Monsoosn is a typical seasonal wind in low altitude climate that changes direction between winter and summer season. Monsoon wind typically flow from the geographical land in winter season carrying cool dry air (dry phase), and to the land in summer carrying warm moist air (wet phase), causing a sweeping change in the rain and temperature patterns of the area.
The “monsoon” word is derived from the Arabic word mausim, meaning changing season. Initially the word monsoon was used to describe winds in the Arabian Sea, but with the passage of time it is employed for seasonally changing wind systems all over the world. The difference in the heating of land and water surface results in pressure difference between land and ocean. (Roger G. Barry, Richard J. Chorley, 1998)
In the winter to maintain the energy balance between land and water heat is transferred by land- sea breezes. This pressure difference gives rise to monsoon. In winter season the air over the land area is colder than over the oceans, generating a large high pressure area over the Siberia, leading to air movement across the Indian Ocean and South China, causing clear skies for South Asia and East. While the exact opposite phenomenon occurs in summer.
In summer monsoon of Southwest Asia wind starts blowing from the ocean the continent with wet shower patterns. The air over the continents land is much warmer than over the oceans, resulting in moisture laden wind movement from the ocean towards the continent. When this humidified air unites with comparatively dry west air flow crossing over the mountains, it starts rising till reaches its saturation point with the development of heavy showers and thunderstorms. (Roger G. Barry, Richard J. Chorley, 1998)
The most prominent pattern of monsoon system is seen in eastern and southern Asia; however it has also been prevailing in Australia, West Africa and the Pacific Ocean. A small scale monsoonal system exists in the southern United States called North American, Mexican or Arizona Monsoon. Between July and September the North American monsoon is circulating over southwest North America bringing remarkable increase in rain showers in normally arid regions of Arizona, New Mexico and northwestern Mexico. The west or northwest winds with similarities to the original Southwest Asian monsoon turn more south or southwest having monsoonal circulation brings moisture from Pacific Ocean, Gulf of California and Gulf of Maxico.
The monsoon is an important aspect of atmospheric circulation. It brings humid air from over the oceans traveling across the land to the mountains being lifted up due to day time heating from the sun, finally causing thunderstorm and turning out into rain over the land. The large areas in the tropics and subtropics are under the influence of monsoons. In highly populated areas of the world like Asia or India, the monsoon is vital for agriculture for harvesting land and thus the food production. From time to time a strong monsoon circulation has been seen bringing flood or if the monsoon is late in a specific year, it has been seen causing droughts.
A very interesting phenomenon similar to monsoon occurs in smaller spatial and temporal scale, the mountain and valley breezes. The reason behind their occurrence is the same as that of monsoon. During the day the valley and area around it becomes warm heating the air from scorching sun which makes it less dense with a propensity to rise and a gentle upslope wind occurs.
This upslope valley wind if carry enough humidity may cause showers and even thunderstorms in warmest part of the day or early afternoon. Exactly opposite to this phenomenon at night slopes cool down quickly causing the air around to cool and blow down from the mountain to the valley forming some sort of gravity wind named mountain breeze. Technically speaking nay kind of down slope wind is called Katabatic or fall wind which is best used for considerably stronger wind than mountain breeze. (Peter Combs, 2000)
The monsoon season prevails from June to September, and is liable for widespread rain leading to flooding across India and Bangladesh. This is a very common phenomenon in this region of world happening time and again. Noticeable monsoon effects are recorded in the history of meteorological study in past.
Bangladesh is a low-lying, poor nation of 144 million people, where seasonal floods and cyclones due to monsoon kill hundreds every year. A powerful cyclone in 1991 killed 139,000 people along the coast with a record worst in a decade monsoon rains. (Peter Combs, 2000)
In June 2007 heavy rains swept across southern India’s Karnataka state western Maharashtra state killing 38 people. This sweep had also caused flooding and wild storms that had hit also the neighboring Pakistan where 228 people were killed in a week.
“Hundreds of people died last year in South Asia in floods and landslides which are common during the monsoon season. But officials in Indian and Bangladesh say that this year's flooding is the worst they have seen in over a decade. "Some 400,000 houses were damaged in floods in 18 of the state's 24 districts," the chief minister of the Indian state of Assam, Tarun Gogoi, told AFP news agency”. “Flood waters are continuing to rise in Bangladesh and eastern India, swamping large stretches of land and affecting millions of people.” http://news.bbc.co.uk/1/hi/world/south_asia/
On August 23 in 2005 hurricane Katrina had developed. It was the one of the five deadliest hurricanes in the history of the United States. Atlantic monsoon season had brought damage in a large extent of the north-central Gulf Coast. Severe loss of life and property had occurred in New Orleans, Louisiana due to flooding of vast area. The hurricane caused much more devastating destruction across the entire Mississippi coast and into Albama as far as 160 km from the storm’s center.
New Orleans has a long history of natural disasters being geographically positioned in a region frequently hit by hurricanes. In the year 1915 a category four hurricane had caused Lake Pontchartrain to overflow killing 275 people in the same area hit badly by the Hurricane Katrina.
Hurricane Betsy with category three occurred in 1965, flooded half of New Orleans and submerged up to 20 feet in some areas, and left 60,000 inhabitants homeless.
Hurricane Camille hit the Mississippi Gulf near to New Orleans in 1969, once more causing destructive flooding in the area and displacement of people. As recorded by National Weather Service report monsoon season throughout 1995-2004 have brought 13.6 tropical storms, 7.8 hurricanes, 3.8 major hurricanes.
References:
http://news.bbc.co.uk/1/hi/world/south_asia/
Peter Combs. Clouds and Climate Change; Focus, Vol. 46, Spring 2000
Roger G. Barry, Richard J. Chorley. Atmosphere, Weather, and Climate; Routledge, 1998
