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Lesson 7-04 Air Pressure

Page history last edited by debra.krohn@gmail.com 11 years, 3 months ago


Lesson 7.04 – Air Pressure

Standard:  ES5.a Students know how differential heating of Earth results in circulation patterns in the atmosphere and oceans that globally distribute the heat.

ES5.b Students know the relationship between the rotation of Earth and the circular motions of ocean currents and air in pressure centers.




Of the various elements of weather and climate, changes in air pressure are the least noticeable. When you listen to a weather report, you probably focus on precipitation, temperature, and humidity. Most people don’t wonder about air pressure. Although you might not perceive hour-to-hour and day-to-day variations in air pressure, they are very important in producing changes in our weather. For example, variations in air pressure from place to place can generate winds like those shown in Figure 1. The winds, in turn, bring change in temperature and humidity. Air pressure is one of the basic weather elements and is an important factor in weather forecasting. Air pressure is closely tied to the other elements of weather in a cause-and-effect relationship.



Figure 1 These palm trees in Corpus Christi, Texas, are buffeted by hurricane-force winds.




Air Pressure Defined

Air pressure is simply the pressure exerted by the weight of air above. Average air pressure at sea level is about 1 kilogram per square centimeter. This pressure is roughly the same pressure that is produced by a column of water 10 meters in height. You can calculate that the air pressure exerted on the top of a 50-centimeter-by-100-centimeter school desk exceeds 5000 kilograms, which is about the mass of a 50-passenger school bus. Why doesn’t the desk collapse under the weight of the air above it? Air pressure is exerted in all directions—down, up, and sideways. The air pressure pushing down on an object exactly balances the air pressure pushing up on the object.



Imagine a tall aquarium that has the same dimensions as the desktop in the previous example. When this aquarium is filled to a height of 10 meters, the water pressure at the bottom equals 1 atmosphere, or 1 kilogram per square centimeter. Now imagine what will happen if this aquarium is placed on top of a student desk so that all the force is directed downward. The desk collapses because the pressure downward is greater than the pressure exerted in the other directions. When the desk is placed inside the aquarium and allowed to sink to the bottom, however, the desk does not collapse in the water because the water pressure is exerted in all directions, not just downward. The desk, like your body, is built to withstand the pressure of 1 atmosphere.



Measuring Air Pressure

When meteorologists measure atmospheric pressure, they use a unit called the millibar. Standard sea-level pressure is 1013.2 millibars. You might have heard the phrase “inches of mercury,” which is used by the media to describe atmospheric pressure. This expression dates from 1643, when Torricelli, a student of the famous Italian scientist Galileo, invented the mercury barometer. A barometer is a device used for measuring air pressure (bar = pressure, metron = measuring instrument).



Torricelli correctly described the atmosphere as a vast ocean of air that exerts pressure on us and all objects around us. To measure this force, he filled a glass tube, closed at one end, with mercury. He then put the tube upside down into a dish of mercury, as shown in Figure 2A. The mercury flowed out of the tube until the weight of the column was balanced by the pressure that the atmosphere exerted on the surface of the mercury in the dish. In other words, the weight of mercury in the column (tube) equaled the weight of the same size column of air that extended from the ground to the top of the atmosphere.

Figure 2 A Mercury Barometer Standard atmospheric pressure at sea level is 29.92 inches of mercury. B Aneroid Barometer The recording mechanism provides a continuous record of pressure changes over time. Applying Concepts Why would a continuous record help weather forecasters?



When air pressure increases, the mercury in the tube rises. When air pressure decreases, so does the height of the mercury column. With some improvements, the mercury barometer is still the standard instrument used today for measuring air pressure.



The need for a smaller and more portable instrument for measuring air pressure led to the development of the aneroid barometer. The aneroid barometer uses a metal chamber with some air removed. This partially emptied chamber is extremely sensitive to variations in air pressure. It changes shape and compresses as the air pressure increases, and it expands as the pressure decreases. One advantage of the aneroid barometer is that it can be easily connected to a recording device, shown in Figure 2B. The device provides a continuous record of pressure changes with the passage of time.



Factors Affecting Wind

As important as vertical motion is, far more air moves horizontally, the phenomenon we call wind. What causes wind?

Wind is the result of horizontal differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure. You may have experienced this flow of air when opening a vacuum-packed can of coffee or tennis balls. The noise you hear is caused by air rushing from the higher pressure outside the can to the lower pressure inside. Wind is nature’s way of balancing such inequalities in air pressure. The unequal heating of Earth’s surface generates pressure differences. Solar radiation is the ultimate energy source for most wind.



If Earth did not rotate, and if there were no friction between moving air and Earth’s surface, air would flow in a straight line from areas of higher pressure to areas of lower pressure. But both factors do exist so the flow of air is not that simple. Three factors combine to control wind: pressure differences, the Coriolis effect, and friction.



Pressure Differences

Wind is created from differences in pressure—the greater these differences are, the greater the wind speed is. Over Earth’s surface, variations in air pressure are determined from barometric readings taken at hundreds of weather stations. These pressure data are shown on a weather map, like the one in Figure 3, using isobars. Isobars are lines on a map that connect places of equal air pressure. The spacing of isobars indicates the amount of pressure change occurring over a given distance. These pressure changes are expressed as the pressure gradient.




