Poland's long-term and short-term weather patterns are made transitional and variable by the collision of diverse air masses above the country's surface. Maritime air moves across Western Europe, Arctic air sweeps down from the North Atlantic Ocean, and subtropical air arrives from the South Atlantic Ocean. Although the Arctic air dominates for much of the year, its conjunction with warmer currents generally moderates temperatures and generates considerable precipitation, clouds, and fog. When the moderating influences are lacking, winter temperatures in mountain valleys may drop to max -20 °C.
The spring arrives slowly in March or April, bringing mainly sunny days after a period of alternating wintry and springtime conditions. Summer, which extends from June to August, is generally less humid than winter. Showers and thunderstorms alternate with dry sunny weather that is generated when southern winds prevail. Early autumn is generally sunny and warm before a period of rainy, colder weather in November begins the transition into winter. Winter, which may last from one to three months, brings frequent snowstorms but relatively low total precipitation.
The range of mean temperatures is 6 °C in the northeast to 10 °C in the southwest, but individual readings in Poland's regions vary widely by season. On the highest mountain peaks, the mean temperature is below 0 °C. The Baltic coast, influenced by moderating west winds, has cooler summers and warmer winters. The other temperature extreme is in the southeast along the border with Ukraine, where the greatest seasonal differences occur and winter temperatures average 4.5 °C below those in western Poland. The growing season is about 40 days longer in the southwest than in the northeast, where spring arrives latest. The hottest cities in Poland are Tarnów, Wrocław and Słubice.
The average annual precipitation for the whole country is 600 mm, but isolated mountain locations receive as much as 1,300 mm per year. The total is slightly higher in the southern uplands than in the central plains. A few areas, notably along the Vistula between Warsaw and the Baltic Sea and in the far northwest, average less than 500 mm. In winter about half the precipitation in the lowlands and the entire amount in the mountains falls as snow. On the average, precipitation in summer is twice that in winter, providing a dependable supply of water for crops.