Wide World of Weather

The El Nino phenonmenon has caused people to think about the importance of weather and its worldwide implications. Weather is not restricted to isolated pockets of activity; the conditions experienced in one area are interrelated with the conditions that are experienced later in other areas. This dynamic condition, called the butterfly principle, states that due to the interrelationship of global weather patterns, even the slightest atmospheric disturbance in one area, such as a butterfly beating its wings, eventually has an effect on weather conditions around the world. Although the butterfly principle is a bit of an exaggeration, it does serve as a helpful visual symbol of global weather interaction.

Early meteorologists theorized about a global weather model. They speculated that warm, less dense air in the equatorial regions rises and is replaced by colder, dense air in the polar regions. Next, they thought, the polar air sinks and slowly makes its way to the tropical latitudes, completing the cycle.

Further research proved the existence of three separate circuits, or cells. It was correct that the warmer air at the equator does rise and drift at high altitudes, toward the north and south poles. However, not all of the rising air makes it to the polar regions. Some air is forced downward at 30 north latitude and at 30 south latitude, a circuit known as the Hadley cell. In the second circuit, called the Ferrel cell, wind at the surface moves north from 30 north latitude to about 60 north latitude. At that point the airflow encounters the southerly flow from the polar regions and is forced upward. And finally, the upwelling air from 60 latitude to the north and south poles is called the polar cell. When the deflection of the Coriolis force from the earth's rotation is accounted for, the complex global picture begins to emerge.

Another area of global weather patterns is the intertropical convergence zone (ITCZ), found at the equator. Here the northeast trade winds from the Northern Hemisphere meet the southeast trade winds of the Southern Hemisphere. Because the converging air has nowhere to go but up, significant areas of thunderstorms can be created. The ITCZ in the western Atlantic ocean is known for spawning powerful hurricanes.

At 30 north latitude, descending air creates the subtropical high. High-pressure descending air can limit the development of horizontal wind flow, and in the early days of sailing the lack of a breeze for extended periods of time sometimes forced drifting ships to lighten their loads by throwing horses overboard. Subsequently, sailors dubbed the region the horse latitudes. The descending dry air also accounts for the deserts found around the world at this latitude.

At 60 north latitude the westerlies of the south meet up with the polar easterlies of the north, forming the polar front. The movement of the polar front can have a dramatic effect, known as the Siberian Express, when the jet stream dives to the south bringing arctic temperatures.

The picture is further complicated by the effects of the earth's continents, which tend to add a touch of chaos to the formula. All of these factors add up to an ever-changing, interrelated global weather pattern that modern meteorologists, with their knowledge of wind flow and their sophisticated computer models, may never be able to fully predict. It seems we'll be able to blame the changeable weather on the forecasters for a long time to come.

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