Monsoon

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Advancing monsoon clouds and showers in Aralvaimozhy, near Nagercoil, India Monsoon clouds near Nagercoil.jpg
Advancing monsoon clouds and showers in Aralvaimozhy, near Nagercoil, India

Monsoon ( /mɒnˈsn/ ) is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, [1] but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. [2] [3] 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 sometimes incorrectly used for locally heavy but short-term rains. [4] [5]

Contents

The major monsoon systems of the world consist of the West African and Asia-Australian monsoons. The inclusion of the North and South American monsoons with incomplete wind reversal has been debated. [6]

The term was first used in English in British India and neighbouring countries to refer to the big seasonal winds blowing from the Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall to the area. [7] [8]

Etymology

Monsoon clouds over Lucknow, Uttar Pradesh, India Monsoon clouds Lucknow.JPG
Monsoon clouds over Lucknow, Uttar Pradesh, India

The etymology of the word monsoon is not wholly certain. [9] The English monsoon came from Portuguese monção, ultimately from Arabic mawsim (موسم "season"), "perhaps partly via early modern Dutch monson." [10]

History

Strengthening of the Asian monsoon has been linked to the uplift of the Tibetan Plateau after the collision of the Indian sub-continent and Asia around 50 million years ago. [11] Because of studies of records from the Arabian Sea and that of the wind-blown dust in the Loess Plateau of China, many geologists believe the monsoon first became strong around 8 million years ago. More recently, studies of plant fossils in China and new long-duration sediment records from the South China Sea led to a timing of the monsoon beginning 15–20 million years ago and linked to early Tibetan uplift. [12] Testing of this hypothesis awaits deep ocean sampling by the Integrated Ocean Drilling Program. [13] The monsoon has varied significantly in strength since this time, largely linked to global climate change, especially the cycle of the Pleistocene ice ages. [14] A study of marine plankton suggested that the Indian Monsoon strengthened around 5 million years ago. Then, during ice periods, the sea level fell and the Indonesian Seaway closed. When this happened, cold waters in the Pacific were impeded from flowing into the Indian Ocean. It is believed that the resulting increase in sea surface temperatures in the Indian Ocean increased the intensity of monsoons. [15]

Five episodes during the Quaternary at 2.22 Ma (PL-1), 1.83 Ma (PL-2), 0.68 Ma (PL-3), 0.45 Ma (PL-4) and 0.04 Ma (PL-5) were identified which showed a weakening of Leeuwin Current (LC). The weakening of the LC would have an effect on the sea surface temperature (SST) field in the Indian Ocean, as the Indonesian through flow generally warms the Indian Ocean. Thus these five intervals could probably be those of considerable lowering of SST in the Indian Ocean and would have influenced Indian monsoon intensity. During the weak LC, there is the possibility of reduced intensity of the Indian winter monsoon and strong summer monsoon, because of change in the Indian Ocean dipole due to reduction in net heat input to the Indian Ocean through the Indonesian through flow. Thus a better understanding of the possible links between El Niño, Western Pacific Warm Pool, Indonesian Throughflow, wind pattern off western Australia, and ice volume expansion and contraction can be obtained by studying the behaviour of the LC during Quaternary at close stratigraphic intervals. [16]

Strength of impact

MatheranPanoramaPointDrySeason.JPG
On May 28, in the dry season
MatheranPanoramaPointMonsoon.JPG
On August 28, in the rainy season
Western Ghats in 2010
This visualization shows the Asian monsoon and how it develops using observational and modeled data. It also shows some of the impacts.

The impact of monsoon on the local weather is different from place to place. In some places there is just a likelihood of having a little more or less rain. In other places, quasi semi-deserts are turned into vivid green grasslands where all sorts of plants and crops can flourish.

The Indian Monsoon turns large parts of India from a kind of semi-desert into green lands. See photos only taken 3 months apart in the Western Ghats. In places like this it is crucial for farmers to have the right timing for putting the seeds on the fields, as it is essential to use all the rain that is available for growing crops.

