Our physical environment has a tremendous impact on living things and on how they have evolved to become what they are. Climate, which is the prevailing weather conditions, including temperature, humidity, wind speed, cloud cover, and rainfall has a significant impact on the types of plants and animals that will be found in a particular area of the earth.
In the tropics, which is the area close to the equator, sunlight is more intense, and the temperature subsequently warmer because solar radiation strikes the earth almost perpendicular to its surface.
For two reasons, solar radiation hitting the earth's surface is less as we move further from the equator. First, sunlight must penetrate more atmosphere before it reaches the surface and more of it is reflected away by water and the atmosphere. At night, thermal radiation rising from the earth's surface is reflected back to the earth's surface by water in the atmosphere. This is called the greenhouse effect.
Secondly, sunlight strikes the surface at smaller angles at higher latitudes (greater distance away from the equator) resulting in the light being spread out over a greater area. If you were to hold a flashlight perpendicular to a wall, the beam would illuminate a small, intense area. If you then moved the flashlight more parallel to the wall, the beam would change to a larger, more diffuse area.
Rainfall and precipitation patterns are largely determined by winds and by ocean currents. Winds and ocean currents in turn, are generated by the amount of sunlight hitting the earth, the earth's rotation, and the tilt of the earth's axis as it orbits the sun.
Here are two excellent animations that help to explain these concepts:
Air circulation animation (Audio - Important)
Seasons animation (Audio - Important)
Differences in the amount of sunlight reaching the earth at different latitudes combined with patterns of air circulation results in major temperature zones.
The heating and cooling of air. causes winds. Warm moist air at the tropics expands and rises, moving both northward and southward from the equator. As the air rises, it also cools and releases its moisture as rain resulting in the creation of rainforests. The cool air sinks at 30 degrees North and South of the equator. As the air descends, it warms once again, and picks up moisture from its surroundings resulting in deserts around the earth at 30 degrees latitude both North and South of the equator. The descending air spreads both north and south.
In addition to moving north and south, winds also move both east and west as a result of the rotation of the earth. Someone standing at the equator, moves farther and therefore faster in a day than someone farther from the equator. This difference in velocity, causes winds to veer. In the northern hemisphere, winds move north and south from latitude 30 degrees. Those moving north speed up and veer east while those moving south slow down and veer west. In the southern hemisphere, winds from latitude 30 degrees move north and west, slowing, or they move south and east, speeding up.
Ocean currents arise from a combination of wind patterns and the earth's rotation and rotate clockwise in the northern hemisphere and counter clockwise in the southern hemisphere. In this figure from your textbook, note how the red lines indicating ocean currents swirl in a clockwise direction north of the equator and counter clockwise south of the equator.
This animation describes climate currents (Audio - Important)
Take a look at this animated weather map for the Pacific Coast courtesy of KCRA3.com . These weather maps are updated frequently so the cloud cover will be different each time you access them. Does the swirling motion of the clouds correspond to the lines in the previous figure? Can you see a counter clockwise swirl in the area above the Gulf of Alaska and a clockwise movement below that? Wind and ocean currents vary seasonally because the latitude receiving the most incident solar radiation changes seasonally with changes in the tilt of the earth's axis relative to the sun. The incline of the earth's axis plus the elliptical shape of the earth's orbit causes the distance from the sun to change resulting in the different seasons.