Page 187 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 6 Atmospheric and Oceanic Circulations 151
would simply move in paths parallel to isobars and at high rates of speed. The effect of surface friction extends to a height of about 500 m; thus, upper-air winds are not affected by the friction force. At the surface, the effect of friction varies with surface texture, wind speed, time of day and year, and atmospheric conditions. In general, rougher surfaces produce more friction.
Summary of Physical Forces on Winds
Winds are a result of the combination of these physi- cal forces (Figure 6.8). When the pressure gradient acts alone, shown in Figure 6.8a, winds flow from areas of high pressure to areas of low pressure. Note the descend- ing, diverging air associated with high pressure and the ascending, converging air associated with low pressure in the side view.
Figure 6.8b illustrates the combined effect of the pres- sure gradient force and the Coriolis force on air currents in the upper atmosphere, above about 1000 m. Together, they produce winds that do not flow directly from high to low, but that flow around the pressure areas, remaining parallel to the isobars. Such winds are geostrophic winds and are characteristic of upper tropospheric circula- tion. (The suffix -strophic means “to turn.”) Geostrophic winds produce the characteristic pattern shown on the upper-air weather map just ahead in Figure 6.12.
Near the surface, friction prevents the equilibrium between the pressure gradient and Coriolis forces that results in geostrophic wind flows in the upper atmo- sphere (Figure 6.8c). Because surface friction decreases wind speed, it reduces the effect of the Coriolis force and causes winds to move across isobars at an angle. Thus, wind flows around pressure centres form enclosed areas called pressure systems, or pressure cells, as illustrated in Figure 6.8c.
High- and Low-Pressure Systems
In the Northern Hemisphere, surface winds spi- ral out from a high-pressure area in a clockwise di- rection, forming an anticyclone, and spiral into a low-pressure area in a counterclockwise direction, forming a cyclone (Figure 6.8). In the Southern Hemi- sphere these circulation patterns are reversed, with winds flowing counterclockwise out of anticyclonic high-pressure cells and clockwise into cyclonic low- pressure cells.
Anticyclones and cyclones have vertical air move- ment in addition to these horizontal patterns. As air moves away from the centres of an anticyclone, it is
replaced by descending, or subsiding (sinking), air. These high-pressure systems are typically characterised by clear skies. As surface air flows toward the centres of a cyclone, it converges and moves upward. These ris- ing motions promote the formation of cloudy and stormy weather, as we will see in Chapters 7 and 8.
Figure 6.9 shows high- and low-pressure systems on a weather map, with a side view of the wind movement around and within each pressure cell. You may have noticed that on weather maps, pressure systems vary in size and shape. Often these cells have elongated shapes and are called low-pressure “troughs” or high-pressure “ridges” (illustrated in Figure 6.12 just ahead).
Atmospheric Patterns of Motion
Atmospheric circulation is categorized at three levels: primary circulation, consisting of general worldwide cir- culation; secondary circulation, consisting of migratory high-pressure and low-pressure systems; and tertiary cir- culation, including local winds and temporal weather pat- terns. Winds that move principally north or south along meridians of longitude are meridional flows. Winds mov- ing east or west along parallels of latitude are zonal flows.
With the concepts related to pressure and wind movement in mind, we are ready to examine primary cir- culation and build a general model of Earth’s circulation patterns. To begin, we should remember the relation- ships between pressure, density, and temperature as they apply to the unequal heating of Earth’s surface (energy surpluses at the equator and energy deficits at the poles). The warmer, less-dense air along the equator rises, cre- ating low pressure at the surface, and the colder, more- dense air at the poles sinks, creating high pressure at the surface. If Earth did not rotate, the result would be a simple wind flow from the poles to the equator, a meridi- onal flow caused solely by pressure gradient. However, Earth does rotate, creating a more complex flow system. On a rotating Earth, the poles-to-equator flow is broken up into latitudinal zones, both at the surface and aloft in the upper-air winds.
Primary Pressure Areas
and Associated Winds
The maps in Figure 6.10 (page 154) show average surface barometric pressures in January and July. Indirectly, these maps indicate prevailing surface winds, which
Georeport 6.3 Coriolis: Not a Force on Sinks or Toilets
A common misconception about the Coriolis force is that it affects water draining out of a sink, tub, or toilet. Moving water or air must cover some distance across space and time before the Coriolis force noticeably deflects it. Long-range artillery
shells and guided missiles do exhibit small amounts of deflection that must be corrected for accuracy. But water movements down a drain are too small in spatial extent to be noticeably affected by this force.