Atmospheric Circulation
The troposphere, the lowest 9 mi (15 km) of 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 Earth's surface is the unequal heating and cooling of 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.
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 then 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 over-board 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 typical model. 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 stronger of the two. It flows eastward to speeds of 250 mph (400 kph) at the center and receives its energy from an accumulation of solar radiation. The subtropical jet stream is weaker and receives its force from an accumulation of westerly momentum.
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.
On a more local level, air movements occur in the form of interacting air masses and frontal systems. Low-pressure cyclones and high-pressure anticyclones travel from the west to east. Low-pressure cells are responsible for instability in the weather, with cold and warm fronts radiating from the center of the cell. These fronts represent the interface of cold and warm air masses, which develop into 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, also known as typhoons or cyclones. These tropical cyclones generate over warm moist ocean surfaces. The rising heat and moisture builds into a massive storm that can extend 1000 mi (1,600 km).
A hurricane tracks westward and will decay when the creative factors are eliminated. This occurs rapidly as the storm travels over land or more gradually as 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 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 countermovements in the circulation complex.
Atmospheric circulation is a simple process with complex results. It is a system of cells within cells. When we observe leaves swirling in the shadow of a building or a bird soaring on an updraft of warm air, the same principles are at work as with larger global units of the same circulation system. It is a system that is worldwide, that reacts to everything it encounters, and that is even interactive with itself.
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