Abstract:

Weather starts with the Sun.

Weather plays an important role in our lives, and all forms of weather are produced by complex, constantly changing conditions in Earth's atmosphere. However, the driving force behind the weather is the Sun. The Sun continually generates energy, which escapes from its surface and flows through space. Solar energy travels 93 million miles (149 million kilometers) to reach Earth. It warms all of Earth's atmosphere, some parts more than others. The area of Earth that receives the Sun's rays most directly, the equatorial region, is heated the most. The poles, conversely, never receive sunlight directly. Sunlight strikes the poles only at a steep angle. Hence, they are warmed the least.

Another factor that determines how much solar energy strikes any particular part of Earth at any time is the season, a period of year characterized by certain weather conditions. Most places in the world have four seasons: winter, summer, spring, and fall. In winter, the Sun shines for the fewest hours per day and never gets very high in the sky. In summer, day is longer than night, and the Sun shines high in the sky. In spring and fall, the Sun rises to an intermediate height, and there are roughly the same number of hours of daylight as darkness.

The change in seasons is caused by a combination of Earth's tilt and its yearly journey around the Sun. Earth's axis of rotation is tilted 23.4 degrees away from the perpendicular. At different points along Earth's orbit around the Sun, the Northern Hemisphere, the half of the earth which lies north of the equator (which includes the United States) is tilted either toward or away from the Sun. For instance, on or about June 21, the first day of summer, the Northern Hemisphere receives more sunlight than on any other day. On or about December 21, the first day of winter, the Southern Hemisphere, the half of the earth that lies south of the equator, receives its greatest amount of sunlight...

The uneven heating of the atmosphere sets the atmosphere in motion. Air moves through the atmosphere in such a way as to even out the distribution of heat around the planet, with warm air moving from the equator to cold areas at the poles and cold air back toward the equator. The movement of air between the equator and the poles is influenced by other factors as well, such as differences in composition of air over land and sea, and Earth's rotation. The result is a complex web of air currents whirling around the globe, the ingredients of weather. (more) K. Lee Lerner. Paris.