The Atmospheric Cycle
The Earth's atmosphere and oceans play the important role in redistributing heat across the surface of the planet. The atmosphere also has chemical cycles involving oxygen and carbon, but those cycles are intimately bound up with the presence of living things on the Earth. The circulation of gases near the Earth's surface—both the short–term variations of weather, and long patterns of climate— is the atmospheric cycle.
The General Circulation of the Atmosphere
The atmosphere circulates in vast rivers of air that cover the globe from the equator to the poles. This circulation is powered by the energy of the Sun. Air in the tropics is heated and rises. If the Earth did not rotate, we would expect to have a situation as shown in figure below. (427) In this figure because the earth is not rotating, the circulation of the atmosphere would take place in convention cells that create high–altitude winds, from the equator to the poles, and low–atlitude winds from the pole to the equator. Therefore warm air would rise at the equator, cool off, and sink at the poles.
Thus, if the Earth did not rotate, in the Northern Hemisphere prevailing winds would flow from north to south. In fact, they do nothing of the kind. The weather patterns in much of the Northern Hemisphere, including North America, move, in general, from west to east—we live in a region of what meteorologists call prevailing westerlies. This behavior of the Earth's atmosphere results from the fact that the Earth rotates. The rotation breaks the north–south atmospheric convention cell that would exist in the absence of rotation into three cells in each hemisphere as sown on 427.Atmospheric convention on the rotating Earth, showing the principal air circulation cells. In addition, the rotation “streches out” the shape of the air circulation pattern in each cell. In the cell nearest the equator, the winds at the surface tend to blow from east to west–the trade winds that drove sailing ships from Europe to North America. SEE THE CONNECTION! In temperate zones the effect is to cause the winds to blow from west to east, creating regions in which weather patterns also unusually move from west to east. Finally, in the Artic and Antarctic, the winds blow once again from east to west.
Similar patterns of atmosphere motion can be seen on all planets in the solar system that have atmosphere. In some cases, like the planet Jupiter, the rapid rotation of the planet and the atmospheric dynamics cause more than 3 convection cells. Jupiter, in fact has no fewer than 11.
Weather and Climate
The world weather refers to daily changes in rainfall, temperature, amounts of sunshine, and other variables that are part of our everyday lives. The term climate on the other hand, refers to long term trends in the cycle of seasons. Thus the city of Seattle many not have hot sunny weather on a particular day, but will have a generally moderate, rainy climate. The weather in a given region results partly from the general circulation in the atmosphere, and partly from local disturbances and variations.
The most important variations underlying the daily weather are differences in atmospheric pressures—in the amount of air actually present at a given location. Air, like any other fluid, can slosh around. Some regions will experience higher pressure, where more air than average accumulates, while other regions will be at lower pressure. When low–pressure area develop, air from the surrounding regions tends to rush in to fill up those lows. In the Northern Hemisphere, the rotation of the Earth forces the air to flow in a clockwise direction around a high–pressure area. The reverse in the Southern Hemisphere. This flow cause the circular pattern you see in satellite weather photos.
Air in low–pressure areas rises, cooling as it does so. At some altitude, the temperature falls to a point where the water droplets condense out. Low–pressure areas are thus often characterized by extensive cloud cover and precipitation. In high–pressure areas, associated with sunny and clear weather. A typical weather map will show two or three high–or–low–pressure area across a continent as North America at any given time, and it is constant interplay and changes among these patterns as they move across the continent that produces the daily weather changes of weather.
Another important determinate of the weather pattern in North America is the position of the jet stream. This high–altitude stream of fast–moving winds marks the boundary between the northern polar cold air mass and the warmer air of the temperate zone. It generally shifts north and south on an annual basis, but small kink in the jet stream can prevent low–pressure area from forming (a process that can produce a drought) and can allow cold air to spill down into the temperate zones (a process nicknamed the Alberta Clipper or Siberian Express), creating cold waves. 429The jet stream is a fast–moving, high–altitude air current above North America.
- The jet stream often follows a relative straight path, with minor undulations.
- Strongly developed undulations may pull a mass of cold arctic air to the south.
