South Asia: Monsoons
INTRODUCTION
Definition: Seasonal shifting of wind directions
The large Eurasian (Europe and Asia) land mass and the equally large Indian Ocean result in differences between the heating capacities of the Eurasian landmass and the Indian Ocean. Because land heats up faster and cools down quicker than water, a seasonal reversal of winds occur. This is called the monsoon from an Arabic word Mqusim which describes a seasonal reversal of winds. Southwesterly winds [sswetmon] blow on shore in South Asia during the Northern Hemisphere summer while northeasterly winds [ssdrymon] blow offshore during the Northern Hemisphere's winter.
As the sun begins its northward movement to its highest point (the Tropic of Cancer [wwtropic] ) during the Spring the land heats up rapidly. This intense heating creates lower pressure with rising air currents. Additional air blows in [sswetmon] from the ocean bringing precipitation. As the precipitation wave advances, first into the Western Ghats [ssphys] of India and then from the Bay of Bengal [sswetmon] into northern India, precipitation arrives and the dry fields become flooded soon. Some of the world’s heaviest rains [ssprecip] occur in monsoon climates [ssclimat] (Am). During one year at Cherripunji in northeastern India, 466 inches of rain was recorded-in one month! Eventually the rains stop as the seasons change. Dry autumn gives way to a cool, dry winter when the huge Siberian High dominates all of the Eurasia’s weather. Winds circulate from the northeast [ssdrymon] out of this high bringing South Asia a dry winter regime.
The monsoons determine the agriculture [ssagric] calendar of farmers throughout the region. Prior to the monsoon onset, rural life is slow as the earth is scorched under the intense sun with air temperatures often exceeding 115 degrees F. With the onset of rain, human activity resumes with soil preparation, weeding and other tasks associated with growing season accomplished into a very short time period. If the monsoon commences later than usual (the monsoon occurs on regular basis in South Asia), the consequences can be severe and can still result in famine to this densely populated region.
OPTIONAL: See: http://geography.about.com/education/scilife/geography/library/weekly/aa102599.htm
DRY (WINTER) MONSOONS [ssdrymon]
The winds from the northeast [ssdrymon] during the winter months are dry because they have lost their moisture on the Asian landmass. As these winds approach the southern tip of India, the location of the state of Tamil Nadu, they do pass over the Bay of Bengal and pick up moisture. Tamil Nadu then receives most of its rainfall during these months [rainfall-Madras]. Toward late Spring and early summer the weather is Hot and dry over most of the subcontinent. March to June: HOT!
summary- dry monssons:
WET (SUMMER) MONSOONS [sswetmon]
As the land surface heats up air is drawn in from the Indian Ocean and Arabian Sea in the east. These winds pick up plenty of moisture and the rains fall first along India's western coast. Later the winds round the southern tip of India and are funneled up the Bay of Bengal into the delta area of the Ganges and Brahmaputra rivers. Later, the rains reach the upper Ganges valley and the capital city of New Delhi receives less moisture than the other areas mentioned and it arrives later in June and July. Locate the cities of Bombay, Calcutta, and New Delhi [sscities] and then check their monthly precipitation to confirm this pattern. [rainfall-Bombay] [rainfall-Calcutta] [rainfall-New Delhi]
The Deccan Plateau [ssphys] to the east of the Western Ghats receives significantly less rainfall than the coasts [ssprecip]. As the summer (wet) monsoons [sswetmon] approach the west coast of India, they rise up the western Ghats (mountains) and the air cools. This cool air is less able to hold moisture an it is released as rainfall. this is called OROGRAPHIC RAINFALL [ssprecip]. Orographic means that it is related to mountains.
By the time the winds make it over the Western Ghats they have lost most of their moisture and very little falls on the Deccan Plateau to the east of the ghats. This reduced rainfall on the leeward side (away from the wind) of mountains is called a RAIN SHADOW EFFECT. [ssprecip]
summary - wet monsoons:
WHY DO WE CARE?
As we well know by now, geography is the study of WHERE?, WHY THERE?, and WHY DO WE CARE? Now that we know where the monsoons occur and why they occur there (as well as when they occur where they do), we'll study why do we care, or what is the effects of these monsoonal patterns.
Look at map of agricultural regions [ssagric]
From what you know of the monsoons and precipitation [ssprecip] patterns, you should be able to better understand these agricultural regions. Rice is a crop that needs plenty of rainfall, therefore rice is grown along the west coast [ssagric] where the summer monsoons arrive first as well as in the Ganges and Brahmaputra river delta and valleys [ssphys]. You'll also notice that rice is grown along the southeast coast in the state of Tamil Nadu. Why? remember that the dry, winter monsoons [ssdrymon] pick up moisture over the Bay of Bengal and it rains in this area during the months of Sept. to March which are dry else where in India. [rainfall-Madras] [sscities]
Drier climate crops [ssagric] are grown on the drier Deccan Plateau and further up the Ganges valley.[ssprecip]
Look at map of where people live [ssecumen]
Since a high percentage of people in this realm depend on agriculture [ssaglab] for their livelihood, we would expect most people to live where there is adequate rainfall and fewer people live in areas of less rainfall.
Be able compare maps of precipitation [ssprecip] to explain the patterns seen on maps of where most people live [ssecumen] [sscities].
Sources:
Rainfall-Bombay: http://www.worldclimate.com/cgi-bin/data.pl?ref=N18E072+2100+43057W
Rainfall-Calcutta: http://www.worldclimate.com/cgi-bin/data.pl?ref=N22E088+2100+42807W
Rainfall -New Delhi: http://www.worldclimate.com/cgi-bin/data.pl?ref=N28E077+2100+42182W
Rainfall-Madras: http://www.worldclimate.com/cgi-bin/data.pl?ref=N13E080+2100+43279W
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