|Part of the nature series|
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. 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.
Character of the rainfall
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 Subsahara, 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 early 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 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.
- Glossary of Meteorology (2013). Rainy season. American Meteorological Society. Retrieved on 2008-12-27.
- Costa Rica Guide (2005). When to Travel to Costa Rica. ToucanGuides. Retrieved on 2008-12-27.
- Michael Pidwirny (2008). CHAPTER 9: Introduction to the Biosphere. PhysicalGeography.net. Retrieved on 2008-12-27.
- "Updated world Köppen-Geiger climate classification map".
- Elisabeth M. Benders-Hyde (2003). World Climates. Blue Planet Biomes. Retrieved on 2008-12-27.
- J . S. 0guntoyinbo and F. 0. Akintola (1983). Rainstorm characteristics affecting water availability for agriculture. IAHS Publication Number 140. Retrieved on 2008-12-27.
- Glossary of Meteorology (2009). Monsoon. American Meteorological Society. Retrieved on 2009-01-16.
- 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.
- J. Horel (2006). Normal Monthly Precipitation, Inches. University of Utah. Retrieved on 2008-03-19.
- Norman W. Junker (2008). West Coast Cold Season Heavy Rainfall Events. Hydrometeorological Prediction Center. Retrieved on 2008-03-01.
- BBC Weather (2009). Country Guide: Italy. British Broadcasting Company. Retrieved on 2008-12-27.
- Patrick Laux et al. (2008): Predicting the regional onset of the rainy season in West Africa. International Journal of Climatology, 28 (3), 329-342.
- Patrick Laux et al. (2009): Modelling daily precipitation features in the Volta Basin of West Africa. International Journal of Climatology, 29 (7), 937-954.,
- David Vandervort (2009). Darfur: getting ready for the rainy season. International Committee of the Red Cross. Retrieved on 2009-02-06.
- Mehari Tesfazgi Mebrhatu, M. Tsubo, and Sue Walker (2004). A Statistical Model for Seasonal Rainfall Forecasting over the Highlands of Eritrea. New directions for a diverse planet: Proceedings of the 4th International Crop Science Congress. Retrieved on 2009-02-08.
- Alex Wynter (2009). Ethiopia: March rainy season "critical" for southern pastoralists. Thomson Reuters Foundation. Retrieved on 2009-02-06.
- The Voice (2009). Botswana: Rainy Season Fills Up Dams. allAfrica.com. Retrieved on 2009-02-06.
- Randy Lascody (2008). The Florida Rain Machine. National Weather Service. Retrieved on 2009-02-06.
- John J. Stransky (1960-01-01). "Site Treatments Have Little Effect During Wet Season in Texas". Tree Planters' Notes 10 (2). Retrieved 2009-08-23.[dead link]
- OCHA Partnership for Humanity (2008). OCHA Field Situation Report: Indonesia – Rainy Season 1 December 2008. United Nations Office for the Coordination of Humanitarian Affairs. Retrieved on 2009-02-06.
- Burarra Gathering (2006). Burarra Gathering. Burarra Gathering. Retrieved on 2009-02-06.
- Tahiti Sun Travel Network (2007). About Bora Bora Island. Retrieved on 2009-02-06.
- Joint Typhoon Warning Center (2006). 2.4 Analysis & Forecasting "Thumb Rules" for the Rainy Season. United States Navy. Retrieved on 2009-02-06.
- Remote Sensing for Migratory Creatures (2002). Phenology and Creature Migration: Dry season and wet season in West Mexico. Arizona Remote Sensing Center. Retrieved on 2009-02-06.
- Horace Burton (2006). The climate of Guyana. Caribbean Institute for Meteorology and Hydrology. The Outfield, August 2006, pp. 3. Retrieved on 2009-02-08.
- 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.
- C. H. Mari, G. Cailley, L. Corre, M. Saunois, J. L. Attie, V. Thouret, and A. Stohl (2007). Biomass burning plumes during the AMMA wet season experiment. Atmospheric Chemistry and Physics Discussions, pp. 17342. Retrieved on 2009-02-06.
- Greek Embassy London (2008). Welcome to Greece. Government of Greece. Retrieved on 2009-02-06.
- D. Bozkurt, O.L. Sen and M. Karaca (2008). Wet season evaluation of RegCM3 performance for Eastern Mediterranean. EGU General Assembly. Retrieved on 2009-02-06.
