The S. W. Monsoons, from Bay of Bengal, first of all strike in Kolkata in West Bengal. These moisture laden winds give a heavy rainfall of 145 cms here.
Jaisalmer lies to the west of Aravallis in Rajasthan. Aravallis lie parallel to S.W. Monsoons of Arabian sea. So these mountains fail to check S. W. monsoons. These winds move northward giving only 12 cms of rainfall.
