Key Concepts

Regional Change

• Since Earth is spherical, it is heated unevenly by sunlight. The equator is much hotter than the poles since light strikes Earth more directly in the tropics.

Seasonal Change

• 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.