Why does the rainfall gets on decrease as we go up the Ganga Valley but increase as we go up the Indus Valley ?

We know that the monsoons in summer rise from Bay of Bengal and Arabian Sea simultaneously. The Bay of Bengal branch causes heavy rainfall in the Brahmaputra valley and Assam hills. The rain bearing winds strike at the lofty peaks of the Himalayas there and get deflected towards the west. Thus, they move up the Ganga Valley but as they proceed westwards, they become drier and cause less rain with every steps towards the west of the plains. Owing to this decreasing trend, Kolkata receives 119 cm, Patna 105 cm, Allahabad 76 cm and Delhi 56 cms rainfall. Distance from the sea also decreases the quantum of the rainfall in the areas proportionately. Places near the sea get more rainfall as compared to the places lying for away from the sea. As we move away from the Bay of Bengal to Punjab, the distance from the sea also becomes more and more leading lesser and lesser rain.

Another branch of the monsoon i.e. southeast monsoon blows from the Arabian sea. It enters in India and blows over Rajasthan but the Aravalli Hills there cannot check them and so, these winds pass on without raining there. At their access to Himachal Pradesh, these monsoon winds strike with the lofty peaks of Himalayas and cause heavy rains there. The Bay of Bengal branch also joins here and thus, the rain goes on increasing as we go up the Indus Valley.