3.4.4. Atmospheric Circulation on 12 July 2010 and 11 August 2015

The analysis showed the different effects of land development on the thermal conditions in the city on selected days; the weather situation in Poland occurring on these days was therefore analysed. On 12 July 2010, Poland was within the weak-gradient high-pressure system, while on 11 August 2015 it was within the reach of the trough of low pressure (Figure 10). Both pressure situations provided an influx of warm air masses from the east and southeast sector. Recorded anomalies indicate the presence of warm air masses, whose temperature was higher by more than 8 ◦C than average in the summer. The 500 hPa isobaric surface on both days over central Poland settled over 140 m higher than average during the summer season. On 11 August, a warm weather front moved from the south to the north

of the country, providing the exchange of air masses over the area. Advection of tropical air masses took place after the weather front (Figure 11). The different thermal conditions in the city during the two selected days were due to the differentiation of atmospheric circulation. On 12 July 2010, the air over the city was additionally warmed up by settling for longer over the urban area with anticyclonic weather. In turn, the passage of the weather front on the second of the days analysed resulted in significantly lower temperature values.

**Figure 10.** SLP and z500 hPa (**a**), SLP and z500 hPa anomalies (**b**) and T850 anomalies (**c**) on 12 July 2010 (**right column**) and 11 August 2015 (**left column**).

**Figure 11.** Weather situation on 12 July 2010 (**a**) and 11 August 2015 (**b**). Source: Meteorology and Water Management–National Research Institute.
