<P> The El Niño - Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics . There is also evidence that global warming is leading to increased precipitation to the eastern portions of North America, while droughts are becoming more frequent in the tropics and subtropics . The eastern half of the contiguous United States east of the 98th meridian, the mountains of the Pacific Northwest, and the Sierra Nevada range are the wetter portions of the nation, with average rainfall exceeding 30 inches (760 mm) per year . The drier areas are the Desert Southwest, Great Basin, valleys of northeast Arizona, eastern Utah, central Wyoming, eastern Oregon and Washington and the northeast of the Olympic Peninsula . Mount Waialeale on the island of Kauaʻi in the Hawaiian Islands averages more than 11,680 millimetres (460 in) of rain over the last 32 years, with a record 17,340 millimetres (683 in) in 1982 . Its summit is considered one of the rainiest spots on earth . It has been promoted in tourist literature for many years as the wettest spot in the world . </P> <P> The annual average rainfall maxima across the continent lie across the northwest from northwest Brazil into northern Peru, Colombia, and Ecuador, then along the Atlantic coast of the Guyanas and far northern Brazil, as well as within the southern half of Chile . Lloró, a town situated in Chocó, Colombia, is probably the place with the largest measured rainfall in the world, averaging 13,300 mm per year (523.6 in). In fact, the whole Department of Chocó is extraordinarily humid . Tutunendo, a small town situated in the same department, is one of the wettest places on earth, averaging 11,394 mm per year (448 in); in 1974 the town received 26,303 mm (86 ft 31⁄2 in), the largest annual rainfall measured in Colombia . Unlike Cherrapunji, which receives most of its rainfall between April and September, Tutunendo receives rain almost uniformly distributed throughout the year . The months of January and February have somewhat less frequent storms . On average, Tutunendo has 280 days with rainfall per year . Over 2⁄3 of the rain (68%) falls during the night . The average relative humidity is 90% and the average temperature is 26.4 ° C. Quibdó, the capital of Chocó, receives the most rain in the world among cities with over 100,000 inhabitants: 9,000 millimetres (350 in) per year . Storms in Chocó can drop 500 mm (19.7 in) of rainfall in a day . This amount is more than falls in many cities in a year's time . The Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina . </P> <P> Aside from the effect on temperature, urban heat islands (UHIs) can produce secondary effects on local meteorology, including the altering of local wind patterns, the development of clouds and fog, the humidity, and the rates of precipitation . The extra heat provided by the UHI leads to greater upward motion, which can induce additional shower and thunderstorm activity . Rainfall rates downwind of cities are increased between 48% and 116% . Partly as a result of this warming, monthly rainfall is about 28% greater between 20 miles (32 km) to 40 miles (64 km) downwind of cities, compared with upwind . Some cities show a total precipitation increase of 51% . Using satellite images, researchers discovered that city climates have a noticeable influence on plant growing seasons up to 10 kilometers (6 mi) away from a city's edges . Growing seasons in 70 cities in eastern North America were about 15 days longer in urban areas compared to rural areas outside of a city's influence . </P>

An area where rainfall is less than 50 cm per year