After the 21st March summer solstice, heat wave starts in north India. Heat wave starts to exceed as the month of mid June comes and North Western India and Pakistan become the belt of low pressure. These low pressures are so powerful that the winds of Southern Hemisphere are attracted towards it crossing the Tropic of Capricorn and South Western monsoon winds become the part of strong monsoon system. In this way monsoon starts in Indian Subcontinent.