The cause of monsoons isnít fully understood, but everyone agrees that air pressure could be one of the causes. For example, in the winter, an Asian monsoon is caused by high pressure made over South Siberia. Also, in the summer a monsoon is caused by low pressure that is made over South Asia as the land gets warmer. Moisturized air that is over an ocean goes towards the lower pressure. The air gets colder as it goes up the side of a mountain. As it goes up the mountain, it canít hold the rain anymore, causing heavy rain.