Two-thirds of Paraguay is within the temperate zone, one-third in the tropical zone. The climate varies from mild to subtropical. During the winter months (roughly May through August), temperatures generally range from 16° to 21° C (60° to 70° F ); nights are occasionally colder. During the summer (October through March), temperatures range from about 25° to 38° C (77° to 100° F ), with extremes of 43° C (109° F ) and above in the west. Paraguay is open to dry, cold polar winds from the south and to hot, humid north winds from southwestern Brazil; sudden sharp drops in temperatures are not uncommon. Rainfall averages about 152 cm (60 in) a year along the eastern frontier with Brazil, gradually diminishing toward the west to an average of 127 cm (50 in) along the Paraguay River and 76 cm (30 in) in the Chaco. Asunción has an annual average of about 130 cm (50 in), which is moderate for its latitude. There is no definite rainy season, although violent thunderstorms sometimes occur in the summer.