Thursday, 29 March 2018


A monsoon is a seasonal change in the wind direction, caused by the shift of the tropical minimum and the changing temperatures over the ocean and land. The monsoon usually brings with it a different kind of weather. 

Long-term mean precipitation by month
The animation below shows the long-term average precipitation by month (mm/day and in/day).

The South-West Monsoon in the summer 
During June and July, the summer monsoon arrives with southerly wind flow driven by a warm air mass with low pressure at the surface that forms over southern Asia as it is warmed by the sun. Air from the relatively higher pressure air mass over the Indian and tropical western Pacific Ocean flows northward toward the low pressure over land, bring with it heavy rains.

The South-West Monsoon are rain-bearing on-shore winds, blowing from sea to land, and bring rains to most parts of the subcontinent. The Monsoon winds are eagerly awaited in most parts of India for their agricultural and economic importance.

The North-East Monsoon in the winter 
During the winter monsoon, a large high pressure zone over Asia drives cool, dry air southward toward the tropics. This provides the monsoon region with its dry season.

Subsequently later in the year, around October, these winds reverse direction and start blowing from north direction. The North-East Monsoon are off-shore winds. Given their land to sea flow, from subcontinent onto the Indian Ocean, they have less moisture and bring rain to only limited parts of India. This is known as the North-East Monsoon. 

Late arrival of the monsoon
A late arrival of the monsoon can be bad for agriculture, as the monsoon rains are necessary for summer crops.

In India, for example, the dry northerly wind flow over India changes direction, and warm humid air from the Indian Ocean flows from the south, gradually overspreading the Indian subcontinent. Widespread heavy rains, and even severe thunderstorms, large hail and tornadoes can accompany the onset (arrival) of the summer monsoon.

The annually monsoon rains flooding large part of Bangladesh, killing thousands of people and destroying farm animals and crops. Bangladesh is vulnerable to the natural disaster of flooding due to being situated on the Ganges Delta and the many tributaries flowing into the Bay of Bengal. 

The coastal flooding twinned with the bursting of Bangladesh's river banks is common and severely affects the landscape and Bangladeshi society. 75% of Bangladesh is less than 10m above sea level and 80% is flood plain, therefore rendering Bangladesh a nation very much at risk of further widespread damage despite its development. Whilst more permanent defences are being built, many embankments are composed purely of soil and turf and made by local farmers. Flooding normally occurs during the monsoon season from June to September during the monsoon. The convectional rainfall of the monsoon is added to by relief rainfall caused by the Himalayas. Melt-water from the Himalayas is also a significant input and flood every year.