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Monsoon is known  for its uncertainties because

'Indian monsoon are dependent on the south west monsoon winds,
.At times due to reversal of temperature or some other climatic disturbances such as EI nino monsoon gets delayed.'
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The alternate dry and wet spells in a place vary in duration,frequency and quantity. They are responsible for floods in one part of the country while they cause draught in some other part. This also cause problems for farmers without irrigation systems as they r completely dependent on monsoon showers
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Class 9 geography- page 35 1 st para left hand side