<p>Solar radiation reaching the Earth&#8217;s surface is a crucial energy source in the climate system and the primary factor regulating the planet energy balance. The amount of solar radiation reaching the Earth surface is conditioned by the atmosphere composition and its transparency that is determined by the content of aerosols, moisture and clouds. The G&#243;rno&#347;l&#261;sko-Zag&#322;&#281;biowska Metropolis (GZM) located in southern Poland, is the most urbanized part of the country and one of the most polluted parts of Europe, which has an impact on the atmosphere transparency and amount of global radiation at the Earth's surface. This study aims to determine the daily and annual variability in global radiation and its relationship with cloudiness, selected cloud types and atmospheric circulation.</p><p>This study is based on unique 10-minute global radiation data measured in the centre of GZM&#160; at the meteorological station of the faculty of Earth Sciences. The data covers the periods between 2002 and 2019. Average radiation intensity was converted into hourly and daily radiation sums expressed in MJ/m<sup>2</sup>. Data on cloudiness were taken from the synoptic station Katowice Muchowiec located 9.6 km far from the meteorological station in GZM. The degree of cloud cover is expressed in a percentage of the sky covered with clouds. To analyse relationships between atmospheric circulation and global radiation, the calendar of circulation types and air masses for southern Poland was used.</p><p>Daily course calculated based on annual data showed that global radiation reached its highest values of 1.5 MJ/m<sup>2</sup> at 10 UTC. The highest hourly sums of global radiation varied seasonally from about 0.5 MJ/m<sup>2</sup> in winter to 2.0 MJ/m<sup>2</sup> in summer. The widest range of variability in particular hours was found in spring (the quartiles: 1.2 - 2.0 MJ/m<sup>2</sup>) and autumn (quartiles: 0.7 to 1.4 MJ/m<sup>2</sup>). It occurred that most cloudiness classes enhanced the global radiation compared to cloudless conditions. The highest radiation sums were recorded during the days with a cloudiness >0 and &#8804;20%. During such days, global radiation was higher by 3.2 MJ/m<sup>2</sup> than during cloudless days and 7.0 MJ/m<sup>2</sup> than the long-term average 2002-2019. Daily global radiation was lower than the long-term average by about 3.0 MJ/m<sup>2</sup> only during days with cloudiness > 80%. Cirrus, cirrostratus, cirrocumulus and cumulus enhanced global radiation by about 40% compared to the long-term average. Altostratus, nimbostratus and stratus reduced the global radiation by about 75% compared to the long-term average. Global radiation also varied depending on circulation types. Extreme values of global radiation were registered under non-advective anticyclonic conditions and during southern advection (maximum 15.0 MJ/m<sup>2</sup>) and during cyclonic types with air advection from the north (minimum 6.8 MJ/m<sup>2</sup>)</p>
Milankovitch, the father of paleoclimate modelling