Wet season

The wet season, also known as the rainy or monsoon season, is the period when a region receives the bulk of its yearly rainfall, spanning at least one month. This phenomenon occurs in scattered areas across the tropics and subtropics. Tourist authorities sometimes refer to it euphemistically as the green season. Under the Köppen climate classification, a month in a tropical climate qualifies as wet if it gets 60 millimeters (2.4 inches) or more of precipitation on average. Unlike savanna and monsoon climates, Mediterranean climates experience wet winters and dry summers. Tropical seasonal forests are defined by alternating dry and rainy months, in contrast to tropical rainforests, which lack distinct seasons due to evenly distributed rainfall year-round. In some places with pronounced rainy seasons, a mid-season break occurs when the Intertropical Convergence Zone or monsoon trough shifts toward higher latitudes during the warm season.

When the wet season falls in summer, rain typically arrives in the late afternoon and early evening. During this time, air and freshwater quality improve, and vegetation grows substantially, leading to crop yields later in the season. Rivers overflow, and some animals retreat to higher ground. Soil nutrients decrease, and erosion increases. In tropical areas where the rainy season coincides with high temperatures, cases of malaria and dengue rise. Some animals have adapted survival strategies for the wet season. Food shortages often follow the previous dry season, as crops have not yet matured. Crops that can be planted successfully during the wet season include cassava, maize, groundnut, millet, rice, and yam. The temperate equivalent of the tropical wet season is spring or autumn.

In regions where heavy rainfall comes with a shift in wind, the wet season is called the monsoon season. Many tropical and subtropical climates follow monsoon rainfall patterns. The rain in the wet season results mainly from daytime heating, which triggers diurnal thunderstorm activity within a moist airmass, so precipitation falls mostly in late afternoon and early evening in savanna and monsoon regions. Much of the daily total rainfall occurs in the first minutes of a downpour, before storms mature into their stratiform stage. Most places have a single wet season, but some tropical areas experience two, as the monsoon trough or Intertropical Convergence Zone passes over them twice a year. Rainforests, with rainfall spread evenly throughout the year, have no wet season.

Savanna climates in Sub-Saharan Africa—including Ghana, Burkina Faso, Darfur, Eritrea, Ethiopia, and Botswana—have a distinct rainy season. Subtropical areas like Florida, South and Southeast Texas, and southern Louisiana in the United States also have one. Monsoon regions include the Indian subcontinent, Southeast Asia (including Indonesia and the Philippines), northern Australia, Polynesia, Central America, western and southern Mexico, the Desert Southwest of the United States, southern Guyana, and northeast Brazil. Northern Guyana has two wet seasons: one in early spring and another in early winter. In western Africa, southern sections have two rainy seasons, while the north has only one. Under the Mediterranean climate, the west coast of the United States, the southwest coast of Australia and South Africa, the Mediterranean coasts of Italy, Spain, Greece, Lebanon, Syria, Algeria, Morocco, Tunisia, and Turkey, as well as inland areas of Western Asia like Jordan, northern Iraq, and most of Iran, experience a wet season in winter. The wet season in Israel’s Negev Desert runs from October through May. The Sonoran Desert, at the boundary between Mediterranean and monsoon climates, receives two rainy seasons, one from each regime. The wet season goes by many local names worldwide; in Mexico, it is called storm season. The First Nations of northern Australia assign different names to short seasons, with the wet season from December to March known as Gudjewg. Its exact meaning is debated, but it is widely linked to severe thunderstorms, flooding, and abundant vegetation growth.

In tropical areas, the arrival of the monsoon lowers high daytime temperatures and raises overnight lows, reducing the daily temperature range. During the wet season, heavy rainfall and, in places like Hong Kong, onshore winds improve air quality. In Brazil, the wet season correlates with weaker trade winds off the ocean. The pH of water becomes more balanced as local aquifers recharge, and water softens because dissolved materials decrease. Erosion increases during rainy periods. Arroyos that are dry at other times fill with runoff, sometimes reaching depths of 10 feet (3.0 meters). Leaching from heavy rainfall depletes soil nutrients. Higher runoff from land affects nearby oceans, making them more stratified and less mixed due to stronger surface currents from the runoff. High rainfall can cause widespread flooding, leading to landslides and mudflows in mountainous areas, with rivers bursting their banks and submerging homes.