The monsoon, or rainy season, refers to the winds that originate over the Indian Ocean and the Arabian Sea and blow towards the southwest coast of India, bringing heavy rainfall to India, Pakistan, Bangladesh, and other regions. These are seasonal winds that are active in the South Asian region for approximately four months, from June to September. The term was first used in the context of British India (present-day India, Pakistan, and Bangladesh) and neighboring countries. It was used to describe the large-scale seasonal winds that blow from the Bay of Bengal and the Arabian Sea, bringing heavy rainfall to the region from the southwest. In hydrology, the term "monsoon" has a broader meaning—it refers to any wind system that brings the majority of rainfall to a region during a particular season. It is important to note that a monsoon does not necessarily mean continuous rainfall. According to this broader definition, other regions of the world, such as North America, South America, sub-Saharan Africa, Australia, and East Asia, can also be considered monsoon regions. The word "monsoon" comes from the Arabic word "mausim," meaning season. The monsoon is entirely dependent on wind patterns. It occurs when the prevailing winds change direction. As these winds move from cooler to warmer regions, they pick up moisture, resulting in rainfall.
Most summer monsoons are characterized by prevailing westerly winds, which bring heavy rainfall. This is because these winds carry a high amount of moisture as they rise. However, the intensity and duration of these monsoons vary from year to year. In contrast, winter monsoons are dominated by easterly winds, which tend to descend and spread out, resulting in generally dry weather.
Monsoon systems around the world
The major monsoon systems in the world include those of West Africa and Asia-Australia. There is still debate about whether the monsoon systems of North America and South America should also be included in this category.
South Asian Monsoon
Indian Monsoon
The monsoon in India depends on the winds that blow from the Indian Ocean and the Arabian Sea towards the Himalayas. When these winds collide with the Western Ghats along India's southwestern coast, they bring heavy rainfall to India and neighboring countries. These winds are active in South Asia from June to September. Generally, the monsoon of any region depends on its climate. India has a tropical climate and is primarily influenced by two types of winds – the northeast monsoon and the southwest monsoon. The northeast monsoon is often called the winter monsoon. These winds blow from land towards the sea, crossing the Indian Ocean, the Arabian Sea, and the Bay of Bengal. Most of the rainfall in India occurs due to the southwest monsoon. The Tropic of Cancer passes through India from east to west, which directly affects the country's climate. Of the three seasons – summer, winter, and monsoon – the monsoon season is often referred to as the rainy season.
Generally, during the monsoon season, the temperature drops, but the humidity (moisture) increases considerably. Humidity plays an important role in meteorology. It is the amount of water vapor present in the atmosphere, which reaches the atmosphere from the earth through various forms of evaporation.
East Asian Monsoon
The East Asian monsoon affects vast areas of Indochina, the Philippines, China, Korea, and Japan. Its main characteristic is a hot, rainy summer and a cool, dry winter. Most of the rainfall occurs within a specific east-west zone, except in eastern China, where the rain extends northeastward to Korea and Japan. This seasonal rainfall is known as May-yu in China, Changma in Korea, and Bai-u in Japan. In southern China and Taiwan, the summer monsoon rains begin with pre-monsoon downpours in early May. From May to August, the summer monsoon progresses, alternating between dry and wet periods. It begins in May over Indochina and the South China Sea, reaches the Yangtze River and Japan by June, and finally extends to northern China and Korea by July. The monsoon season ends in August, and the rainfall retreats southward.
The African Monsoon
The monsoon in West-Sub-Saharan Africa was previously thought to be caused by the seasonal movement of the Intertropical Convergence Zone (ITCZ) and the temperature and humidity differences between the Sahara Desert and the equatorial Atlantic Ocean. It moves northward from the equatorial Atlantic Ocean in February, reaching West Africa by around June 22, and then retreats southward by October. The dry north-westerly trade winds, and their extreme form, the Harmattan, are influenced by the northward movement of the ITCZ, while the southerly winds generated by this movement bring the summer rains. The semi-arid regions of the Sahel and Sudan depend on this pattern for the majority of their annual rainfall.
North American Monsoon
The North American Monsoon (also called NAM) lasts from late June or early July through September. It originates over Mexico and brings rain to the southwestern United States by mid-July. Its influence increases rainfall and humidity in the Sierra Madre Occidental of Mexico, as well as in Arizona, New Mexico, Nevada, Utah, Colorado, western Texas, and California. It extends westward to the peninsular regions and the Transverse Ranges of southern California, but rarely reaches the Pacific coast. The North American Monsoon is also known as the summer monsoon, the southwestern monsoon, the Mexican monsoon, or the Arizona monsoon. It is sometimes called the desert monsoon, as most of its affected area lies within the Mojave and Sonoran Deserts.
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