The summer monsoon is caused by the difference in temperature between the landmass of Asia and the Indian Ocean. During the summer, the landmass of Asia heats up more quickly than the ocean, creating a low-pressure area over the continent. This low-pressure area draws in moist air from the Indian Ocean, which results in heavy rainfall over the Indian subcontinent. The presence of the Himalayan mountain range also influences the summer monsoon by deflecting the moist air towards the Indian subcontinent.