Cloudiness and precipitation
A visible effect of the collisions of air masses above Poland is cloudiness. The number of cloudy days is between 60 and 70 percent, which is relatively high. The most cloudy regions are the lake districts in the north and the Sudetes; the least cloudy are Wielkopolska and the Silesian Lowland. The average number of cloudy days a year, with the sky more than 80% overcast, is 120-160; for sunny days, with cloudiness below 20%, it is 30-50. The heaviest precipitation in Poland was recorded in June 1973 in the Tatra's Hala Gasienicowa. During one rain as much as 30cm of water fell. With Poland's predominantly westerly winds, the highest precipitation occurs on western slopes of mountains and hills. In the Carpathians and Sudetes, the annual precipitation is 800-1400mm. In the lowlands and uplands, it ranges between 400mm and 750mm. Similar levels are recorded in the Pomeranian and Masurian lake districts. This is caused by the proximity of the Baltic Sea, from which humid sea air flows east. The lowest precipitation occurs in the eastern part of Wielkopolska and in Kujawy, a region lying in the rain shadow of the Pomeranian Lake District. Occasionally, Poland witnesses extraordinary precipitation. In 1901, when winds brought dust from Sahara, a black-brown rain fell. 71 years later the same phenomenon was responsible for orange snow in Zakopane. The maximum precipitation is in summer. At this time of the year it is on average 2-3 times higher than in winter (in the Carpathians, as much as four times higher). The smallest seasonal differences are recorded in the coastal lowlands. Winter comes to Poland from the north-east. The average annual number of days with snowfall is 30-40 in the country's western and central part, and over 50 days in the north-east. It snows for 120 days a year in the Karkonosze and for 145 days in the Tatras. Snow stays the longest in the mountains (up to 200 days) and in north-west Poland (90-120 days). The western part of the country has the fewest days with snow cover (40-50).