Process

Monsoons are large-scale sea breezes [17] which occur when the temperature on land is significantly warmer or cooler than the temperature of the ocean. These temperature imbalances happen because oceans and land absorb heat in different ways. Over oceans, the air temperature remains relatively stable for two reasons: water has a relatively high heat capacity (3.9 to 4.2 J g−1 K−1), [18] and because both conduction and convection will equilibrate a hot or cold surface with deeper water (up to 50 metres). In contrast, dirt, sand, and rocks have lower heat capacities (0.19 to 0.35 J g−1 K−1), [19] and they can only transmit heat into the earth by conduction and not by convection. Therefore, bodies of water stay at a more even temperature, while land temperature are more variable.

During warmer months sunlight heats the surfaces of both land and oceans, but land temperatures rise more quickly. As the land's surface becomes warmer, the air above it expands and an area of low pressure develops. Meanwhile, the ocean remains at a lower temperature than the land, and the air above it retains a higher pressure. This difference in pressure causes sea breezes to blow from the ocean to the land, bringing moist air inland. This moist air rises to a higher altitude over land and then it flows back toward the ocean (thus completing the cycle). However, when the air rises, and while it is still over the land, the air cools. This decreases the air's ability to hold water, and this causes precipitation over the land. This is why summer monsoons cause so much rain over land.

In the colder months, the cycle is reversed. Then the land cools faster than the oceans and the air over the land has higher pressure than air over the ocean. This causes the air over the land to flow to the ocean. When humid air rises over the ocean, it cools, and this causes precipitation over the oceans. (The cool air then flows towards the land to complete the cycle.)

Most summer monsoons have a dominant westerly component and a strong tendency to ascend and produce copious amounts of rain (because of the condensation of water vapor in the rising air). The intensity and duration, however, are not uniform from year to year. Winter monsoons, by contrast, have a dominant easterly component and a strong tendency to diverge, subside and cause drought. [20]

Similar rainfall is caused when moist ocean air is lifted upwards by mountains, [21] surface heating, [22] convergence at the surface, [23] divergence aloft, or from storm-produced outflows at the surface. [24] However the lifting occurs, the air cools due to expansion in lower pressure, and this produces condensation.

Global monsoon

Summary table

LocationMonsoon/sub-systemAverage date of arrivalAverage date of withdrawalNotes
Northern MexicoNorth American/Gulf of California-Southwest USAlate May [25] Septemberincomplete wind reversal, waves
Tucson, Arizona, USANorth American/Gulf of California-Southwest USAearly July [25] Septemberincomplete wind reversal, waves
Central AmericaCentral/South American MonsoonApril[ citation needed ]October[ citation needed ]true monsoon
Amazon BrazilSouth American monsoonSeptember[ citation needed ]May[ citation needed ]
Southeast BrazilSouth American monsoonNovember[ citation needed ]March[ citation needed ]
West AfricaWest AfricanJune 22 [26] Sept [27] /October [26] waves
Southeast AfricaSoutheast Africa monsoon w/ HarmattanJan [27] March [27]
Kelantan, MalaysiaIndo-Australian/Borneo-AustralianOctoberMarch
Phuket, ThailandIndo-AustralianFebruary/MarchDecember
Bangkok, ThailandIndo-Australian/Indian-IndochinaApril–MayOctober/Novemberpersistent
Yangon, MyanmarIndo-Australian/Indian-IndochinaMay 25 [28] Nov 1 [28]
Colombo, Sri LankaIndo-AustralianMay 25 [28] Dec 15 [28] persistent
Kerala, IndiaIndian monsoonJun 1 [28] Dec 1 [28] persistent
Jakarta, IndonesiaIndo-Australian/Borneo-AustralianNovemberMarchabrupt
Lahore, PakistanIndian monsoonlate July [28] Sep 1 [28]
Dhaka, BangladeshIndo-Australian/Indian-Indochinamid-JuneOctoberabrupt
Cebu, PhilippinesIndo-Australian/Borneo-AustralianOctoberMarchabrupt
Kaohsiung, TaiwanEast Asian monsoonMay 10 [28]
Taipei, TaiwanEast Asian monsoonMay 20 [28]
Kagoshima, JapanEast Asian monsoonJun 10 [28]
Seoul, South KoreaEast Asian monsoonJuly 10 [28]
Hanoi, VietnamEast Asian monsoonMay 20 [28]
Beijing, ChinaEast Asian monsoonJuly 20 [28]
Karachi, PakistanIndian monsoonJuly 15 [28] August [28]
Mumbai, IndiaIndian monsoonJune 10 [28] Oct 1 [28]
Darwin, AustraliaAustralian monsoonOct [27] April [27]

