Page 181 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 6 atmospheric and Oceanic Circulations 145
system, our atmosphere is shared by all humanity—one person’s or country’s exhalation is another’s inhalation.
In this chapter: We begin with a discussion of wind essentials, including air pressure and the measurement of wind. We examine the driving forces that produce and determine the speed and direction of surface winds: pres­ sure gradients, the Coriolis force, and friction. We then look at the circulation of Earth’s atmosphere, including the principal pressure systems, patterns of global surface winds, upper­atmosphere winds, monsoons, and local winds. Finally, we consider Earth’s wind­driven oceanic currents and explain multiyear oscillations in atmo­ spheric and oceanic flows. The energy driving all this movement comes from one source: the Sun.
Wind Essentials
The large­scale circulation of winds across Earth has fas­ cinated explorers, sailors, and scientists for centuries, al­ though only in the modern era is a clear picture emerging of global winds. Driven by the imbalance between equa­ torial energy surpluses and polar energy deficits, Earth’s atmospheric circulation transfers both energy and mass on a grand scale, determining Earth’s weather patterns and the flow of ocean currents. The atmosphere is the dominant medium for redistributing energy from about 35° latitude to the poles in each hemisphere, whereas ocean currents redistribute more heat in a zone strad­ dling the equator between the 17th parallels in each hemisphere. Atmospheric circulation also spreads air pollutants, whether natural or human­caused, world­ wide, far from their point of origin.
Air Pressure
Air pressure—the weight of the atmosphere described as force per unit area—is key to understanding wind. The molecules that constitute air create air pressure through their motion, size, and number, and this pressure is exerted on all surfaces in contact with air. As we saw in Chapter 3, the number of molecules and their motion are also the fac­ tors that determine the density and temperature of the air.
Pressure Relationships As Chapter 3 discussed, both pressure and density decrease with altitude in the atmo­ sphere. The low density in the upper atmosphere means the molecules are far apart, making collisions between them less frequent and thereby reducing pressure (re­ view Figure 3.2). However, differences in air pressure are noticeable even between sea level and the summits of Earth’s highest mountains.
The subjective experience of “thin air” at altitude is caused by the smaller amount of oxygen available to in­ hale (fewer air molecules means less oxygen). Mountain­ eers feel the effects of thin air as headaches, shortness of breath, and disorientation as less oxygen reaches their brain—these are the symptoms of acute mountain sick- ness. Near the summits of the highest Himalayan peaks, some climbers use oxygen tanks to counteract these effects, which are worsened by ascending quickly with­ out giving the body time to acclimatize, or adapt, to the decrease in oxygen.
Remember from Chapter 5 that temperature is a mea­ sure of the average kinetic energy of molecular motion. When air in the atmosphere is heated, molecular activity increases and temperature rises. With increased activity, the spacing between molecules increases so that density is reduced and air pressure decreases. Therefore, warmer air is less dense, or lighter, than colder air, and exerts less pressure.
The amount of water vapour in the air also affects its density. Moist air is lighter because the molecular weight of water is less than that of the molecules making up dry air. If the same total number of molecules has a higher percentage of water vapour, mass will be less than if the air were dry (that is, than if it were made up entirely of oxygen and nitrogen molecules). As water vapour in the air increases, density decreases, so humid air exerts less pressure than dry air.
The end result over Earth’s surface is that warm, humid air is associated with low pressure and cold, dry air is associated with high pressure. These relationships between pressure, density, temperature, and moisture are important to the discussion ahead.
Air Pressure Measurement In 1643, work by Evange­ lista Torricelli, a pupil of Galileo, on a mine­drainage problem led to the first method for measuring air pres­ sure (Figure 6.2a). Torricelli knew that pumps in the mine were able to “pull” water upward about 10 m but no higher, and that this level fluctuated from day to day. Careful observation revealed that the limitation was not the fault of the pumps but a property of the atmosphere itself. He figured out that air pressure, the weight of the air, varies with weather conditions and that this weight determined the height of the water in the pipe.
To simulate the problem at the mine, Torricelli devised an instrument using a much denser fluid than water—mercury (Hg)—and a glass tube 1 m high. He sealed the glass tube at one end, filled it with mercury, and inverted it into a dish containing mercury, at which point a small space containing a vacuum was formed in
Georeport 6.1 Blowing in the Wind
Dust originating in africa sometimes increases the iron content of the waters off Florida, promoting the toxic algal blooms (Karenia brevis) known as “red tides.” in the amazon, soil samples bear the dust print of these former african soils that
crossed the atlantic. active research on such dust is part of the U.S. navy’s aerosol analysis and Prediction System; see links and the latest navy research at www.nrlmry.navy.mil/7544.html.