Answer:
Over the ocean air masses contain moisture which are responsible for occurring of rainfall over the area.
The air masses which are present over the land are responsible for the dry season over the period of time over an area.
Step-by-step explanation:
United States is surrounded by the water bodies. Land mass get heated due to the sun. The hatred land is responsible for the removal of moisture from the air. This condition causes dry season over the area.
When the air which is present over the ocean comes across the country, these airs contains much moisture to cause precipitation. Therefore, the marshal present in the air is responsible for the precipitation.
Therefore, we can see the overall mechanism of precipitation and dry season in United States.