Specific Air Movement Classifications

Posted by Clinton Maxwell

Earth’s Rotation Causes Circular Weather Effects

Over the centuries, as man has observed and studied climate conditions so critical to life, a number of weather effects have been observed, isolated and classified. All of these effects have their foundation on such basics as the sun’s heat, moving air masses in the atmosphere and the level of water vapor in certain areas at a particular time. In most cases, the uneven or inconsistent heating of air creates a very complex series of wind strengths and direction.

Meteorologists and observers have established some classifications for certain aspects of the changing weather we experience. At present the list includes three significant types of air circulation that are based on the heat of the sun. One of these categories is the Hadley cell, so named because scientist George Hadley was instrumental in isolating and classifying them. This term is applied to air circulation in or very near the tropical regions of the Earth.

Simply stated, a Hadley cell occurs when tropical air (23.5 degrees north and south of the equator) is heated by the sun and begins to rise. This warm air mass spreads, encounters a barrier often called the tropopause, and sinks back to the ground. Generally, the falling air mass touches the Earth at about 30 degrees of latitude, north and south of the equator. As it sinks to the ground, the moving air mass displaces air at the surface. The air mass that moves out flows toward the equator, completing a cycle that can cause weather changes over a large region of the globe. When the same type of circulation occurs at latitudes farther north (30 to 60 degrees) they are known as Ferrel cells. Similar movement of air masses at the poles are called polar Hadley cells.

One of the interesting features of cells such as these is that they do not move in straight lines, north and south. Because the Earth is a planet rotating around the sun and rotating on its axis at the same time, moving air masses experience the Coriolis effect. This is not limited to air movement, however. All free-moving objects and fluids have a “right turn” motion north of the equator and a “left turn” motion south of the equator. Gustave-Gaspard de Coriolis is credited with first identifying this trait nearly 200 years ago.

What does this mean for air movement? The Coriolis effect deflects an air mass, creating what some have called a cyclonic flow. This circular movement applies to air around areas of low pressure, causing large air masses to turn clockwise in the Southern Hemisphere and counter-clockwise in the Northern Hemisphere. But that’s not all. When the air is flowing around a high pressure system, the circular movement is just the opposite! Without the rotation of the earth, air masses would move quickly in a straight line, rapidly equalizing pressure.

There is one place on Earth where the Coriolis effect does not impact air movement ? the equator. Because of this cyclones are not part of the regular experience in life at the equator. Conversely, the Coriolis effect is strongest at the Earth’s poles.

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