- Ron Kahana, Baruch Ziv, Yehouda Enzel, and Uri Dayan (2002). "Synoptic Climatology of Major Floods in the Negev Desert, Israel". International Journal of Climatology 22: 869. doi:10.1002/joc.766.
- Michael J. Plagens (2009). What and Where is the Sonoran Desert? Arizonensis. Retrieved on 2009-02-07.
- Official Web Site of District Sirsa, India (2001). District Sirsa. National Informatice Center. Retrieved on 2008-12-27.
- Mei Zheng (2000). The sources and characteristics of atmospheric particulates during the wet and dry seasons in Hong Kong. University of Rhode Island. Retrieved on 2008-12-27.
- S. I. Efe, F. E. Ogban, M. J. Horsfall, E. E. Akporhonor (2005). Seasonal Variations of Physico-chemical Characteristics in Water Resources Quality in Western Niger Delta Region, Nigeria. Journal of Applied Scientific Environmental Management. Retrieved on 2008-12-27.
- C. D. Haynes, M. G. Ridpath, M. A. J. Williams (1991). Monsoonal Australia. Taylor & Francis, pp. 90. ISBN 978-90-6191-638-3. Retrieved on 2008-12-27.
- United States War Department (1909). Road Notes, Cuba. 1909. United States Department of War. Retrieved on 2009-01-16.
- K.W. Choi and J.H.W. Lee (2000). Wet Season Tidal Circulation and flushing in Three Fathoms Cove. 4th International Conference on Hydro-Science and Engineering. Retrieved on 2008-12-27.
- Overseas Security Advisory Council (2009). Warden Message: Guyana Rainy Season Flood Hazards. Overseas Security Advisory Council. Retrieved on 2009-02-05.
- National Flood Insurance Program (2009). California's Rainy Season. Federal Emergency Management Agency. Retrieved on 2009-02-05.
- AFP (2009). Bali Hit By Wet Season Floods. ABC News. Retrieved on 2009-02-06.
- Jack Ainsworth & Troy Alan Doss. Natural History of Fire & Flood Cycles. California Coastal Commission. Retrieved on 2009-02-05.
- FESA (2007). Flood. Government of Western Australia. Retrieved on 2009-02-06.
- King County Department of Development and Environmental Services (2009). Erosion and Sediment Control for Construction Sites. King County, Washington Government. Retrieved on 2009-02-06.
- Charles Darwin University (2009). Characteristics of tropical savannas. Charles Darwin University. Retrieved on 2008-12-27.
- A. Roberto Frisancho (1993). Human Adaptation and Accommodation. University of Michigan Press, pp. 388. ISBN 978-0-472-09511-7. Retrieved on 2008-12-27.
- Marti J. Van Liere, Eric-Alain D. Ategbo, Jan Hoorweg, Adel P. Den Hartog, and Joseph G. A. J. Hautvast. The significance of socio-economic characteristics for adult seasonal body-weight fluctuations: a study in north-western Benin. British Journal of Nutrition: Cambridge University Press, 1994.
- African Centre of Meteorological Application for Development (2008). Ten Day Climate Bulletin: Dekad of 01 to 10 April, 2008. ACMAD. Retrieved on 2009-02-08.
- John P. McNamara, J. France, D. E. Beever (2000). Modelling Nutrient Utilization in Farm Animals. CABI, pp. 275. ISBN 978-0-85199-449-9. Retrieved on 2009-02-06.
- Dr. Lincoln Brower (2005). Precipitation at the Monarch Overwintering Sites in Mexico. Journey North. Retrieved on 2009-02-06.
- Paul M. Brakefield and Torben B. Larsen (1983). The evolutionary significance of dry and wet season forms in some tropical butterflies. Biological Journal of the Linnean Society, pp. 1-12. Retrieved on 2008-12-27.
- Phil Hall (1989). Crocodiles, Their Ecology, Management, and Conservation. International Union for Conservation of Nature and Natural Resources Crocodile Specialist Group, pp. 167. Retrieved on 2008-12-27.
- San Diego Natural History Museum (2009). Bufo californicus: Arroyo Toad. San Diego Natural History Museum. Retrieved on 2009-01-16.
- Linda Deuver (1978). Dry season, wet season. Audubon Magazine, November 1978, pp. 120-130. Retrieved on 2009-02-06.