The amount of sunlight reaching the surface varies with
the seasons. Most sunlight comes in summer.
Seasonal changes in the regional
heat budget also occur.
Warm, dry air over land during the summer draws cooler, moist air over
the continents as a result of the differential in cooling and heating
rates of the land and the oceans. This is what leads to the Asian monsoons.
The same process occurs in the western United States. In winter, the reverse
situation occurs with cold air from the continent being drawn seaward
by relatively warm air rising over the ocean.