Africa (West African and Southeast African)

Southeast African moonsoon clouds, over Mayotte island. Saison des pluies a Mayotte.jpg
Southeast African moonsoon clouds, over Mayotte island.

The monsoon of western Sub-Saharan Africa is the result of the seasonal shifts of the Intertropical Convergence Zone and the great seasonal temperature and humidity differences between the Sahara and the equatorial Atlantic Ocean. [29] It migrates northward from the equatorial Atlantic in February, reaches western Africa on or near June 22, then moves back to the south by October. [26] The dry, northeasterly trade winds, and their more extreme form, the harmattan, are interrupted by the northern shift in the ITCZ and resultant southerly, rain-bearing winds during the summer. The semiarid Sahel and Sudan depend upon this pattern for most of their precipitation.

North America

Incoming monsoon clouds over Phoenix, Arizona Incoming monsoon clouds over Arizona.jpg
Incoming monsoon clouds over Phoenix, Arizona
Three-second video of a lightning strike within a thunderstorm over Island in the Sky, Canyonlands National Park

The North American monsoon (NAM) occurs from late June or early July into September, originating over Mexico and spreading into the southwest United States by mid-July. It affects Mexico along the Sierra Madre Occidental as well as Arizona, New Mexico, Nevada, Utah, Colorado, West Texas and California. It pushes as far west as the Peninsular Ranges and Transverse Ranges of Southern California, but rarely reaches the coastal strip (a wall of desert thunderstorms only a half-hour's drive away is a common summer sight from the sunny skies along the coast during the monsoon). The North American monsoon is known to many as the Summer, Southwest, Mexican or Arizona monsoon. [30] [31] It is also sometimes called the Desert monsoon as a large part of the affected area are the Mojave and Sonoran deserts. However, it is debatable whether the North and South American weather patterns with incomplete wind reversal should be counted as true monsoons. [6]

Asia

The Asian monsoons may be classified into a few sub-systems, such as the Indian Subcontinental Monsoon which affects the Indian subcontinent and surrounding regions including Nepal, and the East Asian Monsoon which affects southern China, Taiwan, Korea and parts of Japan.

South Asian monsoon

Southwest monsoon
Onset dates and prevailing wind currents of the southwest summer monsoons in India India southwest summer monsoon onset map en.svg
Onset dates and prevailing wind currents of the southwest summer monsoons in India

The southwestern summer monsoons occur from July through September. The Thar Desert and adjoining areas of the northern and central Indian subcontinent heat up considerably during the hot summers. This causes a low pressure area over the northern and central Indian subcontinent. To fill this void, the moisture-laden winds from the Indian Ocean rush into the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas. The Himalayas act like a high wall, blocking the winds from passing into Central Asia, and forcing them to rise. As the clouds rise, their temperature drops, and precipitation occurs. Some areas of the subcontinent receive up to 10,000 mm (390 in) of rain annually.

