Atmospheric Circulation
The troposphere, the lowest 9 miles (15 km) of the Earth's atmosphere, is the layer in which nearly all weather activity takes place. Weather is the result of complex air circulation patterns that can best be described by going from the general to more localized phenomena.
The prime mover of air above the Earth's surface is the unequal heating and cooling of the Earth by the Sun. Air rises as it is heated and descends as it is cooled. The differences in air pressure cause air to circulate, which results in the creation of wind, precipitation and other weather related features.
The Earth's rotation also plays a role in air circulation. Centrifugal force, friction and the apparent Coriolis force are responsible for the circular nature of its flow, as well as for erratic eddies and surges.
On a global scale, there are three circulation belts between the equator and each pole. From 0° to 30° latitude, the trade winds, or tropical easterlies, flow toward the Equator and are deflected to the west by the Earth's rotation as they move across the Earth's surface. The winds rise at the equator, then flow poleward at the tropopause, the boundary between the troposphere and the stratosphere. The trade winds descend back to the surface at 30° latitude. At the Equator, where air from both trade wind belts rises, the lack of cross-surface winds results in the doldrums, an area of calm which historically has been a bane to sailing vessels.
Between 30° and 60° are the mid-latitude, or prevailing westerlies. The circulation pattern of these wind belts is opposite that of the trades. They flow poleward at the Earth's surface, deflecting eastward. They rise at 60°, flow back to the equator, then descend at 30°.
As with the equatorial calm, the Earth's surface at 30° North and South has little lateral wind movement since the circulation of the tropical and mid-latitude belts is downward, then outward at this latitude. These calm regions are referred to as the horse latitudes because sailors who were stranded for lack of wind either had to eat their horses or throw them overboard to lighten the load.
The third set of circulation belts, the polar easterlies, range from 60° to 90° latitude at both ends of the Earth and flow in the same pattern as the tropical easterlies.
This global circulation scheme is only the ideal. Other forces complicate the actual flow. Differences in the type and elevation of surface features have widespread effects.
The jet streams, high-speed winds blowing from the west near the tropopause, play a significant role in determining the weather. The northern and southern hemispheres each have two jet stream wind belts. The polar front jet stream is the strongest of the two. It flows eastward at speeds of up to 250 mph (400 kph) at the center. The somewhat weaker subtropical jet stream lies nearer to the equator at very high elevations (30,000 feet [9000 meters]). Both jet streams receive their energy from the difference in temperature between the pole and the Equator. During the winter, the pole experiences its lowest temperatures while the Equator remains warm, so the jet stream winds blow hardest during the winter months.
On a more local level, air movements occur in the form of interacting air masses, frontal systems, and high and low pressure centers. The air in low pressure areas circulates in a counterclockwise direction in the northern hemisphere. This circulation is termed cyclonic, and low pressure areas are called cyclones. High pressure areas, called anticyclones, circulate in a clockwise direction in the northern hemisphere. In the southern hemisphere, these directions are reversed. Cyclones and anticyclones travel from the west to east. Low pressure cells are associated with rising air masses and more vigorous circulation, which frequently serves to drag cold air from the poleward regions into areas of warmer air. The interface of cold and warm air masses is called a front, and along this boundary can form unstable air masses which can develop into severe storms.
Cold fronts are more active than warm fronts. The upward angle of the cold front line opposes the direction in which it moves, creating friction between the surface and the air, and causing a steeper pressure gradient. The rain band is narrower, but the cumulonimbus clouds that form hold a greater amount of energy and a greater potential for violent weather than the altostratus clouds associated with warm front activity.
Within each cyclonic system are even smaller cyclones. Each storm cell along a front is a cyclone in its own right. In addition to producing heavy rain, hail, high winds and electrical activity, these cells occasionally can produce tornadoes--destructive, whirling funnel-shaped clouds that stretch from the base of a storm cell to the ground. Tornadoes are the most powerful cyclones known on Earth.
Independent of air mass and frontal systems are hurricanes, which are known regionally as typhoons or cyclones. These tropical cyclones form over warm moist ocean surfaces. The rising heat and moisture builds into a massive storm that can extend 1000 miles (1,600 km). A hurricane is fed by the heat energy of the ocean and will begin to decay when this energy source is cut off, as occurs when the storm travels over land or if it encounters lower ocean surface temperatures. A lower tropopause in higher latitudes can also reduce the storm's mass.
An accurate understanding of atmospheric circulation began to emerge during the 1830s when Gustave de Coriolis put forth the theory that as the Earth rotates, an object will appear to move in a deflected path. About twenty years later, American William Ferrel mathematically proved the Coriolis theory, establishing what became known as Ferrel's law.
The ability to make regular unmanned balloon soundings of the atmosphere in the late 1890s and early 1900s made it possible for new details to emerge. A group of Scandinavian meteorologists under the guidance of Vilhelm Bjerknes took full advantage of this new knowledge to develop mathematical and laboratory models of air mass properties.
Bjerknes first proposed the existence of air masses. His son Jacob went on to demonstrate the frontal systems that separate the air masses. Carl-Gustaf Rossby discovered the jet streams and hypothesized detailed movements and counter-movements in the circulation complex.
More recent studies of atmospheric circulation have sought to understand two phenomena important to large portions of the Earth's surface: the Asian monsoon and El Nino (the Southern Oscillation). The monsoons of Asia are a result of a combination of influences from the large Asian land mass and the movements of the inter-tropical front, which straddles the equator. From June to September, when the front runs north of the equator, warm moist winds are drawn northward, bringing heavy rains to India and Southeast Asia. From December to February, the front runs slightly south of the equator, drawing dry cooler air off of the Himalayas and out to sea.
El Nino refers to a change in the atmospheric and oceanic circulation patterns over the equatorial Pacific Ocean that affects large areas of Asia, Africa, Indonesia and North and South America. The prevailing winds in this area usually blow westward, moving the warm surface water of the Pacific toward eastern Asia. This brings warm moist weather to the region which supports the rain forests of Indonesia and southeast Asia. The eastern rim of the Pacific on the other hand experiences upwelling of cool ocean water and cooler, drier weather as a result. Every 3-4 years during an El Nino, this atmospheric circulation is weakened and the weather patterns become reversed: warmer, wetter weather is experienced in the eastern Pacific rim, while the western Pacific rain forests can experience drought. A strong El Nino can cause torrential rains and floods stretching from Chile to California. After about 18 months, the atmospheric circulation pattern switches to its "normal" mode. This switching back and forth is called the Southern Oscillation.
Scientists have recently identified circulation patterns that could be called "atmospheric rivers." These so called rivers are streams of water vapor that flow as high as six miles above the Earth's surface, generally from the near the equator toward higher latitudes. A few hundred miles wide and about a mile deep, atmospheric rivers can be as long as 4000 miles and carry huge amounts of water in its vapor state. This water vapor can feed into mid latitude storms, giving them the moisture and energy to increase in strength.
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