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The rainy season, or monsoon season, is the time of year when most of a region's average annual rainfall occurs. It usually lasts one or more months.〔Glossary of Meteorology (2013). (Rainy season. ) American Meteorological Society. Retrieved on 2008-12-27.〕 The term "green season" is also sometimes used as a euphemism by tourist authorities.〔Costa Rica Guide (2005). (When to Travel to Costa Rica. ) ToucanGuides. Retrieved on 2008-12-27.〕 Areas with wet seasons are dispersed across portions of the tropics and subtropics.〔Michael Pidwirny (2008). (CHAPTER 9: Introduction to the Biosphere. ) PhysicalGeography.net. Retrieved on 2008-12-27.〕 Under the Köppen climate classification, for tropical climates, a wet season month is defined as a month where average precipitation is or more. In contrast to areas with savanna climates and monsoon regimes, Mediterranean climates have wet winters and dry summers. Tropical rainforests do not have dry or wet seasons, since their rainfall is equally distributed throughout the year.〔Elisabeth M. Benders-Hyde (2003). (World Climates. ) Blue Planet Biomes. Retrieved on 2008-12-27.〕 Some areas with pronounced rainy seasons will see a break in rainfall mid-season, when the intertropical convergence zone or monsoon trough moves to higher latitudes in the middle of the warm season.〔J. S. 0guntoyinbo and F. 0. Akintola (1983). (Rainstorm characteristics affecting water availability for agriculture. ) IAHS Publication Number 140. Retrieved on 2008-12-27.〕 When the wet season occurs during a warm season, or summer, precipitation falls mainly during the late afternoon and early evening . In the wet season, air quality improves, fresh water quality improves, and vegetation grows substantially, leading to crop yields late in the season. Rivers overflow their banks, and some animals retreat to higher ground. Soil nutrients diminish and erosion increases. The incidence of malaria increases in areas where the rainy season coincides with high temperatures. Some animals have adaptation and survival strategies for the wet season. Often, the previous dry season leads to food shortages in the wet season, as the crops have yet to mature. ==Character of the rainfall== In areas where the heavy rainfall is associated with a wind shift, the wet season is known as the monsoon.〔Glossary of Meteorology (2009). (Monsoon. ) American Meteorological Society. Retrieved on 2009-01-16.〕 Rainfall in the wet season is mainly due to daytime heating which leads to diurnal thunderstorm activity within a pre-existing moist airmass, so the rain mainly falls in late afternoon and early evening in savannah and monsoon regions. Further, much of the total rainfall each day occurs in the first minutes of the downpour,〔 before the storms mature into their stratiform stage.〔Robert A. Houze Jr (1997). (Stratiform Precipitation in Regions of Convection: A Meteorological Paradox? ) Bulletin of the American Meteorological Society, pp. 2179. Retrieved on 2008-12-27.〕 Most places have only one wet season, but areas of the tropics can have two wet seasons, because the monsoon trough, or Intertropical Convergence Zone, can pass over locations in the tropics twice per year. However, since rain forests have rainfall spread evenly through the year, they do not have a wet season.〔 It is different for places with a Mediterranean climate. In the western United States, during the cold season from September–May, extratropical cyclones from the Pacific Ocean move inland into the region due to a southward migration of the jet stream during the cold season. This shift in the jet stream brings much of the annual precipitation to the region,〔J. Horel (2006). (Normal Monthly Precipitation, Inches. ) University of Utah. Retrieved on 2008-03-19.〕 and sometimes also brings heavy rain and strong low pressure systems.〔Norman W. Junker (2008). (West Coast Cold Season Heavy Rainfall Events. ) Hydrometeorological Prediction Center. Retrieved on 2008-03-01.〕 The peninsula of Italy has weather very similar to the western United States in this regard.〔BBC Weather (2009). (Country Guide: Italy. ) British Broadcasting Company. Retrieved on 2008-12-27.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Wet season」の詳細全文を読む スポンサード リンク
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