The southwest monsoon is generally expected to begin around the beginning of June and fade away by the end of September. The moisture-laden winds on reaching the southernmost point of the Indian Peninsula, due to its topography, become divided into two parts: the Arabian Sea Branch and the Bay of Bengal Branch.

The Arabian Sea Branch of the Southwest Monsoon first hits the Western Ghats of the coastal state of Kerala, India, thus making this area the first state in India to receive rain from the Southwest Monsoon. This branch of the monsoon moves northwards along the Western Ghats (Konkan and Goa) with precipitation on coastal areas, west of the Western Ghats. The eastern areas of the Western Ghats do not receive much rain from this monsoon as the wind does not cross the Western Ghats.

The Bay of Bengal Branch of Southwest Monsoon flows over the Bay of Bengal heading towards North-East India and Bengal, picking up more moisture from the Bay of Bengal. The winds arrive at the Eastern Himalayas with large amounts of rain. Mawsynram, situated on the southern slopes of the Khasi Hills in Meghalaya, India, is one of the wettest places on Earth. After the arrival at the Eastern Himalayas, the winds turns towards the west, travelling over the Indo-Gangetic Plain at a rate of roughly 1–2 weeks per state, [32] pouring rain all along its way. June 1 is regarded as the date of onset of the monsoon in India, as indicated by the arrival of the monsoon in the southernmost state of Kerala.

The monsoon accounts for nearly 80% of the rainfall in India. [33] [34] Indian agriculture (which accounts for 25% of the GDP and employs 70% of the population) is heavily dependent on the rains, for growing crops especially like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can badly affect the economy, as evidenced in the numerous droughts in India in the 1990s.

The monsoon is widely welcomed and appreciated by city-dwellers as well, for it provides relief from the climax of summer heat in June. [35] However, the roads take a battering every year. Often houses and streets are waterlogged and slums are flooded despite drainage systems. A lack of city infrastructure coupled with changing climate patterns causes severe economic loss including damage to property and loss of lives, as evidenced in the 2005 flooding in Mumbai that brought the city to a standstill. Bangladesh and certain regions of India like Assam and West Bengal, also frequently experience heavy floods during this season. Recently, areas in India that used to receive scanty rainfall throughout the year, like the Thar Desert, have surprisingly ended up receiving floods due to the prolonged monsoon season.

The influence of the Southwest Monsoon is felt as far north as in China's Xinjiang. It is estimated that about 70% of all precipitation in the central part of the Tian Shan Mountains falls during the three summer months, when the region is under the monsoon influence; about 70% of that is directly of "cyclonic" (i.e., monsoon-driven) origin (as opposed to "local convection"). [36]

Northeast monsoon
Monsoon clouds in Madhya Pradesh (1) Agriculture and rural farms of India.jpg
Monsoon clouds in Madhya Pradesh

Around September, with the sun fast retreating south, the northern land mass of the Indian subcontinent begins to cool off rapidly. With this air pressure begins to build over northern India, the Indian Ocean and its surrounding atmosphere still holds its heat. This causes cold wind to sweep down from the Himalayas and Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsula. This is known as the Northeast Monsoon or Retreating Monsoon.

While travelling towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka. Cities like Chennai, which get less rain from the Southwest Monsoon, receive rain from this Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu is from the Northeast Monsoon. [37] In Southern Asia, the northeastern monsoons take place from October to December when the surface high-pressure system is strongest. [38] The jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow across southern Asia, creating dry air streams which produce clear skies over India. Meanwhile, a low pressure system known as a monsoon trough develops over South-East Asia and Australasia and winds are directed toward Australia.

East Asian Monsoon

Monsoon floods in Philippines 3933Typhoons Krosa Lekima monsoon tidal flooding in Calumpit, Bulacan 13.jpg
Monsoon floods in Philippines

The East Asian monsoon affects large parts of Indo-China, Philippines, China, Taiwan, Korea and Japan. It is characterised by a warm, rainy summer monsoon and a cold, dry winter monsoon. The rain occurs in a concentrated belt that stretches east-west except in East China where it is tilted east-northeast over Korea and Japan. The seasonal rain is known as Meiyu in China, Jangma in Korea, and Bai-u in Japan, with the latter two resembling frontal rain.

