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The Coriolis force is not perceptible to humans as they walk around because humans move slowly and travel relatively short distances compared to the size and rotation rate of the Earth. However, the Coriolis force significantly affects bodies that move over great distances, such as an air mass or body of water.
The Coriolis force deflects air to the right in the Northern Hemisphere, causing it to follow a curved path instead of a straight line. The amount of deflection differs depending on the latitude. It is greatest at the poles, and diminishes to zero at the equator. The magnitude of Coriolis force also differs with the speed of the moving body—the faster the speed, the greater the deviation. In the Northern Hemisphere, the rotation of the Earth deflects moving air to the right and changes the general circulation pattern of the air.
Pertinent facts about the Coriolis force:
• The Coriolis force deflection is perpendicular to the flow of air.
• The Coriolis force will deflect air to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere.
• The Coriolis force is strongest at the Poles and decreases to zero at the Equator.
• The Coriolis force is zero with calm winds and increases in magnitude as wind speed increases.
• Coriolis force, in combination with other forces involved, will determine the different circulation patterns over the Earth.Pressure Gradient
Pressure gradient is the difference in pressure between high and low pressure areas. It is the rate of change in pressure in
The speed of the Earth’s rotation causes the general flow to break up into three distinct cells in each hemisphere. [Figure 4-10] In the Northern Hemisphere, the warm air at the equator rises upward from the surface, travels northward, and is deflected eastward by the rotation of the Earth. By the time it has traveled one-third of the distance from the equator to the North Pole, it is no longer moving northward, but eastward. This air cools and sinks in a belt-like area at about 30° latitude, creating an area of high pressure as it sinks toward the surface. Then, it flows southward along the surface back toward the equator. Coriolis force bends the flow to the right, thus creating the northeasterly trade winds that prevail from 30° latitude to the equator. Similar forces create circulation cells that encircle the Earth between
4-12
1020
10161012Isobars• The wider the pressure gradient, the weaker the wind.Weak or Flat Pressure Gradient• The closer the spacing of isobars, the stronger the pressure gradient.• The stronger the pressure gradient, the stronger the wind.Strong or Steep Pressure Gradient
Figure 4-11. Principles of pressure gradients.
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1016HIGHLOWSurface WindGradient Wind 1,000 FeetGradient Wind 2,000 FeetGradient Wind 3,000 Feet
Figure 4-12. Examples of variations of wind direction with height.
a direction perpendicular, or across the isobars. Wind speed is directly proportional to the pressure gradient. This means the strongest winds are in the areas where the pressure gradient is the greatest. Since pressure applied to a fluid is exerted equally in all directions throughout the fluid, a pressure gradient exists in the horizontal (along the surface), as well as in the vertical (with altitude) plane in the atmosphere. [Figure 4-11]
The horizontal pressure gradient is steep or strong when the isobars determining the pressure gradient are close together. It is flat or weak when the isobars are far apart. If isobars are considered as depicting atmospheric topography, a high pressure system represents a hill of air, and a low pressure system represents a valley of air. The vertical pressure gradient always indicates a decrease in pressure with altitude, but the rate of pressure decrease (gradient) varies directly with changes in air density with altitude. The vertical cross section through a high and a low depicts the surface pressure gradient.
The pressure gradient force is a force that tries to equalize pressure differences. This is the force that causes high pressure to push air toward low pressure. Thus, air would flow from high to low pressure if the pressure gradient force was the only force acting on it.Surface Friction
Friction is the third component that determines the flow of wind. Because the surface of the Earth is rough, it not only slows the wind down, it also causes the diverging winds from highs and converging winds near lows. Since the Coriolis force varies with the speed of the wind, a reduction in the wind speed by friction means a reduction of the Coriolis force. This results in a momentary disruption of the balance. When the new balance (including friction) is reached, the air flows at an angle across the isobars from high pressure to low pressure. This angle varies from 10° over the ocean to more than 45° over rugged terrain. Frictional effects on the air are greatest near the ground, but the effects are also carried aloft by turbulence. Surface friction is effective in slowing the wind up to an average altitude of 2,000 feet above the ground. Above this level, the effect of friction decreases rapidly and may be considered negligible. Air above 2,000 feet above the ground normally flows parallel to the isobars. [Figures 4-12 and 4-13]Wind Patterns
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