Intertropical Convergence Zone in the context of Hadley Circulation

Rain is a form of precipitation where water droplets that have condensed from atmospheric water vapor fall by gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems.

The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough, or Intertropical Convergence Zone, brings rainy seasons to savannah climes.

The North Atlantic Gyre of the Atlantic Ocean is one of five great oceanic gyres. It is a circular ocean current, with offshoot eddies and sub-gyres, across the North Atlantic from the Intertropical Convergence Zone (calms or doldrums) to the part south of Iceland, and from the east coasts of North America to the west coasts of Europe and Africa.

In turn it is chiefly subdivided into the Gulf Stream flowing northward along the west; its often conflated continuation, the North Atlantic Current across the north; the Canary Current flowing southward along the east; and the Atlantic's North Equatorial Current in the south. The gyre has a pronounced thermohaline circulation, bringing salty water west from the Mediterranean Sea and then north to form the North Atlantic Deep Water.

The climate of Australia is the second driest of any continent, after Antarctica. According to the Bureau of Meteorology (BOM), 80% of the land receives less than 600 mm (24 in) of rainfall annually and 50% has even less than 300 mm (12 in). As a whole, Australia has a very low annual average rainfall of 419 mm (16 in).

This dryness is governed mostly by the subtropical high pressure belt (subtropical ridge), which brings dry air from the upper atmosphere down onto the continent. This high pressure is typically to the south of Australia in the summer and over the north of Australia in the winter. Hence Australia typically has dry summers in the south and dry winters in the north. The Intertropical Convergence Zone also moves south in Australia's summer, bringing the Australian monsoon to parts of northern Australia. The climate is variable, with frequent droughts lasting several seasons, caused in part by the El Niño-Southern Oscillation. Australia has a wide variety of climates due to its large geographical size. The largest part of Australia is desert or semi-arid. Only the south-east and south-west corners have a temperate climate and moderately fertile soil. The northern part of the country has a tropical climate, varying between grasslands and desert, and subject to some of the largest interannual rainfall variability in the world. Australia holds many heat-related records: the continent has the hottest extended region year-round, the areas with the hottest summer climate, and the highest sunshine duration.

A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its location and strength, a tropical cyclone is called a hurricane (/ˈhʌrɪkən, -keɪn/), typhoon (/taɪˈfuːn/), tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a strong tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean. A typhoon is the same thing which occurs in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific, comparable storms are referred to as "tropical cyclones". In modern times, on average around 80 to 90 named tropical cyclones form each year around the world, over half of which develop hurricane-force winds of 65 kn (120 km/h; 75 mph) or more.

Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately condenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are powered primarily by horizontal temperature contrasts. Tropical cyclones are typically between 100 and 2,000 km (62 and 1,243 mi) in diameter. The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, cyclones rarely form within 5° of the equator. South Atlantic tropical cyclones are very rare due to consistently strong wind shear and a weak Intertropical Convergence Zone. In contrast, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea.

A monsoon (/mɒnˈsuːn/) is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ) between its limits to the north and south of the equator. Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase. The term is also sometimes used to describe locally heavy but short-term rains.

The major monsoon systems of the world consist of the West African, Asian–Australian, the North American, and South American monsoons.

The Hadley cell, also known as the Hadley circulation, is a global-scale tropical atmospheric circulation that features air rising near the equator, flowing poleward near the tropopause at a height of 12–15 km (7.5–9.3 mi) above the Earth's surface, cooling and descending in the subtropics at around 30 degrees latitude, and then returning equatorward near the surface. It is a thermally direct circulation within the troposphere that emerges due to differences in insolation and heating between the tropics and the subtropics. On a yearly average, the circulation is characterized by a circulation cell on each side of the equator. The Southern Hemisphere Hadley cell is slightly stronger on average than its northern counterpart, extending slightly beyond the equator into the Northern Hemisphere. During the summer and winter months, the Hadley circulation is dominated by a single, cross-equatorial cell with air rising in the summer hemisphere and sinking in the winter hemisphere. Analogous circulations may occur in extraterrestrial atmospheres, such as on Venus and Mars.