The onset of the summer monsoon is marked by a period of premonsoonal rain over South China and Taiwan in early May. From May through August, the summer monsoon shifts through a series of dry and rainy phases as the rain belt moves northward, beginning over Indochina and the South China Sea (May), to the Yangtze River Basin and Japan (June) and finally to North China and Korea (July). When the monsoon ends in August, the rain belt moves back to South China.

Australia

Monsoonal squall nears Darwin, Northern Territory, Australia Evening monsoonal squall.jpg
Monsoonal squall nears Darwin, Northern Territory, Australia

Also known as the Indo-Australian Monsoon. The rainy season occurs from September to February and it is a major source of energy for the Hadley circulation during boreal winter. The Maritime Continent Monsoon and the Australian Monsoon may be considered to be the same system, the Indo-Australian Monsoon.

It is associated with the development of the Siberian High and the movement of the heating maxima from the Northern Hemisphere to the Southern Hemisphere. North-easterly winds flow down Southeast Asia, are turned north-westerly/westerly by Borneo topography towards Australia. This forms a cyclonic circulation vortex over Borneo, which together with descending cold surges of winter air from higher latitudes, cause significant weather phenomena in the region. Examples are the formation of a rare low-latitude tropical storm in 2001, Tropical Storm Vamei, and the devastating flood of Jakarta in 2007.

The onset of the monsoon over the Maritime Continent tends to follow the heating maxima down Vietnam and the Malay Peninsula (September), to Sumatra, Borneo and the Philippines (October), to Java, Sulawesi (November), Irian Jaya and Northern Australia (December, January). However, the monsoon is not a simple response to heating but a more complex interaction of topography, wind and sea, as demonstrated by its abrupt rather than gradual withdrawal from the region. The Australian monsoon (the "Wet") occurs in the southern summer when the monsoon trough develops over Northern Australia. Over three-quarters of annual rainfall in Northern Australia falls during this time.

Europe

The European Monsoon (more commonly known as the return of the westerlies) is the result of a resurgence of westerly winds from the Atlantic, where they become loaded with wind and rain. [39] These westerly winds are a common phenomenon during the European winter, but they ease as spring approaches in late March and through April and May. The winds pick up again in June, which is why this phenomenon is also referred to as "the return of the westerlies". [40]

The rain usually arrives in two waves, at the beginning of June and again in mid- to late June. The European monsoon is not a monsoon in the traditional sense in that it doesn't meet all the requirements to be classified as such. Instead the return of the westerlies is more regarded as a conveyor belt that delivers a series of low pressure centres to Western Europe where they create unsettled weather. These storms generally feature significantly lower than average temperatures, fierce rain or hail, thunder and strong winds. [41]

The return of the westerlies affects Europe's Northern Atlantic coastline, more precisely Ireland, Great Britain, the Benelux countries, Western Germany, Northern France and parts of Scandinavia.

See also

Related Research Articles

Geography of India geography of the country of India

India lies on the Indian Plate, the northern part of the Indo-Australian Plate, whose continental crust forms the Indian subcontinent. The country is situated north of the equator between 8°04' to 37°06' north latitude and 68°07' to 97°25' east longitude. It is the seventh-largest country in the world, with a total area of 3,287,263 square kilometres (1,269,219 sq mi). India measures 3,214 km (1,997 mi) from north to south and 2,933 km (1,822 mi) from east to west. It has a land frontier of 15,200 km (9,445 mi) and a coastline of 7,516.6 km (4,671 mi).

Orographic lift

Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.

Atmospheric circulation The large-scale movement of air, a process which distributes thermal energy about the Earths surface

Atmospheric circulation is the large-scale movement of air, and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth.

