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The climate of Paraguay consists of a subtropical climate in the Paraneña region and a tropical climate in the Chaco. The Paraneña region has a humid climate, with abundant precipitation throughout the year and only moderate seasonal changes in temperature.
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During the Southern Hemisphere's summer, which corresponds to the northern winter, the dominant influence on the climate is the warm sirocco winds blowing out of the northeast.
During the winter, the dominant wind is the cold pampero from the South Atlantic, which blows across Argentina and is deflected northeastward by the Andes in the southern part of that country. Because of the lack of topographic barriers within Paraguay, these opposite prevailing winds bring about abrupt and irregular changes in the usually moderate weather. Winds are generally brisk. Velocities of 160 kilometres per hour (99 mph) have been reported in southern locations, and the town of Encarnación was once leveled by a tornado.
Paraneña region
The Paraneña region has only two distinct seasons: summer from October to March and winter from May to August. In transitional months of April and September, temperatures below the midsummer averages include nocturnal minima that may dip below freezing.
Winter
During July, the coldest month of the subtropical winter, temperature averages about 17.87 °C (64.17 °F) in Asunción and slightly cooler on the Paraná Plateau. Temperature varies only insignificantly with latitude. The number of days with temperatures falling below freezing ranges from as few as 3 to as many as 16 yearly, and with even wider variations deep in the interior. Some very mild winters with winds blowing constantly from the north feature little frost.
During a cold winter, however, tongues of Antarctic air bring subfreezing temperatures to all areas. No part of the Paraneña region is entirely free from the possibility of frost and consequent damage to crops, and various locations report snow flurries occasionally.
Summer
Moist tropical air keeps the weather warm in the Paraneña region from October through March. In Asunción the temperature averages about 27.96 °C (82.33 °F) during January, the warmest month.
During the summer, daytime temperatures fairly commonly reach 38 °C (100 °F). Frequent waves of cool air from the south, however, cause weather that alternates between clear, humid conditions and storms.
Precipitation
Fairly evenly distributed rain falls in the Paraneña region. Although local meteorological conditions play a contributing role, rain usually falls in dominant tropical air masses. The least rain falls in August, when averages in various parts of the region range from 100 millimetres (3.9 in) to 200 millimetres (7.9 in). The two periods of maximum precipitation are March through May and October to November.
For the region as a whole, the difference between the driest and the wettest months ranges from 100 millimetres (3.9 in) to 180 millimetres (7.1 in). The annual average rainfall is 1,270 millimetres (50 in), although the average on the Paraná Plateau is 250 millimetres (9.8 in) to 380 millimetres (15 in) greater. All subregions may experience considerable variations from year to year. Asunción has recorded as much as 208 millimetres (8.2 in) and as little as 560 millimetres (22 in) of annual rainfall; Puerto Bertoni on the Paraná Plateau has recorded as much as 3,300 millimetres (130 in) and as little as 790 millimetres (31 in).
Chaco
In contrast to the Paraneña region, the Chaco has a tropical wet-and-dry climate bordering on semi-arid. The Chaco experiences seasons that alternately flood and parch the land, yet seasonal variations in temperature are modest. Chaco temperatures are usually high, the averages dropping only slightly in winter. Even at night the air is stifling despite the usually present breezes. Rainfall is light, varying from 500 to 1,000 millimeters (19.7 to 39.4 in) per year, except in the higher land to the northwest where it is somewhat greater. Rainfall is concentrated in the summer months, and extensive areas that are deserts in winter become summer swamps.