According to Southwest Climate Change, the sun heats the land and water differently, forcing the winds to "tug of war," ultimately switching directions and delivering cooler, moister air from over the ocean. At the end of the monsoon season, the winds reverse. The dry air from the desert moves in, bringing with it a return of hot, humid conditions.
The monsoon is one of the most important factors determining where we get our water. Without these frequent rainstorms, many parts of India would be suffering from severe drought.
The monsoon also has great impact on climate everywhere else in the world that gets significant rainfall during this time. For example, when the monsoon declines, so too does the global temperature because less water is evaporating from lakes and rivers around the world. Conversely, when the monsoon increases its activity, then there is more evaporation and heat is absorbed by water molecules in the atmosphere leading to warmer temperatures worldwide.
In addition to these global effects, the monsoon has major consequences for India's geography and economy. The arrival and departure of the monsoon determine where water can be found in India. If the monsoon is late, then the reservoirs will be low; if it is early, then the floods will be high. This leads to economic losses since farmers need at least some amount of water for their crops.
A Shift in the Wind All winds are caused by pressure differences between two points. This pressure imbalance occurs during monsoons when temperatures across broad landmasses, such as India and Asia, are much warmer or colder than those over nearby oceans. The resulting air movement is called a "monsoon-associated flow". During a monsoon season, the wind usually changes direction daily. Although the cause of this shift is not fully understood, it is believed to be related to the movement of large areas of warm air over cooler regions on either side of the equator.
During a monsoon season, the wind usually shifts direction each day around noon. The reason for this is that, near the equator, the sun rises in the east and sets in the west. Thus, any wind coming from the north or south would meet the equal force of the earth's gravity heading back toward the origin of the wind. At midday, however, the sun is due west of the earth at 90 degrees west longitude. Because the sun no longer acts as a force against these directions of wind, they become the new dominant winds at that time of day.
For example, if the sun were still rising in the east at noon, any wind coming from that direction would have to fight against the pull of gravity all day long, which would be very difficult for anything but a strong wind.
Monsoon winds are variations in trade wind direction caused by differential heating of the continent and sea. In comparison to the water, the landmass warms up and cools down quickly. Monsoon winds go from high-pressure areas to low-pressure areas. Changes in air pressure cause the direction of monsoon winds to shift. For example, when the high-pressure area moves to the east, then the normal course of the winds would be west. But because it is a monsoon, the normal course will be south.
The Indus River Valley has been affected by both the Indian and the Chinese Monsoons. The Indus receives moisture from the Indian Ocean through the Gulf of Cambay and flows into the Arabian Sea. The Ganges River Valley has been affected by the Indian Monsoon for as long as records have been kept (about 600 BC to the present). But in addition, the valley also receives moisture from the Bay of Bengal through the Gulf of Meghna. So in total, the Ganges River Valley is affected by two monsoons. This shows that even though India is located at the end of the Asian continent, it is not isolated enough to prevent influence from other oceans.
In conclusion, monsoon winds change their direction because the landmass heats up or cools down faster than the water. These rapid temperature changes cause the winds to shift too.
The sun's warmth of the land and the Pacific Ocean at differing speeds drives the monsoon, with land surfaces warming faster than the ocean. As hot air rises, the heated land generates low-pressure zones. Once this pattern is established over the region, the winds will change to fill the void. The rains come as clouds move in from the oceans.
The monsoon affects large areas of North America, Asia, and Africa. It is one of the most important factors determining where we get our water, along with global ice caps and deserts. During the monsoon season, much of India experiences their rainy season; this leads to many opportunities for entertainment (such as cricket) and business (such as tourism).
In North America, the southwestern monsoon influences weather across much of South America and parts of Mexico. In addition to rain, it can also bring snow as far north as California. In Africa, the northern part of the continent receives moisture from the monsoon, while south of about 25 degrees latitude, the desert wind blows away any moisture that reaches that area.
The word "monsoon" comes from the Greek moneys, which means "month," and souns, which means "west." Thus, the term monsoon describes the western month breeze or wind.
During the summer months in the Northern Hemisphere, the sun is high in the sky and heat waves are common.
So, what is the source of this seasonal variation in wind direction? Monsoons are similar to massive sea/land winds in certain ways. At night, the converse happens: the land cools faster than the neighboring water, and the circulation reverses, resulting in an onshore flow known as the land breeze. The difference between the two types of winds is how they are generated: with monsoons coming from the ocean, waves are needed for them to blow, while land breezes can travel great distances without any significant rise or fall in terrain.
When these winds hit land, they can be very strong and have considerable impact on weather and climate. For example, when a monsoon hits India, it can cause flooding in nearby Bangladesh because the rain was blown in that direction. In addition, the wet wind blows away heat that would otherwise evaporate from the ground or be carried away by clouds forming over oceans, which has a cooling effect on regions near the equator.
Land breezes are also responsible for bringing rain to areas that don't normally get much precipitation. For example, southern California gets its rainy season during the winter months, but most storms that reach the region come from the Pacific Ocean. However, if you look at the maps, you will see that there are areas north of Los Angeles where it doesn't seem like it should be raining; this is because the land breeze is responsible for bringing moisture from the ocean up into those regions.