Low-pressure area region where the atmospheric pressure is lower than that of surrounding locations

A low-pressure area, low, depression or cyclone is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies. A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.

Tibetan Plateau a plateau in Central Asia

The Tibetan Plateau, also known in China as the Qinghai–Tibet Plateau or the Qing–Zang Plateau or Himalayan Plateau, is a vast elevated plateau in Central Asia and East Asia, covering most of the Tibet Autonomous Region and Qinghai in western China, as well as Ladakh and Lahaul & Spiti in India. It stretches approximately 1,000 kilometres (620 mi) north to south and 2,500 kilometres (1,600 mi) east to west. It is the world's highest and largest plateau, with an area of 2,500,000 square kilometres (970,000 sq mi). With an average elevation exceeding 4,500 metres (14,800 ft) and being surrounded by imposing mountain ranges that harbor the world's two highest summits, Mount Everest and K2, the Tibetan Plateau is often referred to as "the Roof of the World".

Trade winds permanent east-to-west prevailing winds that flow in the Earths equatorial region

The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, strengthening during the winter and when the Arctic oscillation is in its warm phase. Trade winds have been used by captains of sailing ships to cross the world's oceans for centuries and enabled colonial expansion into the Americas and trade routes to become established across the Atlantic and Pacific oceans.

Prevailing winds

The prevailing wind in a region of the Earth's surface is a surface wind that blows predominantly from a particular direction. The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface. A region's prevailing and dominant winds are the result of global patterns of movement in the Earth's atmosphere. In general, winds are predominantly easterly at low latitudes globally. In the mid-latitudes, westerly winds are dominant, and their strength is largely determined by the polar cyclone. In areas where winds tend to be light, the sea breeze/land breeze cycle is the most important cause of the prevailing wind; in areas which have variable terrain, mountain and valley breezes dominate the wind pattern. Highly elevated surfaces can induce a thermal low, which then augments the environmental wind flow.

Climate of India Climate of india

The climate of India comprises a wide range of weather conditions across a vast geographic scale and varied topography, making generalisations difficult. Based on the Köppen system, India hosts six major climatic subtypes, ranging from arid deserts in the west, alpine tundra and glaciers in the north, and humid tropical regions supporting rainforests in the southwest and the island territories. Many regions have starkly different microclimates, making it one of the most climatically diverse countries in the world. The country's meteorological department follows the international standard of four climatological seasons with some local adjustments: winter, summer, a monsoon means rainy season, and a post-monsoon period.

North Indian Ocean tropical cyclone type of tropical cyclone located in North Indian Ocean and measured by India Meteorological Department (IMD) scale.

In the Indian Ocean north of the equator, tropical cyclones can form throughout the year on either side of India. On the east side is the Bay of Bengal, and on the west side is the Arabian Sea.

Monsoon trough

The monsoon trough is a portion of the Intertropical Convergence Zone in the Western Pacific, as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and northern hemispheres.

The climate of Delhi is an overlap between monsoon-influenced humid subtropical and semi-arid, with high variation between summer and winter temperatures and precipitation. Delhi's version of a humid subtropical climate is markedly different from many other humid subtropical cities such as Sao Paulo, New Orleans and Brisbane in that the city features dust storms and wildfire haze due to its semi-arid climate.

Natural disasters in India

Natural disasters in India, many of them related to the climate of India, cause massive losses of life and property. Droughts, flash floods, cyclones, avalanches, landslides brought by torrential rains, and snowstorms pose the greatest threats. A natural disaster might be caused by earthquakes, flooding, volcanic eruption, landslides, hurricanes etc. In order to be classified as a disaster it will have profound environmental effect and/or human loss and frequently incurs financial loss. Other dangers include frequent summer dust storms, which usually track from north to south; they cause extensive property damage in North India and deposit large amounts of dust from arid regions. Hail is also common in parts of India, causing severe damage to standing crops such as rice and wheat and many more crops.

