Excerpt and illustrations from The Biosphere by Pearson Education, Inc.
Uneven Heating of Earth’s Surface
Energy from the sun warms Earth’s surface. But because of Earth’s spherical shape, different locations on Earth’s surface receive different amounts of solar energy. Near the equator, the sun’s rays strike the surface most directly. At latitudes farther from the equator the rays strike Earth’s surface at lower angles, causing the same amount of solar energy to be spread over a larger area (Figure 34-6). As a result, regions on Earth’s surface farther from the equator absorb less heat and generally experience cooler temperatures than regions closer to the equator. These temperature differences also drive global air and water movements.
Wind, Precipitation, and Ocean Currents
The uneven heating of Earth’s surface by the sun is also a driving force behind global patterns of winds and precipitation (rain, snow, and sleet). When air is warmed it can absorb more moisture, and it also tends to rise. Thus air near the equator, heated by the direct rays of the sun, absorbs moisture and rises. Higher in the atmosphere the air cools again, forming clouds that produce rainfall. This pattern means that many areas of Earth close to the equator tend to have warm temperatures and heavy rainfall year-round. Most rain forests are found in this part of the world.
On an August day, people in Los Angeles’ San Fernando Valley might face temperatures of 38°C (100°F), while just 30 kilometers away, people at the beach are enjoying the comfortable 24°C (75°F) outdoor air. What causes this difference within such a small geographic area? Local climate variations may be influenced by nearby large water bodies. Oceans and large lakes affect climate because water tends to absorb and release heat more gradually than most other substances (such as soil and rock). Because of the heat-absorbing ability of water, areas near the shore tend to be cooler in the summer than inland areas. In turn, the release of heat from water to the air generally results in milder winters near the shore than inland.
Mountains also affect local climate. First, air temperature declines by about 6°C with every 1000-meter increase in elevation. This is why it becomes cooler as you hike up a mountain. Second, mountains near a coast can block the flow of cool, moist air from the ocean, causing different climates on opposite sides of the mountain range. For example, in California, Oregon, and Washington, precipitation falls on the western, coastal side of the mountain ranges. The eastern side of the mountains can be dry and even desert-like.