The wet season (sometimes called the rainy season or monsoon season) is the time of year when most of a region's average annual

rainfall Rain is a form of precipitation where water droplets that have condensed from atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. ...

occurs. Generally, the season lasts at least one month. The term ''green season'' is also sometimes used as a

euphemism A euphemism ( ) is when an expression that could offend or imply something unpleasant is replaced with one that is agreeable or inoffensive. Some euphemisms are intended to amuse, while others use bland, inoffensive terms for concepts that the u ...

by tourist authorities. Areas with wet seasons are dispersed across portions of the

tropics The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

and

subtropics The subtropical zones or subtropics are geographical and climate zones immediately to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately ...

. Under the

Köppen climate classification The Köppen climate classification divides Earth climates into five main climate groups, with each group being divided based on patterns of seasonal precipitation and temperature. The five main groups are ''A'' (tropical), ''B'' (arid), ''C'' (te ...

, for

tropical climate Tropical climate is the first of the five major climate groups in the Köppen climate classification identified with the letter A. Tropical climates are defined by a monthly average temperature of or higher in the coolest month, featuring hot te ...

s, a wet season month is defined as a month where average precipitation is or more. In contrast to areas with

savanna A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) biome and ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach th ...

climates and

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

regimes,

Mediterranean climate A Mediterranean climate ( ), also called a dry summer climate, described by Köppen and Trewartha as ''Cs'', is a temperate climate type that occurs in the lower mid-latitudes (normally 30 to 44 north and south latitude). Such climates typic ...

s have wet winters and dry summers. Dry and rainy months are characteristic of

tropical seasonal forest Seasonal tropical forest, also known as moist deciduous, semi-evergreen seasonal, tropical mixed or monsoon forest, typically contains a range of tree species: only some of which drop some or all of their leaves during the dry season. This tropic ...

s: in contrast to

tropical rainforest Tropical rainforests are dense and warm rainforests with high rainfall typically found between 10° north and south of the Equator. They are a subset of the tropical forest biome that occurs roughly within the 28° latitudes (in the torrid zo ...

s, which 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 The Intertropical Convergence Zone (ITCZ , or ICZ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the t ...

monsoon trough The monsoon trough is a convergence zone between the wind patterns of the southern and northern hemispheres. It is a portion of the Intertropical Convergence Zone in the Western Pacific,Bin WangThe Asian Monsoon.Retrieved 2008-05-03. and is dep ...

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 Summer or summertime is the hottest and brightest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, daylight hours are the longest and darkness hours are the shortest, with day ...

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

falls mainly during the late afternoon and early evening. In the wet season,

air quality Air pollution is the presence of substances in the Atmosphere of Earth, air that are harmful to humans, other living beings or the environment. Pollutants can be Gas, gases like Ground-level ozone, ozone or nitrogen oxides or small particles li ...

improves, fresh

water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...

improves, and

vegetation Vegetation is an assemblage of plants and the ground cover they provide. It is a general term, without specific reference to particular Taxon, taxa, life forms, structure, Spatial ecology, spatial extent, or any other specific Botany, botanic ...

grows substantially, leading to crop yields late in the season. Rivers overflow their banks, and some animals retreat to higher ground.

Soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

nutrients diminish and erosion increases. The incidence of

malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...

and dengue increases in areas where the rainy season coincides with high temperatures, particularly in tropical areas. 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. Crops which can be successfully planted during the wet or rainy season are cassava, maize, peanut, groundnut, millet, rice and Yam (vegetable), yam. The temperate counterpart to the tropical wet season is Spring (season), spring or autumn.

Character of the rainfall

In areas where the heavy rainfall is associated with a wind shift, the wet season is known as the

season. Many tropical and subtropical climates experience monsoon rainfall patterns. 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 savanna and monsoon regions. Much of the total rainfall each day occurs in the first minutes of the downpour, before the storms mature into their stratiform stage. Most places have only one wet season, but areas of the tropics can have two wet seasons, because the monsoon trough, or

, 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.

Areas affected

Areas with a

climate in Sub-Saharan Africa, such as Ghana, Burkina Faso, Darfur, Eritrea, Ethiopia, and Botswana have a distinct rainy season. Also subtropical areas like Florida, South Texas, South and Southeast Texas, Southeast Texas, and southern Louisiana in the United States have a rainy season. Monsoon regions include the Indian subcontinent, Southeast Asia (including Indonesia and Philippines), northern sections of Australia, Polynesia, Central America, western and southern Mexico, the Desert Southwest of the United States, southern Guyana, and northeast Brazil.James Brian Elsner (1988).
Analysis of Wet Season Rainfall Over the Nordeste of Brazil, South America
. University Of Wisconsin-Milwaukee. Retrieved on 2009-02-06. Northern Guyana has two wet seasons: one in early spring and the other in early winter. In western Africa, there are two rainy seasons across southern sections, but only one across the north. Within the Mediterranean climate regime, the west coast of the United States, the southwest coast of Australia and South Africa, the Mediterranean Basin, Mediterranean coastline of Italy, Spain, Greece, Lebanon, Syria, Algeria, Morocco, Tunisia, and Turkey, as well as areas further inland in Western Asia which include Jordan, Northern Iraq and most parts of Iran, 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, in Mexico it is known as "storm season". Different names are given to the various short "seasons" of the year by the First Nations of Northern Australia: the wet season typically experienced there from December to March is called ''Gudjewg''. The precise meaning of the word is disputed, although it is widely accepted to relate to the severe thunderstorms, flooding, and abundant vegetation growth commonly experienced at this time.

Effects

In tropical areas, when the monsoon arrives, high daytime high temperatures drop and overnight low temperatures increase, thus reducing diurnal temperature variation. During the wet season, a combination of heavy rainfall and, in some places such as Hong Kong, an onshore wind, improve

. In Brazil, the wet season is correlated with 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 the concentration of dissolved materials reduces during the rainy season. Erosion is also increased during rainy periods. Arroyo (watercourse), Arroyos that are dry at other times of the year fill with runoff, in some cases with water as deep as .United States War Department (1909)
Road Notes, Cuba. 1909.
United States Department of War. Retrieved on 2009-01-16. Leaching of soils during periods of heavy rainfall depletes nutrients. The higher runoff from land masses affects nearby ocean areas, which are more Ocean stratification, stratified, or less mixed, due to stronger surface currents forced by the heavy rainfall runoff.

Floods

High rainfall can cause widespread flooding, which can lead to landslides and mudflows in mountainous areas. Such floods cause rivers to burst their banks and submerge homes. The Ghaggar-Hakra River, which only flows during India's monsoon season, can flood and severely damage local crops. Floods can be exacerbated by fires that occurred during the previous dry season, which cause soils which are sandy or composed of loam to become hydrophobic, or water repellent. In various ways governments may help people deal with wet season floods. Flood plain mapping identifies which areas are more prone to flooding. Instructions on controlling erosion through outreach are also provided by telephone or the internet.

Life adaptations

Humans

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.

Animals

Cows calve, or give birth, at the beginning of the wet season. The onset of the rainy season signals the departure of the monarch butterfly from Mexico. Tropical species of butterfly, 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.Linda Deuver (1978)
Dry season, wet season.
Audubon Magazine, November 1978, pp. 120–130. Retrieved on 2009-02-06.

References

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