Located entirely within the temperate zone, Uruguay has a climate that is fairly uniform nationwide. Seasonal variations are pronounced, but extremes in temperature are rare. As would be expected by its abundance of water, high humidity and fog are common. The absence of mountains, which act as weather barriers, makes all locations vulnerable to high winds and rapid changes in weather as fronts or storms sweep across the country.
Seasons are fairly well defined, and in most of Uruguay spring is usually damp, cool, and windy; summers are warm; autumns are mild; and winters are chilly and uncomfortably damp. Northwestern Uruguay, however, is farther from large bodies of water and therefore has warmer summers and milder and drier winters than the rest of the country. Average highs and lows in summer (January) in Montevideo are 28° C and 17° C, respectively, with an absolute maximum of 43° C; comparable numbers for Artigas in the northwest are 33° C and 18° C, with the highest temperature ever recorded (42° C).
Winter (July) average highs and lows in Montevideo are 14° C and 6° C, respectively, although the high humidity makes the temperatures feel colder; the lowest temperature registered in Montevideo is -4° C. Averages in July of a high of 18° C and a low of 7° C in Artigas confirm the milder winters in northwestern Uruguay, but even here temperatures have dropped to a subfreezing -4° C. Rainfall is fairly evenly distributed throughout the year, and annual amounts increase from southeast to northwest.
Montevideo averages 950 millimeters annually, and Artigas receives 1,235 millimeters in an average year. As in most temperate climates, rainfall results from the passage of cold fronts in winter, falling in overcast drizzly spells, and summer thunderstorms are frequent.
High winds are a disagreeable characteristic of the weather, particularly during the winter and spring, and wind shifts are sudden and pronounced. A winter warm spell can be abruptly broken by a strong pampero, a chilly and occasionally violent wind blowing north from the Argentine pampas. Summer winds off the ocean, however, have the salutary effect of tempering warm daytime temperatures.