Q What is the lowest barometric pressure ever recorded?

A All of the lowest recorded barometric pressures have been associated with strong hurricanes. The record for the United States is 888 millibars (26.20 inches) measured during Hurricane Gilbert in September 1988. The world’s record, 870 millibars (25.70 inches), occurred during Typhoon Tip, a Pacific hurricane, in October 1979. Although tornadoes undoubtedly have produced even lower pressures, they have not been accurately measured.

Figure 3 Isobars The distribution of air pressure is shown on weather maps using isobar lines. Wind flags indicate wind speed and direction. Winds blow toward the station circles. Interpreting Visuals Use the data on this map to explain which pressure cell, high or low, has the fastest wind speeds.



A steep pressure gradient, like a steep hill, causes greater acceleration of a parcel of air. A less steep pressure gradient causes a slower acceleration. Closely spaced isobars indicate a steep pressure gradient and high winds. Widely spaced isobars indicate a weak pressure gradient and light winds. The pressure gradient is the driving force of wind. The pressure gradient has both magnitude and direction. Its magnitude is reflected in the spacing of isobars. The direction of force is always from areas of higher pressure to areas of lower pressure and at right angles to the isobars. Friction affects wind speed and direction. The Coriolis effect affects wind direction only.



Coriolis Effect

The weather map in Figure 3 shows typical air movements associated with high- and low-pressure systems. Air moves out of the regions of higher pressure and into the regions of lower pressure. However, the wind does not cross the isobars at right angles as you would expect based solely on the pressure gradient. This change in movement results from Earth’s rotation and has been named the Coriolis effect.



The Coriolis effect describes how Earth’s rotation affects moving objects. All free-moving objects or fluids, including the wind, are deflected to the right of their path of motion in the Northern Hemisphere. In the Southern Hemisphere, they are deflected to the left. The reason for this deflection is illustrated in Figure 4. Imagine the path of a rocket launched from the North Pole toward a target located on the equator. The true path of this rocket is straight, and the path would appear to be straight to someone out in space looking down at Earth. However, to someone standing on Earth, it would look as if the rocket swerved off its path and landed 15 degrees to the west of its target.



This slight change in direction happens because Earth would have rotated 15 degrees to the east under the rocket during a one-hour flight. The counterclockwise rotation of the Northern Hemisphere causes path deflection to the right. In the Southern Hemisphere, the clockwise rotation produces a similar deflection, but to the left of the path of motion.



Figure 4 The Coriolis Effect Because Earth rotates 15° each hour, the rocket’s path is curved and veers to the right from the North Pole to the equator. Calculating How many degrees does Earth rotate in one day?



The apparent shift in wind direction is attributed to the Coriolis effect. This deflection: 1) is always directed at right angles to the direction of airflow; 2) affects only wind direction and not wind speed; 3) is affected by wind speed—the stronger the wind, the greater the deflection; and 4) is strongest at the poles and weakens toward the equator, becoming nonexistent at the equator.




The effect of friction on wind is important only within a few kilometers of Earth’s surface. Friction acts to slow air movement, which changes wind direction. To illustrate friction’s effect on wind direction, first think about a situation in which friction does not play a role in wind’s direction.



When air is above the friction layer, the pressure gradient causes air to move across the isobars. As soon as air starts to move, the Coriolis effect acts at right angles to this motion. The faster the wind speed, the greater the deflection is. The pressure gradient and Coriolis effect balance in high-altitude air, and wind generally flows parallel to isobars, as shown in Figure 5A. The most prominent features of airflow high above the friction layer are the jet streams. Jet streams are fast-moving rivers of air that travel between 120 and 240 kilometers per hour in a west-to-east direction. One such jet stream is situated over the polar front, which is the zone separating the cool polar air from warm subtropical air. Jet streams originally were encountered by high-flying bombers during World War II.



Figure 5 A Upper-level wind flow is balanced by the Coriolis effect and pressure gradient forces. B Friction causes surface winds to cross isobars and move toward lower pressure areas.



For air close to Earth’s surface, the roughness of the terrain determines the angle of airflow across the isobars. Over the smooth ocean surface, friction is low, and the angle of airflow is small. Over rugged terrain, where the friction is higher, winds move more slowly and cross the isobars at greater angles. As shown in Figure 5B, friction causes wind to flow across the isobars at angles as great as 45 degrees. Slower wind speeds caused by friction decrease the Coriolis effect.




  1. Take notes on the above information.
  2. Click here (http://videos.howstuffworks.com/hsw/16848-hands-on-weather-ii-a-look-at-air-pressure-video.htm) to watch a video about measuring air pressure.
  3. Click here (http://videos.howstuffworks.com/hsw/19241-simply-science-air-currents-video.htm) to watch a video explaining the Coriolis Effect and wind patterns around earth.
  4. Read Section 19.1 in the online textbook https://www.pearsonsuccessnet.com/snpapp/login.do?method=login




  1. Turn in your notes.
  2. Take the 7.04 Quiz in Quia.

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