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The monsoon season or rainy season, is the time of year, covering one or more months, when most of the average annual rainfall in a region occurs. The term green season is also sometimes used as a euphemism by tourist authorities. Areas with wet seasons are dispersed across portions of the tropics and subtropics. Under the Köppen climate classification, for tropical climates, a wet season month is defined as a month where average precipitation is 60 millimetres (2.4 in) or more.  In contrast to areas with savanna climates and monsoon regimes, mediterranean climates have wet winters and dry summers. Tropical rainforests technically do not have dry or wet seasons, since their rainfall is equally distributed throughout the year. Some areas with pronounced rainy seasons will see a break in rainfall mid-season, when the intertropical convergence zone or monsoon trough moves poleward of their location during the middle of the warm season.
When the wet season occurs during a warm season, or summer, precipitation falls mainly during the late afternoon and early evening hours. The wet season is a time when air quality improves, freshwater quality improves, and vegetation grows substantially, leading to crop yields late in the season. Floods cause rivers to overflow their banks, and some animals to 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. Animals have adaptation and survival strategies for the wetter regime. Often, the previous dry season leads to food shortages in the wet season, as the crops have yet to mature.
In areas where the heavy rainfall is associated with a wind shift, the wet season becomes known as the monsoon. Since rainfall during the wet season is predominantly due to daytime heating which leads to diurnal thunderstorm activity within a pre-existing moist airmass, rainfall is mainly focused during the late afternoon and early evening hours within savannah and monsoon regimes. This also leads to much of the total rainfall each day falling during the initial minutes of the downpour, before the storms mature into their stratiform stage. While most locations have only one wet season, areas of the tropics can experience two wet seasons as the monsoon trough, or Intertropical Convergence Zone, can pass over locations in the tropics twice per year. Since rain forests have equitable rainfall throughout the year, they do not technically have a wet season.
The situation is different for locations within the Mediterranean climate regime. In the western United States, during the cold season from September through 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, and also brings the potential for heavy rain and strong low pressure systems. The peninsula of Italy experiences very similar weather to the western United States in this regard.
Areas with a savanna climate in Sub-Saharan Africa, such as Ghana, Burkina Faso, Darfur, Eritrea, Ethiopia, and Botswana have a distinct rainy season. Also within the savannah climate regime, Florida and East Texas have a rainy season. Monsoon regions include southeast Asia (including Indonesia and Philippines), northern sections of Australia's North, Polynesia, Central America, western and southern Mexico, the Desert Southwest of the United States, southern Guyana, portions of northeast Brazil.
Northern Guyana experiences two wet seasons: one in late spring and the other in early winter. In western Africa, there are two rainy seasons across southern sections with only one across the north. Within the Mediterranean climate regime, the west coast of the United States and the Mediterranean coastline of Italy, Greece, and Turkey experience a wet season in the winter months. Similarly, the wet season in the Negev desert of Israel extends from October through May. At the boundary between the Mediterranean and monsoon climates lies the Sonoran desert, which receives the two rainy seasons associated with each climate regime. The wet season is known by many different local names throughout the world. For example, the wet season period of the year in Mexico is known as storm season, with other tropical locations across the globe giving the wet season period of the year in the area they live a name in the local language of the area. Arguably, the most notable example of this is found in the list of names given to the various short "seasons" of the year by the Aboriginal tribes of Northern Australia: the name given to the wet season typically experienced in Northern Australia from December to March by the various Aboriginal tribes living in the region is Gudjewg. The precise meaning of the word is fairly disputable, although it's widely accepted to relate to the severe thunderstorms, flooding, and abundant vegetation growth commonly experienced at this time.
In tropical areas, when the monsoon arrives daytime high temperatures drop and overnight low temperatures increase. During the wet season, a combination of heavy rainfall and in some areas, such as Hong Kong, a wind more off the ocean, significantly improve air quality. In Brazil, the wet season is correlated to weaker trade winds off the ocean. The pH level of water becomes more balanced due to the charging of local aquifers during the wet season. Water also softens, as dissolved materials lower in concentration during the rainy season. Erosion is also increased during rainy periods. Arroyos that are dry at other times of the year fill with runoff, in some cases with water as deep as 10 feet (3.0 m). Leaching of soils during periods of heavy rainfall depletes nutrients. The excessive runoff from land masses significantly impacts nearby ocean areas, which are more stratified, or less mixed, due to stronger surface currents forced by the heavy rainfall runoff.
Widespread flooding can occur if rainfall becomes excessive, which can lead to landslides and mudflows in mountainous areas. Such floods cause rivers to leave their banks and homes to go underwater. Floods can be exacerbated by fires during the previous dry season, which cause soils which are sandy or composed of loam to become hydrophobic, or repellent of water. There are various ways government organizations help their residents deal with wet season floods. Flood plain mapping is conducted, which helps diagnose what areas are more prone to flooding. Instructions on how to control erosion through outreach is also done via telephone or the internet.
The wet season is the main period of vegetation growth within the Savanna climate regime. However, this also means that wet season is a time for food shortages before crops reach their full maturity. This causes seasonal weight changes for people in developing countries, with a drop occurring during the wet season until the time of the first harvest, when weights rebound. Malaria incidence increases during periods of high temperature and heavy rainfall.
Cows calve, or give birth, at the beginning of the beor wet season. The onset of the rainy season signals the departure of the Monarch butterfly from Mexico. Tropical species of butterflies show larger dot markings on their wings to fend off possible predators and are more active during the wet season than the dry season. Within the tropics and warmer areas of the subtropics, decreased salinity of near shore wetlands due to the rains causes an increase in crocodile nesting. Other species, such as the arroyo toad, spawn within the couple of months after the seasonal rains. Armadillos and rattlesnakes seek higher ground.
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