Introduction:
This page explains about Air Pressure, and how it controls the weather. Factors affecting wind shows how wind is changed by certain forces of the earth. One such factor is Pressure Gradient Force, another is Coriolis Effect, and the last is Friction with the Earth's surface. Cyclonic and Anticyclonic winds determines how pressure systems form, and the circulation around them. Finally, Weather Generalizations about Highs and Lows gives an overview of the weather associated with these systems. At the bottom of the page, along with the bottom frame, is a link to view the bibliography for this and all other pages.
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Air moves in two directions. Vertically and Horizontally. Vertical air movement results in cloud formation, but far more air moves horizontally. This phenomenon is what we call wind. Wind is the result of horizontal differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure. Wind is affected by three major factors, one of which is the pressure gradient force. Pressure differences create wind, and the greater these differences, the greater the wind spee. Over Earth's surface, variations in air pressure are determined from baroetric readings taken at hundreds of weather stations. These pressure data are shown on a weather map using isobars, or lines of equal pressure. Figure 1 shows isobars on a weather map. The spacing of isobars indicates the amount of pressure change occuring over a given distance and is expressed as the pressure gradient. In simple terms, it can be described as the slope of a hill. The steeper the slope, the closer the lines would be on a map showing altitude. Same with pressure. The closer the lines on an isobar map, the more force, and consequently higher velocity winds, is exhibited on the air. Wind is forced by the gradient always at a right angle to the isobars.
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Another factor affecting wind is the coriolis effect. Wind doesn't naturally move at right angles to the isobars, rather it deviates due to the Earth's rotation. This has been named the the Coriolis effect after the French scientist who first thoroughly described it. All free-moving objects or fluids, including the wind, are deflected to the right of their path of motion in the Northern Hemisphere, and to the left in the Southern Hemisphere. For the rest of the lesson, only the Northern Hemisphere phenomenon will be referenced. The easiest way to imagine this is if a plane where to fly from the north pole to the equator, moving in a straight path. If the plane took 6 hours to make this flight, the earth would have rotated 90 degrees. To an observer the plane would have appeared to drift to the west, when in fact it just stayed straight. That is why planes must adjust their flight pattern to accomodate for the Coriolis effect.
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The last factor affecting wind is the friction with the Earth's surface. This is only important within the first few kilometers of the Earth's surface, where the air is most dense and tightly packed. To understand how friction works, lets consider a situation in which it has no role. Above the friction layer, the pressure gradient force and Coriolis effect work together to direct the flow of air. Under these conditions, the pressure gradient force causes air to start moving across the isobars, As soon as the air starts to move, the Coriolis effect acts at right angles to this motion. The faster the wind speed, the greater the deflection. Eventually, the Coriolis effect will balance the pressure gradient force and the wind will blow parallel to the isobars. Upper-air winds generally take this path and are called geostrophic winds. Because of the lack of friction with Earth's surface, geostrophic winds travel at higher speeds than do surface winds. The most prominent features of upper-level flow are the jet streams. Below 600 meters (2000 feet), friction complicates the airflow just described. Recall that the Coriolis effect is proportional to wind speed. Friction lowers the wind speed, so it reduces the Coriolis effect. Because the pressure gradient force is not affected by wind speed, it wins the tug of war between itself and Coriolis effect. The result is a movement of air at an angle across the isobars toward the area of lower pressure. The roughness of terrain determines the angle of airflow across the isobars. Over the smooth ocean surface, friction is low and the angle is small. Over rugged terrain, where friction is higher, the angle that air makes as it flows across the isobars can be as great as 45 degrees.
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Cyclones, or lows, are centers of low pressure. The pressure decreases from the outer isobars toward the center in a cyclone. Due to the factors affecting wind, air around a cyclone is blown in a counterclockwise direction. The air moves from a high pressure system to a low, and with the pressure gradient force and the Coriolis effect, the wind is bent around the low flowing towards the center. A 3D image of a cyclone would look like a bowl, where the outer edge is up in the high atmosphere, and the center is low to the ground.
Anticyclones, or highs, are centers of high pressure. The pressure increases from the outer isobars toward the center in an anticyclone. The circulation around a high is the opposite of a low, which would be clockwise. Air flows outward from the center, rather than toward. A 3D image of a cyclone would look like an overturned bowl, or a convex lens, where the highest point is in the center.
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Weather generalizations about Highs and Lows
Rising air is associated with cloud formation and precipitation, whereas subsidence produces clear skies. The air flows inward in a cyclone, causing the pressure to increase. To eliviate this stress caused by convergence, the air rises into the atmosphere. This vertical movement of air can cause cloud formation, if the air contains enough water vapor in order for it to condense and form water droplets. Most often, where there is convergence, there are storms. Low pressure systems are usually associated with bad weather. When you hear that the pressure is "30.05 and dropping", this means that a low pressure system is approaching, and you might expect a storm, or at least clouds. The opposite happens around a high. The air is diverging, since it is leaving the high pressure to flow to the low pressure. This causes the air above the anticyclone to subside to fill the void left by the departing air. Since there is no upward movement of air, clouds do not form, and anticyclones are associated with fair weather. Figure 2 shows convergence and divergence as it relates to high and low pressure systems.
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