Western Disturbance continuous rainfall and flood

A Western Disturbance is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the northwestern parts of the Indian subcontinent. It is a non-monsoonal precipitation pattern driven by the westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean. Extratropical storms are a global phenomena with moisture usually carried in the upper atmosphere, unlike their tropical counterparts where the moisture is carried in the lower atmosphere. In the case of the Indian subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas.

Thermal low

Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments. Thermal lows occur near the Sonoran Desert, on the Mexican plateau, in California's Great Central Valley, the Sahara, over north-west Argentina in South America, over the Kimberley region of north-west Australia, the Iberian peninsula, and the Tibetan plateau.

North American Monsoon california monsoons

The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon, is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern United States and northwestern Mexico, typically occurring between July and mid September. During the monsoon, thunderstorms are fueled by daytime heating and build up during the late afternoon-early evening. Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily. The monsoon typically loses its energy by mid-September when drier and cooler conditions are reestablished over the region. Geographically, the North American monsoon precipitation region is centered over the Sierra Madre Occidental in the Mexican states of Sinaloa, Durango, Sonora and Chihuahua.

Climate of Vietnam

Vietnam's climate, being located in the tropics and strongly influenced by the South China Sea has a monsoon-influenced tropical climate typical of that of mainland Southeast Asia. In the north, the climate is monsoonal with four distinct seasons while in the south, the climate is tropical monsoon with two seasons. In addition temperate climate exists in mountainous areas, which are found in Sa Pa, Da Lat while a more continental climate exists in Lai Chau Province and Son La Province. The diverse topography, wide range of latitudes, and influences from the South China Sea lead to climatic conditions varying significantly between regions. 20% of Vietnam's total surface area is low-elevation coastal area making the country highly vulnerable to climate change effects and the rising sea levels in particular.

Indian Monsoon Current The seasonally varying ocean current regime found in the tropical regions of the northern Indian Ocean

The Indian Monsoon Current refers to the seasonally varying ocean current regime found in the tropical regions of the northern Indian Ocean. During winter, the flow of the upper ocean is directed westward from near the Indonesian Archipelago to the Arabian Sea. During the summer, the direction reverses, with eastward flow extending from Somalia into the Bay of Bengal. These variations are due to changes in the wind stress associated with the Indian monsoon. The seasonally reversing open ocean currents that pass south of India are referred to as the Winter Monsoon Current and the Summer Monsoon Current. The Somali Current, which is strongly linked to the Indian monsoon, is also discussed in this article.

Earth rainfall climatology

Earth rainfall climatology Is the study of rainfall, a sub-field of Meteorology. Formally, a wider study includes water falling as ice crystals, i.e. hail, sleet, snow.

Climate of Asia

The climate of Asia is dry across southeast sections, and dry across much of the interior. Some of the largest daily temperature ranges on Earth occur in western sections of Asia. The monsoon circulation dominates across southern and eastern sections, due to the presence of the Himalayas forcing the formation of a thermal low which draws in moisture during the summer. Southwestern sections of the continent experience low relief as a result of the subtropical high pressure belt; they are hot in the summer, warm to cool in winter, and may snow at higher altitudes. Siberia is one of the coldest places in the Northern Hemisphere, and can act as a source of arctic air mass for North America. The most active place on Earth for tropical cyclone activity lies northeast of the Philippines and south of Japan, and the phase of the El Nino-Southern Oscillation modulates where in Asia landfall is more likely to occur.

Monsoon of South Asia Monsoon in Asian Countries

The monsoon of South Asia is among several geographically distributed global monsoons. It affects the Indian subcontinent, where it is one of the oldest and most anticipated weather phenomena and an economically important pattern every year from June through September, but it is only partly understood and notoriously difficult to predict. Several theories have been proposed to explain the origin, process, strength, variability, distribution, and general vagaries of the monsoon, but understanding and predictability are still evolving.

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Further reading