Global climate is greatly influenced by the structure and behavior of the Hadley circulation. The prevailing trade winds are a manifestation of the lower branches of the Hadley circulation, converging air and moisture in the tropics to form the Intertropical Convergence Zone (ITCZ) where the Earth's heaviest rains are located. Shifts in the ITCZ associated with the seasonal variability of the Hadley circulation cause monsoons. The sinking branches of the Hadley cells give rise to the oceanic subtropical ridges and suppress rainfall; many of the Earth's deserts and arid regions are located in the subtropics coincident with the position of the sinking branches. The Hadley circulation is also a key mechanism for the meridional transport of heat, angular momentum and moisture, contributing to the subtropical jet stream, the moist tropics and maintaining a global thermal equilibrium.

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, and is depicted by a line on a weather map showing the locations of minimum sea level pressure.

Westerly monsoon winds lie in its equatorward portion while easterly trade winds exist poleward of the trough. Right along its axis, heavy rains can be found which usher in the peak of a location's respective rainy season. The monsoon trough plays a role in creating many of the world's rainforests.

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 occurs. Generally, the season lasts at least one month. 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. Dry and rainy months are characteristic of tropical seasonal forests: in contrast to tropical rainforests, which 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 to higher latitudes in the middle of the warm season.

A tropical wave (also called easterly wave, tropical easterly wave, and African easterly wave), in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. Tropical waves form in the easterly flow along the equatorial side of the subtropical ridge or belt of high air pressure which lies north and south of the Intertropical Convergence Zone (ITCZ). Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They can lead to the formation of tropical cyclones in the north Atlantic and northeastern Pacific basins. A tropical wave study is aided by Hovmöller diagrams, a graph of meteorological data.

West-moving waves can also form from the tail end of frontal zones in the subtropics and tropics, and may be referred to as easterly waves, but the waves are not properly called tropical waves. They are a form of inverted trough that shares many characteristics of a tropical wave.

The North Equatorial Current (NEC) is a westward wind-driven current mostly located near the equator, but the location varies from different oceans. The NEC in the Pacific and the Atlantic is about 5°-20°N, while the NEC in the Indian Ocean is very close to the equator. It ranges from the sea surface down to 400 m in the western Pacific.

The NEC is driven by the north-hemisphere easterly trade wind. In couple with NEC, there is another current called South Equatorial Current (SEC), generated by the easterly trade wind in the southern hemisphere. Despite the well-coupled name of the two equatorial currents, the distribution of the NEC and the SEC is not in symmetry at the equator, but slightly northward to the equator. This asymmetric distribution is aligned to the location of the Intertropical Convergence Zone (ITCZ), which is the area that the northeast and the southeast trade wind converge.

The Australian monsoon (AUM), also known as the Australian summer monsoon (ASM), and the Australian-Indonesian monsoon (AIM), is a monsoon system that increases thunderstorms and rainfall over many areas of Indonesia and northern Australia, from the far northern tropics of the region to the semi-arid zone of Australia, typically between November and mid-March, which is the wet season of many parts of northern Australia and Indonesia.

The origin of the Australian monsoon (AUM) is comparable the North African monsoon, since both develop from the seasonal motion of the Intertropical Convergence Zone (ITCZ) and the connected meridional shift in the overturning Hadley circulation, which lead to a pronounced rainfall seasonality. From the end of the 19th century, the force of the Australian monsoon has been measured by the summer precipitation at Darwin.

The Harmattan is a season in West Africa that occurs between the end of November and the middle of March. It is characterized by a dry and dusty northeasterly trade wind, of the same name, which blows from the Sahara over West Africa into the Gulf of Guinea. The name is related to the word haramata in the Twi language. The temperature is cold usually at night but can be very hot in certain places during the day. Generally, temperature differences can depend on local circumstances.

The Harmattan blows during the dry season, which occurs during the months with the lowest sun. In this season, the subtropical ridge of high pressure stays over the central Sahara and the low-pressure Intertropical Convergence Zone (ITCZ) stays over the Gulf of Guinea. On its passage over the Sahara, the Harmattan picks up fine dust and sand particles (between 0.5 and 10 micrometres). It is also known as the "doctor wind", because of its invigorating dryness compared with oppressive humid tropical air.