e., that lasting from 21 July to 7 August, 1994, the average Tmax was 32.1 ◦C, while the average Tmin was 16.1 ◦C.
